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   (a) The FSF's Back-Cover Text is: "You are free to copy and modify
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INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* Gdb: (gdb).                     The GNU debugger.
* gdbserver: (gdb) Server.        The GNU debugging server.
END-INFO-DIR-ENTRY

   This file documents the GNU debugger GDB.

   This is the Tenth Edition, of ‘Debugging with GDB: the GNU
Source-Level Debugger’ for GDB (GDB) Version
18.0.50.20260610-ddc575edf10.

   Copyright © 1988-2026 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software" and "Free Software Needs Free
Documentation", with the Front-Cover Texts being "A GNU Manual," and
with the Back-Cover Texts as in (a) below.

   (a) The FSF's Back-Cover Text is: "You are free to copy and modify
this GNU Manual.  Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom."


File: gdb.info,  Node: Analyze Collected Data,  Next: Tracepoint Variables,  Prev: Set Tracepoints,  Up: Tracepoints

13.2 Using the Collected Data
=============================

After the tracepoint experiment ends, you use GDB commands for examining
the trace data.  The basic idea is that each tracepoint collects a trace
“snapshot” every time it is hit and another snapshot every time it
single-steps.  All these snapshots are consecutively numbered from zero
and go into a buffer, and you can examine them later.  The way you
examine them is to “focus” on a specific trace snapshot.  When the
remote stub is focused on a trace snapshot, it will respond to all GDB
requests for memory and registers by reading from the buffer which
belongs to that snapshot, rather than from _real_ memory or registers of
the program being debugged.  This means that *all* GDB commands
(‘print’, ‘info registers’, ‘backtrace’, etc.)  will behave as if we
were currently debugging the program state as it was when the tracepoint
occurred.  Any requests for data that are not in the buffer will fail.

* Menu:

* tfind::                       How to select a trace snapshot
* tdump::                       How to display all data for a snapshot
* save tracepoints::            How to save tracepoints for a future run


File: gdb.info,  Node: tfind,  Next: tdump,  Up: Analyze Collected Data

13.2.1 ‘tfind N’
----------------

The basic command for selecting a trace snapshot from the buffer is
‘tfind N’, which finds trace snapshot number N, counting from zero.  If
no argument N is given, the next snapshot is selected.

   Here are the various forms of using the ‘tfind’ command.

‘tfind start’
     Find the first snapshot in the buffer.  This is a synonym for
     ‘tfind 0’ (since 0 is the number of the first snapshot).

‘tfind none’
     Stop debugging trace snapshots, resume _live_ debugging.

‘tfind end’
     Same as ‘tfind none’.

‘tfind’
     No argument means find the next trace snapshot or find the first
     one if no trace snapshot is selected.

‘tfind -’
     Find the previous trace snapshot before the current one.  This
     permits retracing earlier steps.

‘tfind tracepoint NUM’
     Find the next snapshot associated with tracepoint NUM.  Search
     proceeds forward from the last examined trace snapshot.  If no
     argument NUM is given, it means find the next snapshot collected
     for the same tracepoint as the current snapshot.

‘tfind pc ADDR’
     Find the next snapshot associated with the value ADDR of the
     program counter.  Search proceeds forward from the last examined
     trace snapshot.  If no argument ADDR is given, it means find the
     next snapshot with the same value of PC as the current snapshot.

‘tfind outside ADDR1, ADDR2’
     Find the next snapshot whose PC is outside the given range of
     addresses (exclusive).

‘tfind range ADDR1, ADDR2’
     Find the next snapshot whose PC is between ADDR1 and ADDR2
     (inclusive).

‘tfind line [FILE:]N’
     Find the next snapshot associated with the source line N.  If the
     optional argument FILE is given, refer to line N in that source
     file.  Search proceeds forward from the last examined trace
     snapshot.  If no argument N is given, it means find the next line
     other than the one currently being examined; thus saying ‘tfind
     line’ repeatedly can appear to have the same effect as stepping
     from line to line in a _live_ debugging session.

   The default arguments for the ‘tfind’ commands are specifically
designed to make it easy to scan through the trace buffer.  For
instance, ‘tfind’ with no argument selects the next trace snapshot, and
‘tfind -’ with no argument selects the previous trace snapshot.  So, by
giving one ‘tfind’ command, and then simply hitting <RET> repeatedly you
can examine all the trace snapshots in order.  Or, by saying ‘tfind -’
and then hitting <RET> repeatedly you can examine the snapshots in
reverse order.  The ‘tfind line’ command with no argument selects the
snapshot for the next source line executed.  The ‘tfind pc’ command with
no argument selects the next snapshot with the same program counter (PC)
as the current frame.  The ‘tfind tracepoint’ command with no argument
selects the next trace snapshot collected by the same tracepoint as the
current one.

   In addition to letting you scan through the trace buffer manually,
these commands make it easy to construct GDB scripts that scan through
the trace buffer and print out whatever collected data you are
interested in.  Thus, if we want to examine the PC, FP, and SP registers
from each trace frame in the buffer, we can say this:

     (gdb) tfind start
     (gdb) while ($trace_frame != -1)
     > printf "Frame %d, PC = %08X, SP = %08X, FP = %08X\n", \
               $trace_frame, $pc, $sp, $fp
     > tfind
     > end

     Frame 0, PC = 0020DC64, SP = 0030BF3C, FP = 0030BF44
     Frame 1, PC = 0020DC6C, SP = 0030BF38, FP = 0030BF44
     Frame 2, PC = 0020DC70, SP = 0030BF34, FP = 0030BF44
     Frame 3, PC = 0020DC74, SP = 0030BF30, FP = 0030BF44
     Frame 4, PC = 0020DC78, SP = 0030BF2C, FP = 0030BF44
     Frame 5, PC = 0020DC7C, SP = 0030BF28, FP = 0030BF44
     Frame 6, PC = 0020DC80, SP = 0030BF24, FP = 0030BF44
     Frame 7, PC = 0020DC84, SP = 0030BF20, FP = 0030BF44
     Frame 8, PC = 0020DC88, SP = 0030BF1C, FP = 0030BF44
     Frame 9, PC = 0020DC8E, SP = 0030BF18, FP = 0030BF44
     Frame 10, PC = 00203F6C, SP = 0030BE3C, FP = 0030BF14

   Or, if we want to examine the variable ‘X’ at each source line in the
buffer:

     (gdb) tfind start
     (gdb) while ($trace_frame != -1)
     > printf "Frame %d, X == %d\n", $trace_frame, X
     > tfind line
     > end

     Frame 0, X = 1
     Frame 7, X = 2
     Frame 13, X = 255


File: gdb.info,  Node: tdump,  Next: save tracepoints,  Prev: tfind,  Up: Analyze Collected Data

13.2.2 ‘tdump’
--------------

This command takes no arguments.  It prints all the data collected at
the current trace snapshot.

     (gdb) trace 444
     (gdb) actions
     Enter actions for tracepoint #2, one per line:
     > collect $regs, $locals, $args, gdb_long_test
     > end

     (gdb) tstart

     (gdb) tfind line 444
     #0  gdb_test (p1=0x11, p2=0x22, p3=0x33, p4=0x44, p5=0x55, p6=0x66)
     at gdb_test.c:444
     444        printp( "%s: arguments = 0x%X 0x%X 0x%X 0x%X 0x%X 0x%X\n", )

     (gdb) tdump
     Data collected at tracepoint 2, trace frame 1:
     d0             0xc4aa0085       -995491707
     d1             0x18     24
     d2             0x80     128
     d3             0x33     51
     d4             0x71aea3d        119204413
     d5             0x22     34
     d6             0xe0     224
     d7             0x380035 3670069
     a0             0x19e24a 1696330
     a1             0x3000668        50333288
     a2             0x100    256
     a3             0x322000 3284992
     a4             0x3000698        50333336
     a5             0x1ad3cc 1758156
     fp             0x30bf3c 0x30bf3c
     sp             0x30bf34 0x30bf34
     ps             0x0      0
     pc             0x20b2c8 0x20b2c8
     fpcontrol      0x0      0
     fpstatus       0x0      0
     fpiaddr        0x0      0
     p = 0x20e5b4 "gdb-test"
     p1 = (void *) 0x11
     p2 = (void *) 0x22
     p3 = (void *) 0x33
     p4 = (void *) 0x44
     p5 = (void *) 0x55
     p6 = (void *) 0x66
     gdb_long_test = 17 '\021'

     (gdb)

   ‘tdump’ works by scanning the tracepoint's current collection actions
and printing the value of each expression listed.  So ‘tdump’ can fail,
if after a run, you change the tracepoint's actions to mention variables
that were not collected during the run.

   Also, for tracepoints with ‘while-stepping’ loops, ‘tdump’ uses the
collected value of ‘$pc’ to distinguish between trace frames that were
collected at the tracepoint hit, and frames that were collected while
stepping.  This allows it to correctly choose whether to display the
basic list of collections, or the collections from the body of the
while-stepping loop.  However, if ‘$pc’ was not collected, then ‘tdump’
will always attempt to dump using the basic collection list, and may
fail if a while-stepping frame does not include all the same data that
is collected at the tracepoint hit.


File: gdb.info,  Node: save tracepoints,  Prev: tdump,  Up: Analyze Collected Data

13.2.3 ‘save tracepoints FILENAME’
----------------------------------

This command saves all current tracepoint definitions together with
their actions and passcounts, into a file ‘FILENAME’ suitable for use in
a later debugging session.  To read the saved tracepoint definitions,
use the ‘source’ command (*note Command Files::).  The
‘save-tracepoints’ command is a deprecated alias for ‘save tracepoints’


File: gdb.info,  Node: Tracepoint Variables,  Next: Trace Files,  Prev: Analyze Collected Data,  Up: Tracepoints

13.3 Convenience Variables for Tracepoints
==========================================

‘(int) $trace_frame’
     The current trace snapshot (a.k.a. “frame”) number, or -1 if no
     snapshot is selected.

‘(int) $tracepoint’
     The tracepoint for the current trace snapshot.

‘(int) $trace_line’
     The line number for the current trace snapshot.

‘(char []) $trace_file’
     The source file for the current trace snapshot.

‘(char []) $trace_func’
     The name of the function containing ‘$tracepoint’.

   Note: ‘$trace_file’ is not suitable for use in ‘printf’, use ‘output’
instead.

   Here's a simple example of using these convenience variables for
stepping through all the trace snapshots and printing some of their
data.  Note that these are not the same as trace state variables, which
are managed by the target.

     (gdb) tfind start

     (gdb) while $trace_frame != -1
     > output $trace_file
     > printf ", line %d (tracepoint #%d)\n", $trace_line, $tracepoint
     > tfind
     > end


File: gdb.info,  Node: Trace Files,  Prev: Tracepoint Variables,  Up: Tracepoints

13.4 Using Trace Files
======================

In some situations, the target running a trace experiment may no longer
be available; perhaps it crashed, or the hardware was needed for a
different activity.  To handle these cases, you can arrange to dump the
trace data into a file, and later use that file as a source of trace
data, via the ‘target tfile’ command.

‘tsave [ -r ] FILENAME’
     Save the trace data to FILENAME.  By default, this command assumes
     that FILENAME refers to the host filesystem, so if necessary GDB
     will copy raw trace data up from the target and then save it.  If
     the target supports it, you can also supply the optional argument
     ‘-r’ ("remote") to direct the target to save the data directly into
     FILENAME in its own filesystem, which may be more efficient if the
     trace buffer is very large.  (Note, however, that ‘target tfile’
     can only read from files accessible to the host.)

‘target tfile FILENAME’
     Use the file named FILENAME as a source of trace data.  Commands
     that examine data work as they do with a live target, but it is not
     possible to run any new trace experiments.  ‘tstatus’ will report
     the state of the trace run at the moment the data was saved, as
     well as the current trace frame you are examining.  FILENAME must
     be on a filesystem accessible to the host.

     The FILENAME argument supports escaping and quoting, see *note
     Filenames As Command Arguments: Filename Arguments.


File: gdb.info,  Node: Overlays,  Next: Languages,  Prev: Tracepoints,  Up: Top

14 Debugging Programs That Use Overlays
***************************************

If your program is too large to fit completely in your target system's
memory, you can sometimes use “overlays” to work around this problem.
GDB provides some support for debugging programs that use overlays.

* Menu:

* How Overlays Work::              A general explanation of overlays.
* Overlay Commands::               Managing overlays in GDB.
* Automatic Overlay Debugging::    GDB can find out which overlays are
                                   mapped by asking the inferior.
* Overlay Sample Program::         A sample program using overlays.


File: gdb.info,  Node: How Overlays Work,  Next: Overlay Commands,  Up: Overlays

14.1 How Overlays Work
======================

Suppose you have a computer whose instruction address space is only 64
kilobytes long, but which has much more memory which can be accessed by
other means: special instructions, segment registers, or memory
management hardware, for example.  Suppose further that you want to
adapt a program which is larger than 64 kilobytes to run on this system.

   One solution is to identify modules of your program which are
relatively independent, and need not call each other directly; call
these modules “overlays”.  Separate the overlays from the main program,
and place their machine code in the larger memory.  Place your main
program in instruction memory, but leave at least enough space there to
hold the largest overlay as well.

   Now, to call a function located in an overlay, you must first copy
that overlay's machine code from the large memory into the space set
aside for it in the instruction memory, and then jump to its entry point
there.

         Data             Instruction            Larger
     Address Space       Address Space        Address Space
     +-----------+       +-----------+        +-----------+
     |           |       |           |        |           |
     +-----------+       +-----------+        +-----------+<-- overlay 1
     | program   |       |   main    |   .----| overlay 1 | load address
     | variables |       |  program  |   |    +-----------+
     | and heap  |       |           |   |    |           |
     +-----------+       |           |   |    +-----------+<-- overlay 2
     |           |       +-----------+   |    |           | load address
     +-----------+       |           |   |  .-| overlay 2 |
                         |           |   |  | |           |
              mapped --->+-----------+   |  | +-----------+
              address    |           |   |  | |           |
                         |  overlay  | <-'  | |           |
                         |   area    |  <---' +-----------+<-- overlay 3
                         |           | <---.  |           | load address
                         +-----------+     `--| overlay 3 |
                         |           |        |           |
                         +-----------+        |           |
                                              +-----------+
                                              |           |
                                              +-----------+

                         A code overlay

   The diagram (*note A code overlay::) shows a system with separate
data and instruction address spaces.  To map an overlay, the program
copies its code from the larger address space to the instruction address
space.  Since the overlays shown here all use the same mapped address,
only one may be mapped at a time.  For a system with a single address
space for data and instructions, the diagram would be similar, except
that the program variables and heap would share an address space with
the main program and the overlay area.

   An overlay loaded into instruction memory and ready for use is called
a “mapped” overlay; its “mapped address” is its address in the
instruction memory.  An overlay not present (or only partially present)
in instruction memory is called “unmapped”; its “load address” is its
address in the larger memory.  The mapped address is also called the
“virtual memory address”, or “VMA”; the load address is also called the
“load memory address”, or “LMA”.

   Unfortunately, overlays are not a completely transparent way to adapt
a program to limited instruction memory.  They introduce a new set of
global constraints you must keep in mind as you design your program:

   • Before calling or returning to a function in an overlay, your
     program must make sure that overlay is actually mapped.  Otherwise,
     the call or return will transfer control to the right address, but
     in the wrong overlay, and your program will probably crash.

   • If the process of mapping an overlay is expensive on your system,
     you will need to choose your overlays carefully to minimize their
     effect on your program's performance.

   • The executable file you load onto your system must contain each
     overlay's instructions, appearing at the overlay's load address,
     not its mapped address.  However, each overlay's instructions must
     be relocated and its symbols defined as if the overlay were at its
     mapped address.  You can use GNU linker scripts to specify
     different load and relocation addresses for pieces of your program;
     see *note (ld.info)Overlay Description::.

   • The procedure for loading executable files onto your system must be
     able to load their contents into the larger address space as well
     as the instruction and data spaces.

   The overlay system described above is rather simple, and could be
improved in many ways:

   • If your system has suitable bank switch registers or memory
     management hardware, you could use those facilities to make an
     overlay's load area contents simply appear at their mapped address
     in instruction space.  This would probably be faster than copying
     the overlay to its mapped area in the usual way.

   • If your overlays are small enough, you could set aside more than
     one overlay area, and have more than one overlay mapped at a time.

   • You can use overlays to manage data, as well as instructions.  In
     general, data overlays are even less transparent to your design
     than code overlays: whereas code overlays only require care when
     you call or return to functions, data overlays require care every
     time you access the data.  Also, if you change the contents of a
     data overlay, you must copy its contents back out to its load
     address before you can copy a different data overlay into the same
     mapped area.


File: gdb.info,  Node: Overlay Commands,  Next: Automatic Overlay Debugging,  Prev: How Overlays Work,  Up: Overlays

14.2 Overlay Commands
=====================

To use GDB's overlay support, each overlay in your program must
correspond to a separate section of the executable file.  The section's
virtual memory address and load memory address must be the overlay's
mapped and load addresses.  Identifying overlays with sections allows
GDB to determine the appropriate address of a function or variable,
depending on whether the overlay is mapped or not.

   GDB's overlay commands all start with the word ‘overlay’; you can
abbreviate this as ‘ov’ or ‘ovly’.  The commands are:

‘overlay off’
     Disable GDB's overlay support.  When overlay support is disabled,
     GDB assumes that all functions and variables are always present at
     their mapped addresses.  By default, GDB's overlay support is
     disabled.

‘overlay manual’
     Enable “manual” overlay debugging.  In this mode, GDB relies on you
     to tell it which overlays are mapped, and which are not, using the
     ‘overlay map-overlay’ and ‘overlay unmap-overlay’ commands
     described below.

‘overlay map-overlay OVERLAY’
‘overlay map OVERLAY’
     Tell GDB that OVERLAY is now mapped; OVERLAY must be the name of
     the object file section containing the overlay.  When an overlay is
     mapped, GDB assumes it can find the overlay's functions and
     variables at their mapped addresses.  GDB assumes that any other
     overlays whose mapped ranges overlap that of OVERLAY are now
     unmapped.

‘overlay unmap-overlay OVERLAY’
‘overlay unmap OVERLAY’
     Tell GDB that OVERLAY is no longer mapped; OVERLAY must be the name
     of the object file section containing the overlay.  When an overlay
     is unmapped, GDB assumes it can find the overlay's functions and
     variables at their load addresses.

‘overlay auto’
     Enable “automatic” overlay debugging.  In this mode, GDB consults a
     data structure the overlay manager maintains in the inferior to see
     which overlays are mapped.  For details, see *note Automatic
     Overlay Debugging::.

‘overlay load-target’
‘overlay load’
     Re-read the overlay table from the inferior.  Normally, GDB
     re-reads the table GDB automatically each time the inferior stops,
     so this command should only be necessary if you have changed the
     overlay mapping yourself using GDB.  This command is only useful
     when using automatic overlay debugging.

‘overlay list-overlays’
‘overlay list’
     Display a list of the overlays currently mapped, along with their
     mapped addresses, load addresses, and sizes.

   Normally, when GDB prints a code address, it includes the name of the
function the address falls in:

     (gdb) print main
     $3 = {int ()} 0x11a0 <main>
When overlay debugging is enabled, GDB recognizes code in unmapped
overlays, and prints the names of unmapped functions with asterisks
around them.  For example, if ‘foo’ is a function in an unmapped
overlay, GDB prints it this way:

     (gdb) overlay list
     No sections are mapped.
     (gdb) print foo
     $5 = {int (int)} 0x100000 <*foo*>
When ‘foo’'s overlay is mapped, GDB prints the function's name normally:

     (gdb) overlay list
     Section .ov.foo.text, loaded at 0x100000 - 0x100034,
             mapped at 0x1016 - 0x104a
     (gdb) print foo
     $6 = {int (int)} 0x1016 <foo>

   When overlay debugging is enabled, GDB can find the correct address
for functions and variables in an overlay, whether or not the overlay is
mapped.  This allows most GDB commands, like ‘break’ and ‘disassemble’,
to work normally, even on unmapped code.  However, GDB's breakpoint
support has some limitations:

   • You can set breakpoints in functions in unmapped overlays, as long
     as GDB can write to the overlay at its load address.
   • GDB can not set hardware or simulator-based breakpoints in unmapped
     overlays.  However, if you set a breakpoint at the end of your
     overlay manager (and tell GDB which overlays are now mapped, if you
     are using manual overlay management), GDB will re-set its
     breakpoints properly.


File: gdb.info,  Node: Automatic Overlay Debugging,  Next: Overlay Sample Program,  Prev: Overlay Commands,  Up: Overlays

14.3 Automatic Overlay Debugging
================================

GDB can automatically track which overlays are mapped and which are not,
given some simple co-operation from the overlay manager in the inferior.
If you enable automatic overlay debugging with the ‘overlay auto’
command (*note Overlay Commands::), GDB looks in the inferior's memory
for certain variables describing the current state of the overlays.

   Here are the variables your overlay manager must define to support
GDB's automatic overlay debugging:

‘_ovly_table’:
     This variable must be an array of the following structures:

          struct
          {
            /* The overlay's mapped address.  */
            unsigned long vma;

            /* The size of the overlay, in bytes.  */
            unsigned long size;

            /* The overlay's load address.  */
            unsigned long lma;

            /* Non-zero if the overlay is currently mapped;
               zero otherwise.  */
            unsigned long mapped;
          }

‘_novlys’:
     This variable must be a four-byte signed integer, holding the total
     number of elements in ‘_ovly_table’.

   To decide whether a particular overlay is mapped or not, GDB looks
for an entry in ‘_ovly_table’ whose ‘vma’ and ‘lma’ members equal the
VMA and LMA of the overlay's section in the executable file.  When GDB
finds a matching entry, it consults the entry's ‘mapped’ member to
determine whether the overlay is currently mapped.

   In addition, your overlay manager may define a function called
‘_ovly_debug_event’.  If this function is defined, GDB will silently set
a breakpoint there.  If the overlay manager then calls this function
whenever it has changed the overlay table, this will enable GDB to
accurately keep track of which overlays are in program memory, and
update any breakpoints that may be set in overlays.  This will allow
breakpoints to work even if the overlays are kept in ROM or other
non-writable memory while they are not being executed.


File: gdb.info,  Node: Overlay Sample Program,  Prev: Automatic Overlay Debugging,  Up: Overlays

14.4 Overlay Sample Program
===========================

When linking a program which uses overlays, you must place the overlays
at their load addresses, while relocating them to run at their mapped
addresses.  To do this, you must write a linker script (*note
(ld.info)Overlay Description::).  Unfortunately, since linker scripts
are specific to a particular host system, target architecture, and
target memory layout, this manual cannot provide portable sample code
demonstrating GDB's overlay support.

   However, the GDB source distribution does contain an overlaid
program, with linker scripts for a few systems, as part of its test
suite.  The program consists of the following files from
‘gdb/testsuite/gdb.base’:

‘overlays.c’
     The main program file.
‘ovlymgr.c’
     A simple overlay manager, used by ‘overlays.c’.
‘foo.c’
‘bar.c’
‘baz.c’
‘grbx.c’
     Overlay modules, loaded and used by ‘overlays.c’.
‘d10v.ld’
‘m32r.ld’
     Linker scripts for linking the test program on the ‘d10v-elf’ and
     ‘m32r-elf’ targets.

   You can build the test program using the ‘d10v-elf’ GCC
cross-compiler like this:

     $ d10v-elf-gcc -g -c overlays.c
     $ d10v-elf-gcc -g -c ovlymgr.c
     $ d10v-elf-gcc -g -c foo.c
     $ d10v-elf-gcc -g -c bar.c
     $ d10v-elf-gcc -g -c baz.c
     $ d10v-elf-gcc -g -c grbx.c
     $ d10v-elf-gcc -g overlays.o ovlymgr.o foo.o bar.o \
                       baz.o grbx.o -Wl,-Td10v.ld -o overlays

   The build process is identical for any other architecture, except
that you must substitute the appropriate compiler and linker script for
the target system for ‘d10v-elf-gcc’ and ‘d10v.ld’.


File: gdb.info,  Node: Languages,  Next: Symbols,  Prev: Overlays,  Up: Top

15 Using GDB with Different Languages
*************************************

Although programming languages generally have common aspects, they are
rarely expressed in the same manner.  For instance, in ANSI C,
dereferencing a pointer ‘p’ is accomplished by ‘*p’, but in Modula-2, it
is accomplished by ‘p^’.  Values can also be represented (and displayed)
differently.  Hex numbers in C appear as ‘0x1ae’, while in Modula-2 they
appear as ‘1AEH’.

   Language-specific information is built into GDB for some languages,
allowing you to express operations like the above in your program's
native language, and allowing GDB to output values in a manner
consistent with the syntax of your program's native language.  The
language you use to build expressions is called the “working language”.

* Menu:

* Setting::                     Switching between source languages
* Show::                        Displaying the language
* Checks::                      Type and range checks
* Supported Languages::         Supported languages
* Unsupported Languages::       Unsupported languages


File: gdb.info,  Node: Setting,  Next: Show,  Up: Languages

15.1 Switching Between Source Languages
=======================================

There are two ways to control the working language--either have GDB set
it automatically, or select it manually yourself.  You can use the ‘set
language’ command for either purpose.  On startup, GDB defaults to
setting the language automatically.  The working language is used to
determine how expressions you type are interpreted, how values are
printed, etc.

   In addition to the working language, every source file that GDB knows
about has its own working language.  For some object file formats, the
compiler might indicate which language a particular source file is in.
However, most of the time GDB infers the language from the name of the
file.  The language of a source file controls whether C++ names are
demangled--this way ‘backtrace’ can show each frame appropriately for
its own language.  There is no way to set the language of a source file
from within GDB, but you can set the language associated with a filename
extension.  *Note Displaying the Language: Show.

   This is most commonly a problem when you use a program, such as
‘cfront’ or ‘f2c’, that generates C but is written in another language.
In that case, make the program use ‘#line’ directives in its C output;
that way GDB will know the correct language of the source code of the
original program, and will display that source code, not the generated C
code.

* Menu:

* Filenames::                   Filename extensions and languages.
* Manually::                    Setting the working language manually
* Automatically::               Having GDB infer the source language


File: gdb.info,  Node: Filenames,  Next: Manually,  Up: Setting

15.1.1 List of Filename Extensions and Languages
------------------------------------------------

If a source file name ends in one of the following extensions, then GDB
infers that its language is the one indicated.

‘.ada’
‘.ads’
‘.adb’
‘.a’
     Ada source file.

‘.c’
     C source file

‘.C’
‘.cc’
‘.cp’
‘.cpp’
‘.cxx’
‘.c++’
     C++ source file

‘.d’
     D source file

‘.m’
     Objective-C source file

‘.f’
‘.F’
     Fortran source file

‘.mod’
     Modula-2 source file

‘.s’
‘.S’
     Assembler source file.  This actually behaves almost like C, but
     GDB does not skip over function prologues when stepping.

   In addition, you may set the language associated with a filename
extension.  *Note Displaying the Language: Show.


File: gdb.info,  Node: Manually,  Next: Automatically,  Prev: Filenames,  Up: Setting

15.1.2 Setting the Working Language
-----------------------------------

If you allow GDB to set the language automatically, expressions are
interpreted the same way in your debugging session and your program.

   If you wish, you may set the language manually.  To do this, issue
the command ‘set language LANG’, where LANG is the name of a language,
such as ‘c’ or ‘modula-2’.  For a list of the supported languages, type
‘set language’.

   Setting the language manually prevents GDB from updating the working
language automatically.  This can lead to confusion if you try to debug
a program when the working language is not the same as the source
language, when an expression is acceptable to both languages--but means
different things.  For instance, if the current source file were written
in C, and GDB was parsing Modula-2, a command such as:

     print a = b + c

might not have the effect you intended.  In C, this means to add ‘b’ and
‘c’ and place the result in ‘a’.  The result printed would be the value
of ‘a’.  In Modula-2, this means to compare ‘a’ to the result of ‘b+c’,
yielding a ‘BOOLEAN’ value.


File: gdb.info,  Node: Automatically,  Prev: Manually,  Up: Setting

15.1.3 Having GDB Infer the Source Language
-------------------------------------------

To have GDB set the working language automatically, use ‘set language
local’ or ‘set language auto’.  GDB then infers the working language.
That is, when your program stops in a frame (usually by encountering a
breakpoint), GDB sets the working language to the language recorded for
the function in that frame.  If the language for a frame is unknown
(that is, if the function or block corresponding to the frame was
defined in a source file that does not have a recognized extension), the
current working language is not changed, and GDB issues a warning.

   This may not seem necessary for most programs, which are written
entirely in one source language.  However, program modules and libraries
written in one source language can be used by a main program written in
a different source language.  Using ‘set language auto’ in this case
frees you from having to set the working language manually.

   The warning is enabled by default, but it can be controlled via a
setting:

‘set warn-language-frame-mismatch [on|off]’
     Enable or disable the warning that is issued when the current
     language is set to a value that does not match the current frame.

‘show warn-language-frame-mismatch’
     Show whether the frame-mismatch warning will be issued.


File: gdb.info,  Node: Show,  Next: Checks,  Prev: Setting,  Up: Languages

15.2 Displaying the Language
============================

The following commands help you find out which language is the working
language, and also what language source files were written in.

‘show language’
     Display the current working language.  This is the language you can
     use with commands such as ‘print’ to build and compute expressions
     that may involve variables in your program.

‘info frame’
     Display the source language for this frame.  This language becomes
     the working language if you use an identifier from this frame.
     *Note Information about a Frame: Frame Info, to identify the other
     information listed here.

‘info source’
     Display the source language of this source file.  *Note Examining
     the Symbol Table: Symbols, to identify the other information listed
     here.

   In unusual circumstances, you may have source files with extensions
not in the standard list.  You can then set the extension associated
with a language explicitly:

‘set extension-language EXT LANGUAGE’
     Tell GDB that source files with extension EXT are to be assumed as
     written in the source language LANGUAGE.

‘info extensions’
     List all the filename extensions and the associated languages.


File: gdb.info,  Node: Checks,  Next: Supported Languages,  Prev: Show,  Up: Languages

15.3 Type and Range Checking
============================

Some languages are designed to guard you against making seemingly common
errors through a series of compile- and run-time checks.  These include
checking the type of arguments to functions and operators and making
sure mathematical overflows are caught at run time.  Checks such as
these help to ensure a program's correctness once it has been compiled
by eliminating type mismatches and providing active checks for range
errors when your program is running.

   By default GDB checks for these errors according to the rules of the
current source language.  Although GDB does not check the statements in
your program, it can check expressions entered directly into GDB for
evaluation via the ‘print’ command, for example.

* Menu:

* Type Checking::               An overview of type checking
* Range Checking::              An overview of range checking


File: gdb.info,  Node: Type Checking,  Next: Range Checking,  Up: Checks

15.3.1 An Overview of Type Checking
-----------------------------------

Some languages, such as C and C++, are strongly typed, meaning that the
arguments to operators and functions have to be of the correct type,
otherwise an error occurs.  These checks prevent type mismatch errors
from ever causing any run-time problems.  For example,

     int klass::my_method(char *b) { return  b ? 1 : 2; }

     (gdb) print obj.my_method (0)
     $1 = 2
but
     (gdb) print obj.my_method (0x1234)
     Cannot resolve method klass::my_method to any overloaded instance

   The second example fails because in C++ the integer constant ‘0x1234’
is not type-compatible with the pointer parameter type.

   For the expressions you use in GDB commands, you can tell GDB to not
enforce strict type checking or to treat any mismatches as errors and
abandon the expression; When type checking is disabled, GDB successfully
evaluates expressions like the second example above.

   Even if type checking is off, there may be other reasons related to
type that prevent GDB from evaluating an expression.  For instance, GDB
does not know how to add an ‘int’ and a ‘struct foo’.  These particular
type errors have nothing to do with the language in use and usually
arise from expressions which make little sense to evaluate anyway.

   GDB provides some additional commands for controlling type checking:

‘set check type on’
‘set check type off’
     Set strict type checking on or off.  If any type mismatches occur
     in evaluating an expression while type checking is on, GDB prints a
     message and aborts evaluation of the expression.

‘show check type’
     Show the current setting of type checking and whether GDB is
     enforcing strict type checking rules.


File: gdb.info,  Node: Range Checking,  Prev: Type Checking,  Up: Checks

15.3.2 An Overview of Range Checking
------------------------------------

In some languages (such as Modula-2), it is an error to exceed the
bounds of a type; this is enforced with run-time checks.  Such range
checking is meant to ensure program correctness by making sure
computations do not overflow, or indices on an array element access do
not exceed the bounds of the array.

   For expressions you use in GDB commands, you can tell GDB to treat
range errors in one of three ways: ignore them, always treat them as
errors and abandon the expression, or issue warnings but evaluate the
expression anyway.

   A range error can result from numerical overflow, from exceeding an
array index bound, or when you type a constant that is not a member of
any type.  Some languages, however, do not treat overflows as an error.
In many implementations of C, mathematical overflow causes the result to
"wrap around" to lower values--for example, if M is the largest integer
value, and S is the smallest, then

     M + 1 ⇒ S

   This, too, is specific to individual languages, and in some cases
specific to individual compilers or machines.  *Note Supported
Languages: Supported Languages, for further details on specific
languages.

   GDB provides some additional commands for controlling the range
checker:

‘set check range auto’
     Set range checking on or off based on the current working language.
     *Note Supported Languages: Supported Languages, for the default
     settings for each language.

‘set check range on’
‘set check range off’
     Set range checking on or off, overriding the default setting for
     the current working language.  A warning is issued if the setting
     does not match the language default.  If a range error occurs and
     range checking is on, then a message is printed and evaluation of
     the expression is aborted.

‘set check range warn’
     Output messages when the GDB range checker detects a range error,
     but attempt to evaluate the expression anyway.  Evaluating the
     expression may still be impossible for other reasons, such as
     accessing memory that the process does not own (a typical example
     from many Unix systems).

‘show check range’
     Show the current setting of the range checker, and whether or not
     it is being set automatically by GDB.


File: gdb.info,  Node: Supported Languages,  Next: Unsupported Languages,  Prev: Checks,  Up: Languages

15.4 Supported Languages
========================

GDB supports C, C++, D, Go, Objective-C, Fortran, OpenCL C, Pascal,
Rust, assembly, Modula-2, and Ada.  Some GDB features may be used in
expressions regardless of the language you use: the GDB ‘@’ and ‘::’
operators, and the ‘{type}addr’ construct (*note Expressions:
Expressions.) can be used with the constructs of any supported language.

   The following sections detail to what degree each source language is
supported by GDB.  These sections are not meant to be language tutorials
or references, but serve only as a reference guide to what the GDB
expression parser accepts, and what input and output formats should look
like for different languages.  There are many good books written on each
of these languages; please look to these for a language reference or
tutorial.

* Menu:

* C::                           C and C++
* D::                           D
* Go::                          Go
* Objective-C::                 Objective-C
* OpenCL C::                    OpenCL C
* Fortran::                     Fortran
* Pascal::                      Pascal
* Rust::                        Rust
* Modula-2::                    Modula-2
* Ada::                         Ada


File: gdb.info,  Node: C,  Next: D,  Up: Supported Languages

15.4.1 C and C++
----------------

Since C and C++ are so closely related, many features of GDB apply to
both languages.  Whenever this is the case, we discuss those languages
together.

   The C++ debugging facilities are jointly implemented by the C++
compiler and GDB.  Therefore, to debug your C++ code effectively, you
must compile your C++ programs with a supported C++ compiler, such as
GNU ‘g++’, or the HP ANSI C++ compiler (‘aCC’).

* Menu:

* C Operators::                 C and C++ operators
* C Constants::                 C and C++ constants
* C Plus Plus Expressions::     C++ expressions
* C Defaults::                  Default settings for C and C++
* C Checks::                    C and C++ type and range checks
* Debugging C::                 GDB and C
* Debugging C Plus Plus::       GDB features for C++
* Decimal Floating Point::      Numbers in Decimal Floating Point format


File: gdb.info,  Node: C Operators,  Next: C Constants,  Up: C

15.4.1.1 C and C++ Operators
............................

Operators must be defined on values of specific types.  For instance,
‘+’ is defined on numbers, but not on structures.  Operators are often
defined on groups of types.

   For the purposes of C and C++, the following definitions hold:

   • _Integral types_ include ‘int’ with any of its storage-class
     specifiers; ‘char’; ‘enum’; and, for C++, ‘bool’.

   • _Floating-point types_ include ‘float’, ‘double’, and ‘long double’
     (if supported by the target platform).

   • _Pointer types_ include all types defined as ‘(TYPE *)’.

   • _Scalar types_ include all of the above.

The following operators are supported.  They are listed here in order of
increasing precedence:

‘,’
     The comma or sequencing operator.  Expressions in a comma-separated
     list are evaluated from left to right, with the result of the
     entire expression being the last expression evaluated.

‘=’
     Assignment.  The value of an assignment expression is the value
     assigned.  Defined on scalar types.

‘OP=’
     Used in an expression of the form ‘A OP= B’, and translated to
     ‘A = A OP B’.  ‘OP=’ and ‘=’ have the same precedence.  The
     operator OP is any one of the operators ‘|’, ‘^’, ‘&’, ‘<<’, ‘>>’,
     ‘+’, ‘-’, ‘*’, ‘/’, ‘%’.

‘?:’
     The ternary operator.  ‘A ? B : C’ can be thought of as: if A then
     B else C.  The argument A should be of an integral type.

‘||’
     Logical OR.  Defined on integral types.

‘&&’
     Logical AND.  Defined on integral types.

‘|’
     Bitwise OR.  Defined on integral types.

‘^’
     Bitwise exclusive-OR.  Defined on integral types.

‘&’
     Bitwise AND.  Defined on integral types.

‘==, !=’
     Equality and inequality.  Defined on scalar types.  The value of
     these expressions is 0 for false and non-zero for true.

‘<, >, <=, >=’
     Less than, greater than, less than or equal, greater than or equal.
     Defined on scalar types.  The value of these expressions is 0 for
     false and non-zero for true.

‘<<, >>’
     left shift, and right shift.  Defined on integral types.

‘@’
     The GDB "artificial array" operator (*note Expressions:
     Expressions.).

‘+, -’
     Addition and subtraction.  Defined on integral types,
     floating-point types and pointer types.

‘*, /, %’
     Multiplication, division, and modulus.  Multiplication and division
     are defined on integral and floating-point types.  Modulus is
     defined on integral types.

‘++, --’
     Increment and decrement.  When appearing before a variable, the
     operation is performed before the variable is used in an
     expression; when appearing after it, the variable's value is used
     before the operation takes place.

‘*’
     Pointer dereferencing.  Defined on pointer types.  Same precedence
     as ‘++’.

‘&’
     Address operator.  Defined on variables.  Same precedence as ‘++’.

     For debugging C++, GDB implements a use of ‘&’ beyond what is
     allowed in the C++ language itself: you can use ‘&(&REF)’ to
     examine the address where a C++ reference variable (declared with
     ‘&REF’) is stored.

‘-’
     Negative.  Defined on integral and floating-point types.  Same
     precedence as ‘++’.

‘!’
     Logical negation.  Defined on integral types.  Same precedence as
     ‘++’.

‘~’
     Bitwise complement operator.  Defined on integral types.  Same
     precedence as ‘++’.

‘., ->’
     Structure member, and pointer-to-structure member.  For
     convenience, GDB regards the two as equivalent, choosing whether to
     dereference a pointer based on the stored type information.
     Defined on ‘struct’ and ‘union’ data.

‘.*, ->*’
     Dereferences of pointers to members.

‘[]’
     Array indexing.  ‘A[I]’ is defined as ‘*(A+I)’.  Same precedence as
     ‘->’.

‘()’
     Function parameter list.  Same precedence as ‘->’.

‘::’
     C++ scope resolution operator.  Defined on ‘struct’, ‘union’, and
     ‘class’ types.

‘::’
     Doubled colons also represent the GDB scope operator (*note
     Expressions: Expressions.).  Same precedence as ‘::’, above.

   If an operator is redefined in the user code, GDB usually attempts to
invoke the redefined version instead of using the operator's predefined
meaning.


File: gdb.info,  Node: C Constants,  Next: C Plus Plus Expressions,  Prev: C Operators,  Up: C

15.4.1.2 C and C++ Constants
............................

GDB allows you to express the constants of C and C++ in the following
ways:

   • Integer constants are a sequence of digits.  Octal constants are
     specified by a leading ‘0’ (i.e. zero), and hexadecimal constants
     by a leading ‘0x’ or ‘0X’.  Constants may also end with a letter
     ‘l’, specifying that the constant should be treated as a ‘long’
     value.

   • Floating point constants are a sequence of digits, followed by a
     decimal point, followed by a sequence of digits, and optionally
     followed by an exponent.  An exponent is of the form:
     ‘e[[+]|-]NNN’, where NNN is another sequence of digits.  The ‘+’ is
     optional for positive exponents.  A floating-point constant may
     also end with a letter ‘f’ or ‘F’, specifying that the constant
     should be treated as being of the ‘float’ (as opposed to the
     default ‘double’) type; or with a letter ‘l’ or ‘L’, which
     specifies a ‘long double’ constant.

   • Enumerated constants consist of enumerated identifiers, or their
     integral equivalents.

   • Character constants are a single character surrounded by single
     quotes (‘'’), or a number--the ordinal value of the corresponding
     character (usually its ASCII value).  Within quotes, the single
     character may be represented by a letter or by “escape sequences”,
     which are of the form ‘\NNN’, where NNN is the octal representation
     of the character's ordinal value; or of the form ‘\X’, where ‘X’ is
     a predefined special character--for example, ‘\n’ for newline.

     Wide character constants can be written by prefixing a character
     constant with ‘L’, as in C. For example, ‘L'x'’ is the wide form of
     ‘x’.  The target wide character set is used when computing the
     value of this constant (*note Character Sets::).

   • String constants are a sequence of character constants surrounded
     by double quotes (‘"’).  Any valid character constant (as described
     above) may appear.  Double quotes within the string must be
     preceded by a backslash, so for instance ‘"a\"b'c"’ is a string of
     five characters.

     Wide string constants can be written by prefixing a string constant
     with ‘L’, as in C. The target wide character set is used when
     computing the value of this constant (*note Character Sets::).

   • Pointer constants are an integral value.  You can also write
     pointers to constants using the C operator ‘&’.

   • Array constants are comma-separated lists surrounded by braces ‘{’
     and ‘}’; for example, ‘{1,2,3}’ is a three-element array of
     integers, ‘{{1,2}, {3,4}, {5,6}}’ is a three-by-two array, and
     ‘{&"hi", &"there", &"fred"}’ is a three-element array of pointers.


File: gdb.info,  Node: C Plus Plus Expressions,  Next: C Defaults,  Prev: C Constants,  Up: C

15.4.1.3 C++ Expressions
........................

GDB expression handling can interpret most C++ expressions.

     _Warning:_ GDB can only debug C++ code if you use the proper
     compiler and the proper debug format.  Currently, GDB works best
     when debugging C++ code that is compiled with the most recent
     version of GCC possible.  The DWARF debugging format is preferred;
     GCC defaults to this on most popular platforms.  Other compilers
     and/or debug formats are likely to work badly or not at all when
     using GDB to debug C++ code.  *Note Compilation::.

  1. Member function calls are allowed; you can use expressions like

          count = aml->GetOriginal(x, y)

  2. While a member function is active (in the selected stack frame),
     your expressions have the same namespace available as the member
     function; that is, GDB allows implicit references to the class
     instance pointer ‘this’ following the same rules as C++.  ‘using’
     declarations in the current scope are also respected by GDB.

  3. You can call overloaded functions; GDB resolves the function call
     to the right definition, with some restrictions.  GDB does not
     perform overload resolution involving user-defined type
     conversions, calls to constructors, or instantiations of templates
     that do not exist in the program.  It also cannot handle ellipsis
     argument lists or default arguments.

     It does perform integral conversions and promotions, floating-point
     promotions, arithmetic conversions, pointer conversions,
     conversions of class objects to base classes, and standard
     conversions such as those of functions or arrays to pointers; it
     requires an exact match on the number of function arguments.

     Overload resolution is always performed, unless you have specified
     ‘set overload-resolution off’.  *Note GDB Features for C++:
     Debugging C Plus Plus.

     You must specify ‘set overload-resolution off’ in order to use an
     explicit function signature to call an overloaded function, as in
          p 'foo(char,int)'('x', 13)

     The GDB command-completion facility can simplify this; see *note
     Command Completion: Completion.

  4. GDB understands variables declared as C++ lvalue or rvalue
     references; you can use them in expressions just as you do in C++
     source--they are automatically dereferenced.

     In the parameter list shown when GDB displays a frame, the values
     of reference variables are not displayed (unlike other variables);
     this avoids clutter, since references are often used for large
     structures.  The _address_ of a reference variable is always shown,
     unless you have specified ‘set print address off’.

  5. GDB supports the C++ name resolution operator ‘::’--your
     expressions can use it just as expressions in your program do.
     Since one scope may be defined in another, you can use ‘::’
     repeatedly if necessary, for example in an expression like
     ‘SCOPE1::SCOPE2::NAME’.  GDB also allows resolving name scope by
     reference to source files, in both C and C++ debugging (*note
     Program Variables: Variables.).

  6. GDB performs argument-dependent lookup, following the C++
     specification.


File: gdb.info,  Node: C Defaults,  Next: C Checks,  Prev: C Plus Plus Expressions,  Up: C

15.4.1.4 C and C++ Defaults
...........................

If you allow GDB to set range checking automatically, it defaults to
‘off’ whenever the working language changes to C or C++.  This happens
regardless of whether you or GDB selects the working language.

   If you allow GDB to set the language automatically, it recognizes
source files whose names end with ‘.c’, ‘.C’, or ‘.cc’, etc, and when
GDB enters code compiled from one of these files, it sets the working
language to C or C++.  *Note Having GDB Infer the Source Language:
Automatically, for further details.


File: gdb.info,  Node: C Checks,  Next: Debugging C,  Prev: C Defaults,  Up: C

15.4.1.5 C and C++ Type and Range Checks
........................................

By default, when GDB parses C or C++ expressions, strict type checking
is used.  However, if you turn type checking off, GDB will allow certain
non-standard conversions, such as promoting integer constants to
pointers.

   Range checking, if turned on, is done on mathematical operations.
Array indices are not checked, since they are often used to index a
pointer that is not itself an array.


File: gdb.info,  Node: Debugging C,  Next: Debugging C Plus Plus,  Prev: C Checks,  Up: C

15.4.1.6 GDB and C
..................

The ‘set print union’ and ‘show print union’ commands apply to the
‘union’ type.  When set to ‘on’, any ‘union’ that is inside a ‘struct’
or ‘class’ is also printed.  Otherwise, it appears as ‘{...}’.

   The ‘@’ operator aids in the debugging of dynamic arrays, formed with
pointers and a memory allocation function.  *Note Expressions:
Expressions.


File: gdb.info,  Node: Debugging C Plus Plus,  Next: Decimal Floating Point,  Prev: Debugging C,  Up: C

15.4.1.7 GDB Features for C++
.............................

Some GDB commands are particularly useful with C++, and some are
designed specifically for use with C++.  Here is a summary:

‘breakpoint menus’
     When you want a breakpoint in a function whose name is overloaded,
     GDB has the capability to display a menu of possible breakpoint
     locations to help you specify which function definition you want.
     *Note Ambiguous Expressions: Ambiguous Expressions.

‘rbreak REGEX’
     Setting breakpoints using regular expressions is helpful for
     setting breakpoints on overloaded functions that are not members of
     any special classes.  *Note Setting Breakpoints: Set Breaks.

‘catch throw’
‘catch rethrow’
‘catch catch’
     Debug C++ exception handling using these commands.  *Note Setting
     Catchpoints: Set Catchpoints.

‘ptype TYPENAME’
     Print inheritance relationships as well as other information for
     type TYPENAME.  *Note Examining the Symbol Table: Symbols.

‘info vtbl EXPRESSION.’
     The ‘info vtbl’ command can be used to display the virtual method
     tables of the object computed by EXPRESSION.  This shows one entry
     per virtual table; there may be multiple virtual tables when
     multiple inheritance is in use.

‘demangle NAME’
     Demangle NAME.  *Note Symbols::, for a more complete description of
     the ‘demangle’ command.

‘set print demangle’
‘show print demangle’
‘set print asm-demangle’
‘show print asm-demangle’
     Control whether C++ symbols display in their source form, both when
     displaying code as C++ source and when displaying disassemblies.
     *Note Print Settings: Print Settings.

‘set print object’
‘show print object’
     Choose whether to print derived (actual) or declared types of
     objects.  *Note Print Settings: Print Settings.

‘set print vtbl’
‘show print vtbl’
     Control the format for printing virtual function tables.  *Note
     Print Settings: Print Settings.  (The ‘vtbl’ commands do not work
     on programs compiled with the HP ANSI C++ compiler (‘aCC’).)

‘set overload-resolution on’
     Enable overload resolution for C++ expression evaluation.  The
     default is on.  For overloaded functions, GDB evaluates the
     arguments and searches for a function whose signature matches the
     argument types, using the standard C++ conversion rules (see *note
     C++ Expressions: C Plus Plus Expressions, for details).  If it
     cannot find a match, it emits a message.

‘set overload-resolution off’
     Disable overload resolution for C++ expression evaluation.  For
     overloaded functions that are not class member functions, GDB
     chooses the first function of the specified name that it finds in
     the symbol table, whether or not its arguments are of the correct
     type.  For overloaded functions that are class member functions,
     GDB searches for a function whose signature _exactly_ matches the
     argument types.

‘show overload-resolution’
     Show the current setting of overload resolution.

‘Overloaded symbol names’
     You can specify a particular definition of an overloaded symbol,
     using the same notation that is used to declare such symbols in
     C++: type ‘SYMBOL(TYPES)’ rather than just SYMBOL.  You can also
     use the GDB command-line word completion facilities to list the
     available choices, or to finish the type list for you.  *Note
     Command Completion: Completion, for details on how to do this.

‘Breakpoints in template functions’

     Similar to how overloaded symbols are handled, GDB will ignore
     template parameter lists when it encounters a symbol which includes
     a C++ template.  This permits setting breakpoints on families of
     template functions or functions whose parameters include template
     types.

     The ‘-qualified’ flag may be used to override this behavior,
     causing GDB to search for a specific function or type.

     The GDB command-line word completion facility also understands
     template parameters and may be used to list available choices or
     finish template parameter lists for you.  *Note Command Completion:
     Completion, for details on how to do this.

‘Breakpoints in functions with ABI tags’

     The GNU C++ compiler introduced the notion of ABI "tags", which
     correspond to changes in the ABI of a type, function, or variable
     that would not otherwise be reflected in a mangled name.  See
     <https://developers.redhat.com/blog/2015/02/05/gcc5-and-the-c11-abi/>
     for more detail.

     The ABI tags are visible in C++ demangled names.  For example, a
     function that returns a std::string:

          std::string function(int);

     when compiled for the C++11 ABI is marked with the ‘cxx11’ ABI tag,
     and GDB displays the symbol like this:

          function[abi:cxx11](int)

     You can set a breakpoint on such functions simply as if they had no
     tag.  For example:

          (gdb) b function(int)
          Breakpoint 2 at 0x40060d: file main.cc, line 10.
          (gdb) info breakpoints
          Num     Type           Disp Enb Address    What
          1       breakpoint     keep y   0x0040060d in function[abi:cxx11](int)
                                                     at main.cc:10

     On the rare occasion you need to disambiguate between different ABI
     tags, you can do so by simply including the ABI tag in the function
     name, like:

          (gdb) b ambiguous[abi:other_tag](int)


File: gdb.info,  Node: Decimal Floating Point,  Prev: Debugging C Plus Plus,  Up: C

15.4.1.8 Decimal Floating Point format
......................................

GDB can examine, set and perform computations with numbers in decimal
floating point format, which in the C language correspond to the
‘_Decimal32’, ‘_Decimal64’ and ‘_Decimal128’ types as specified by the
extension to support decimal floating-point arithmetic.

   There are two encodings in use, depending on the architecture: BID
(Binary Integer Decimal) for x86 and x86-64, and DPD (Densely Packed
Decimal) for PowerPC and S/390.  GDB will use the appropriate encoding
for the configured target.

   Because of a limitation in ‘libdecnumber’, the library used by GDB to
manipulate decimal floating point numbers, it is not possible to convert
(using a cast, for example) integers wider than 32-bit to decimal float.

   In addition, in order to imitate GDB's behavior with binary floating
point computations, error checking in decimal float operations ignores
underflow, overflow and divide by zero exceptions.

   In the PowerPC architecture, GDB provides a set of pseudo-registers
to inspect ‘_Decimal128’ values stored in floating point registers.  See
*note PowerPC: PowerPC. for more details.


File: gdb.info,  Node: D,  Next: Go,  Prev: C,  Up: Supported Languages

15.4.2 D
--------

GDB can be used to debug programs written in D and compiled with GDC,
LDC or DMD compilers.  Currently GDB supports only one D specific
feature -- dynamic arrays.


File: gdb.info,  Node: Go,  Next: Objective-C,  Prev: D,  Up: Supported Languages

15.4.3 Go
---------

GDB can be used to debug programs written in Go and compiled with
‘gccgo’ or ‘6g’ compilers.

   Here is a summary of the Go-specific features and restrictions:

‘The current Go package’
     The name of the current package does not need to be specified when
     specifying global variables and functions.

     For example, given the program:

          package main
          var myglob = "Shall we?"
          func main () {
            // ...
          }

     When stopped inside ‘main’ either of these work:

          (gdb) p myglob
          (gdb) p main.myglob

‘Builtin Go types’
     The ‘string’ type is recognized by GDB and is printed as a string.

‘Builtin Go functions’
     The GDB expression parser recognizes the ‘unsafe.Sizeof’ function
     and handles it internally.

‘Restrictions on Go expressions’
     All Go operators are supported except ‘&^’.  The Go ‘_’ "blank
     identifier" is not supported.  Automatic dereferencing of pointers
     is not supported.


File: gdb.info,  Node: Objective-C,  Next: OpenCL C,  Prev: Go,  Up: Supported Languages

15.4.4 Objective-C
------------------

This section provides information about some commands and command
options that are useful for debugging Objective-C code.  See also *note
info classes: Symbols, and *note info selectors: Symbols, for a few more
commands specific to Objective-C support.

* Menu:

* Method Names in Commands::
* The Print Command with Objective-C::


File: gdb.info,  Node: Method Names in Commands,  Next: The Print Command with Objective-C,  Up: Objective-C

15.4.4.1 Method Names in Commands
.................................

The following commands have been extended to accept Objective-C method
names as line specifications:

   • ‘clear’
   • ‘break’
   • ‘info line’
   • ‘jump’
   • ‘list’

   A fully qualified Objective-C method name is specified as

     -[CLASS METHODNAME]

   where the minus sign is used to indicate an instance method and a
plus sign (not shown) is used to indicate a class method.  The class
name CLASS and method name METHODNAME are enclosed in brackets, similar
to the way messages are specified in Objective-C source code.  For
example, to set a breakpoint at the ‘create’ instance method of class
‘Fruit’ in the program currently being debugged, enter:

     break -[Fruit create]

   To list ten program lines around the ‘initialize’ class method,
enter:

     list +[NSText initialize]

   In the current version of GDB, the plus or minus sign is required.
In future versions of GDB, the plus or minus sign will be optional, but
you can use it to narrow the search.  It is also possible to specify
just a method name:

     break create

   You must specify the complete method name, including any colons.  If
your program's source files contain more than one ‘create’ method,
you'll be presented with a numbered list of classes that implement that
method.  Indicate your choice by number, or type ‘0’ to exit if none
apply.

   As another example, to clear a breakpoint established at the
‘makeKeyAndOrderFront:’ method of the ‘NSWindow’ class, enter:

     clear -[NSWindow makeKeyAndOrderFront:]


File: gdb.info,  Node: The Print Command with Objective-C,  Prev: Method Names in Commands,  Up: Objective-C

15.4.4.2 The Print Command With Objective-C
...........................................

The print command has also been extended to accept methods.  For
example:

     print -[OBJECT hash]

will tell GDB to send the ‘hash’ message to OBJECT and print the result.
Also, an additional command has been added, ‘print-object’ or ‘po’ for
short, which is meant to print the description of an object.  However,
this command may only work with certain Objective-C libraries that have
a particular hook function, ‘_NSPrintForDebugger’, defined.


File: gdb.info,  Node: OpenCL C,  Next: Fortran,  Prev: Objective-C,  Up: Supported Languages

15.4.5 OpenCL C
---------------

This section provides information about GDBs OpenCL C support.

* Menu:

* OpenCL C Datatypes::
* OpenCL C Expressions::
* OpenCL C Operators::


File: gdb.info,  Node: OpenCL C Datatypes,  Next: OpenCL C Expressions,  Up: OpenCL C

15.4.5.1 OpenCL C Datatypes
...........................

GDB supports the builtin scalar and vector datatypes specified by OpenCL
1.1.  In addition the half- and double-precision floating point data
types of the ‘cl_khr_fp16’ and ‘cl_khr_fp64’ OpenCL extensions are also
known to GDB.


File: gdb.info,  Node: OpenCL C Expressions,  Next: OpenCL C Operators,  Prev: OpenCL C Datatypes,  Up: OpenCL C

15.4.5.2 OpenCL C Expressions
.............................

GDB supports accesses to vector components including the access as
lvalue where possible.  Since OpenCL C is based on C99 most C
expressions supported by GDB can be used as well.


File: gdb.info,  Node: OpenCL C Operators,  Prev: OpenCL C Expressions,  Up: OpenCL C

15.4.5.3 OpenCL C Operators
...........................

GDB supports the operators specified by OpenCL 1.1 for scalar and vector
data types.


File: gdb.info,  Node: Fortran,  Next: Pascal,  Prev: OpenCL C,  Up: Supported Languages

15.4.6 Fortran
--------------

GDB can be used to debug programs written in Fortran.  Note, that not
all Fortran language features are available yet.

   Some Fortran compilers (GNU Fortran 77 and Fortran 95 compilers among
them) append an underscore to the names of variables and functions.
When you debug programs compiled by those compilers, you will need to
refer to variables and functions with a trailing underscore.

   Fortran symbols are usually case-insensitive, so GDB by default uses
case-insensitive matching for Fortran symbols.  You can change that with
the ‘set case-insensitive’ command, see *note Symbols::, for the
details.

* Menu:

* Fortran Types::               Fortran builtin types
* Fortran Operators::           Fortran operators and expressions
* Fortran Intrinsics::          Fortran intrinsic functions
* Special Fortran Commands::    Special GDB commands for Fortran


File: gdb.info,  Node: Fortran Types,  Next: Fortran Operators,  Up: Fortran

15.4.6.1 Fortran Types
......................

In Fortran the primitive data-types have an associated ‘KIND’ type
parameter, written as ‘TYPE*KINDPARAM’, ‘TYPE*KINDPARAM’, or in the
GDB-only dialect ‘TYPE_KINDPARAM’.  A concrete example would be
‘‘Real*4’’, ‘‘Real(kind=4)’’, and ‘‘Real_4’’.  The kind of a type can be
retrieved by using the intrinsic function ‘KIND’, see *note Fortran
Intrinsics::.

   Generally, the actual implementation of the ‘KIND’ type parameter is
compiler specific.  In GDB the kind parameter is implemented in
accordance with its use in the GNU ‘gfortran’ compiler.  Here, the kind
parameter for a given TYPE specifies its size in memory -- a Fortran
‘Integer*4’ or ‘Integer(kind=4)’ would be an integer type occupying 4
bytes of memory.  An exception to this rule is the ‘Complex’ type for
which the kind of the type does not specify its entire size, but the
size of each of the two ‘Real’'s it is composed of.  A ‘Complex*4’ would
thus consist of two ‘Real*4’s and occupy 8 bytes of memory.

   For every type there is also a default kind associated with it, e.g.
‘Integer’ in GDB will internally be an ‘Integer*4’ (see the table below
for default types).  The default types are the same as in GNU compilers
but note, that the GNU default types can actually be changed by compiler
flags such as ‘-fdefault-integer-8’ and ‘-fdefault-real-8’.

   Not every kind parameter is valid for every type and in GDB the
following type kinds are available.

‘Integer’
     ‘Integer*1’, ‘Integer*2’, ‘Integer*4’, ‘Integer*8’, and ‘Integer’ =
     ‘Integer*4’.

‘Logical’
     ‘Logical*1’, ‘Logical*2’, ‘Logical*4’, ‘Logical*8’, and ‘Logical’ =
     ‘Logical*4’.

‘Real’
     ‘Real*4’, ‘Real*8’, ‘Real*16’, and ‘Real’ = ‘Real*4’.

‘Complex’
     ‘Complex*4’, ‘Complex*8’, ‘Complex*16’, and ‘Complex’ =
     ‘Complex*4’.


File: gdb.info,  Node: Fortran Operators,  Next: Fortran Intrinsics,  Prev: Fortran Types,  Up: Fortran

15.4.6.2 Fortran Operators and Expressions
..........................................

Operators must be defined on values of specific types.  For instance,
‘+’ is defined on numbers, but not on characters or other non-
arithmetic types.  Operators are often defined on groups of types.

‘**’
     The exponentiation operator.  It raises the first operand to the
     power of the second one.

‘:’
     The range operator.  Normally used in the form of array(low:high)
     to represent a section of array.

‘%’
     The access component operator.  Normally used to access elements in
     derived types.  Also suitable for unions.  As unions aren't part of
     regular Fortran, this can only happen when accessing a register
     that uses a gdbarch-defined union type.
‘::’
     The scope operator.  Normally used to access variables in modules
     or to set breakpoints on subroutines nested in modules or in other
     subroutines (internal subroutines).


File: gdb.info,  Node: Fortran Intrinsics,  Next: Special Fortran Commands,  Prev: Fortran Operators,  Up: Fortran

15.4.6.3 Fortran Intrinsics
...........................

Fortran provides a large set of intrinsic procedures.  GDB implements an
incomplete subset of those procedures and their overloads.  Some of
these procedures take an optional ‘KIND’ parameter, see *note Fortran
Types::.

‘ABS(A)’
     Computes the absolute value of its argument A.  Currently not
     supported for ‘Complex’ arguments.

‘ALLOCATE(ARRAY)’
     Returns whether ARRAY is allocated or not.

‘ASSOCIATED(POINTER [, TARGET])’
     Returns the association status of the pointer POINTER or, if TARGET
     is present, whether POINTER is associated with the target TARGET.

‘CEILING(A [, KIND])’
     Computes the least integer greater than or equal to A.  The
     optional parameter KIND specifies the kind of the return type
     ‘Integer(KIND)’.

‘CMPLX(X [, Y [, KIND]])’
     Returns a complex number where X is converted to the real
     component.  If Y is present it is converted to the imaginary
     component.  If Y is not present then the imaginary component is set
     to ‘0.0’ except if X itself is of ‘Complex’ type.  The optional
     parameter KIND specifies the kind of the return type
     ‘Complex(KIND)’.

‘FLOOR(A [, KIND])’
     Computes the greatest integer less than or equal to A.  The
     optional parameter KIND specifies the kind of the return type
     ‘Integer(KIND)’.

‘KIND(A)’
     Returns the kind value of the argument A, see *note Fortran
     Types::.

‘LBOUND(ARRAY [, DIM [, KIND]])’
     Returns the lower bounds of an ARRAY, or a single lower bound along
     the DIM dimension if present.  The optional parameter KIND
     specifies the kind of the return type ‘Integer(KIND)’.

‘LOC(X)’
     Returns the address of X as an ‘Integer’.

‘MOD(A, P)’
     Computes the remainder of the division of A by P.

‘MODULO(A, P)’
     Computes the A modulo P.

‘RANK(A)’
     Returns the rank of a scalar or array (scalars have rank ‘0’).

‘SHAPE(A)’
     Returns the shape of a scalar or array (scalars have shape ‘()’).

‘SIZE(ARRAY[, DIM [, KIND]])’
     Returns the extent of ARRAY along a specified dimension DIM, or the
     total number of elements in ARRAY if DIM is absent.  The optional
     parameter KIND specifies the kind of the return type
     ‘Integer(KIND)’.

‘UBOUND(ARRAY [, DIM [, KIND]])’
     Returns the upper bounds of an ARRAY, or a single upper bound along
     the DIM dimension if present.  The optional parameter KIND
     specifies the kind of the return type ‘Integer(KIND)’.


File: gdb.info,  Node: Special Fortran Commands,  Prev: Fortran Intrinsics,  Up: Fortran

15.4.6.4 Special Fortran Commands
.................................

GDB has some commands to support Fortran-specific features, such as
displaying common blocks.

‘info common [COMMON-NAME]’
     This command prints the values contained in the Fortran ‘COMMON’
     block whose name is COMMON-NAME.  With no argument, the names of
     all ‘COMMON’ blocks visible at the current program location are
     printed.
‘set fortran repack-array-slices [on|off]’
‘show fortran repack-array-slices’
     When taking a slice from an array, a Fortran compiler can choose to
     either produce an array descriptor that describes the slice in
     place, or it may repack the slice, copying the elements of the
     slice into a new region of memory.

     When this setting is on, then GDB will also repack array slices in
     some situations.  When this setting is off, then GDB will create
     array descriptors for slices that reference the original data in
     place.

     GDB will never repack an array slice if the data for the slice is
     contiguous within the original array.

     GDB will always repack string slices if the data for the slice is
     non-contiguous within the original string as GDB does not support
     printing non-contiguous strings.

     The default for this setting is ‘off’.


File: gdb.info,  Node: Pascal,  Next: Rust,  Prev: Fortran,  Up: Supported Languages

15.4.7 Pascal
-------------

Debugging Pascal programs which use sets, subranges, file variables, or
nested functions does not currently work.  GDB does not support entering
expressions, printing values, or similar features using Pascal syntax.

   The Pascal-specific command ‘set print pascal_static-members’
controls whether static members of Pascal objects are displayed.  *Note
pascal_static-members: Print Settings.


File: gdb.info,  Node: Rust,  Next: Modula-2,  Prev: Pascal,  Up: Supported Languages

15.4.8 Rust
-----------

GDB supports the Rust Programming Language (https://www.rust-lang.org/).
Type- and value-printing, and expression parsing, are reasonably
complete.  However, there are a few peculiarities and holes to be aware
of.

   • Linespecs (*note Location Specifications::) are never relative to
     the current crate.  Instead, they act as if there were a global
     namespace of crates, somewhat similar to the way ‘extern crate’
     behaves.

     That is, if GDB is stopped at a breakpoint in a function in crate
     ‘A’, module ‘B’, then ‘break B::f’ will attempt to set a breakpoint
     in a function named ‘f’ in a crate named ‘B’.

     As a consequence of this approach, linespecs also cannot refer to
     items using ‘self::’ or ‘super::’.

   • Because GDB implements Rust name-lookup semantics in expressions,
     it will sometimes prepend the current crate to a name.  For
     example, if GDB is stopped at a breakpoint in the crate ‘K’, then
     ‘print ::x::y’ will try to find the symbol ‘K::x::y’.

     However, since it is useful to be able to refer to other crates
     when debugging, GDB provides the ‘extern’ extension to circumvent
     this.  To use the extension, just put ‘extern’ before a path
     expression to refer to the otherwise unavailable "global" scope.

     In the above example, if you wanted to refer to the symbol ‘y’ in
     the crate ‘x’, you would use ‘print extern x::y’.

   • The Rust expression evaluator does not support "statement-like"
     expressions such as ‘if’ or ‘match’, or lambda expressions.

   • Tuple expressions are not implemented.

   • The Rust expression evaluator does not currently implement the
     ‘Drop’ trait.  Objects that may be created by the evaluator will
     never be destroyed.

   • GDB does not implement type inference for generics.  In order to
     call generic functions or otherwise refer to generic items, you
     will have to specify the type parameters manually.

   • In an expression context, completing a generic function name will
     give syntactically invalid results.  This happens because Rust
     requires the ‘::’ operator between the function name and its
     generic arguments.  For example, GDB might provide a completion
     like ‘crate::f<u32>’, where the parser would require
     ‘crate::f::<u32>’.

   • As of this writing, the Rust compiler (version 1.8) has a few holes
     in the debugging information it generates.  These holes prevent
     certain features from being implemented by GDB:

        • Method calls cannot be made via traits.

        • Operator overloading is not implemented.

        • When debugging in a monomorphized function, you cannot use the
          generic type names.

        • The type ‘Self’ is not available.

        • ‘use’ statements are not available, so some names may not be
          available in the crate.


File: gdb.info,  Node: Modula-2,  Next: Ada,  Prev: Rust,  Up: Supported Languages

15.4.9 Modula-2
---------------

The extensions made to GDB to support Modula-2 only support output from
the GNU Modula-2 compiler (which is currently being developed).  Other
Modula-2 compilers are not currently supported, and attempting to debug
executables produced by them is most likely to give an error as GDB
reads in the executable's symbol table.

* Menu:

* M2 Operators::                Built-in operators
* Built-In Func/Proc::          Built-in functions and procedures
* M2 Constants::                Modula-2 constants
* M2 Types::                    Modula-2 types
* M2 Defaults::                 Default settings for Modula-2
* Deviations::                  Deviations from standard Modula-2
* M2 Checks::                   Modula-2 type and range checks
* M2 Scope::                    The scope operators ‘::’ and ‘.’
* GDB/M2::                      GDB and Modula-2


File: gdb.info,  Node: M2 Operators,  Next: Built-In Func/Proc,  Up: Modula-2

15.4.9.1 Operators
..................

Operators must be defined on values of specific types.  For instance,
‘+’ is defined on numbers, but not on structures.  Operators are often
defined on groups of types.  For the purposes of Modula-2, the following
definitions hold:

   • _Integral types_ consist of ‘INTEGER’, ‘CARDINAL’, and their
     subranges.

   • _Character types_ consist of ‘CHAR’ and its subranges.

   • _Floating-point types_ consist of ‘REAL’.

   • _Pointer types_ consist of anything declared as ‘POINTER TO TYPE’.

   • _Scalar types_ consist of all of the above.

   • _Set types_ consist of ‘SET’ and ‘BITSET’ types.

   • _Boolean types_ consist of ‘BOOLEAN’.

The following operators are supported, and appear in order of increasing
precedence:

‘,’
     Function argument or array index separator.

‘:=’
     Assignment.  The value of VAR ‘:=’ VALUE is VALUE.

‘<, >’
     Less than, greater than on integral, floating-point, or enumerated
     types.

‘<=, >=’
     Less than or equal to, greater than or equal to on integral,
     floating-point and enumerated types, or set inclusion on set types.
     Same precedence as ‘<’.

‘=, <>, #’
     Equality and two ways of expressing inequality, valid on scalar
     types.  Same precedence as ‘<’.  In GDB scripts, only ‘<>’ is
     available for inequality, since ‘#’ conflicts with the script
     comment character.

‘IN’
     Set membership.  Defined on set types and the types of their
     members.  Same precedence as ‘<’.

‘OR’
     Boolean disjunction.  Defined on boolean types.

‘AND, &’
     Boolean conjunction.  Defined on boolean types.

‘@’
     The GDB "artificial array" operator (*note Expressions:
     Expressions.).

‘+, -’
     Addition and subtraction on integral and floating-point types, or
     union and difference on set types.

‘*’
     Multiplication on integral and floating-point types, or set
     intersection on set types.

‘/’
     Division on floating-point types, or symmetric set difference on
     set types.  Same precedence as ‘*’.

‘DIV, MOD’
     Integer division and remainder.  Defined on integral types.  Same
     precedence as ‘*’.

‘-’
     Negative.  Defined on ‘INTEGER’ and ‘REAL’ data.

‘^’
     Pointer dereferencing.  Defined on pointer types.

‘NOT’
     Boolean negation.  Defined on boolean types.  Same precedence as
     ‘^’.

‘.’
     ‘RECORD’ field selector.  Defined on ‘RECORD’ data.  Same
     precedence as ‘^’.

‘[]’
     Array indexing.  Defined on ‘ARRAY’ data.  Same precedence as ‘^’.

‘()’
     Procedure argument list.  Defined on ‘PROCEDURE’ objects.  Same
     precedence as ‘^’.

‘::, .’
     GDB and Modula-2 scope operators.

     _Warning:_ Set expressions and their operations are not yet
     supported, so GDB treats the use of the operator ‘IN’, or the use
     of operators ‘+’, ‘-’, ‘*’, ‘/’, ‘=’, , ‘<>’, ‘#’, ‘<=’, and ‘>=’
     on sets as an error.


File: gdb.info,  Node: Built-In Func/Proc,  Next: M2 Constants,  Prev: M2 Operators,  Up: Modula-2

15.4.9.2 Built-in Functions and Procedures
..........................................

Modula-2 also makes available several built-in procedures and functions.
In describing these, the following metavariables are used:

A
     represents an ‘ARRAY’ variable.

C
     represents a ‘CHAR’ constant or variable.

I
     represents a variable or constant of integral type.

M
     represents an identifier that belongs to a set.  Generally used in
     the same function with the metavariable S.  The type of S should be
     ‘SET OF MTYPE’ (where MTYPE is the type of M).

N
     represents a variable or constant of integral or floating-point
     type.

R
     represents a variable or constant of floating-point type.

T
     represents a type.

V
     represents a variable.

X
     represents a variable or constant of one of many types.  See the
     explanation of the function for details.

   All Modula-2 built-in procedures also return a result, described
below.

‘ABS(N)’
     Returns the absolute value of N.

‘ADR(N)’
     Returns the memory address of N.

‘CAP(C)’
     If C is a lower case letter, it returns its upper case equivalent,
     otherwise it returns its argument.

‘CHR(I)’
     Returns the character whose ordinal value is I.

‘DEC(V)’
     Decrements the value in the variable V by one.  Returns the new
     value.

‘DEC(V,I)’
     Decrements the value in the variable V by I.  Returns the new
     value.

‘EXCL(M,S)’
     Removes the element M from the set S.  Returns the new set.

‘FLOAT(I)’
     Returns the floating point equivalent of the integer I.

‘HIGH(A)’
     Returns the index of the last member of A.

‘INC(V)’
     Increments the value in the variable V by one.  Returns the new
     value.

‘INC(V,I)’
     Increments the value in the variable V by I.  Returns the new
     value.

‘INCL(M,S)’
     Adds the element M to the set S if it is not already there.
     Returns the new set.

‘MAX(T)’
     Returns the maximum value of the type T.

‘MIN(T)’
     Returns the minimum value of the type T.

‘ODD(I)’
     Returns boolean TRUE if I is an odd number.

‘ORD(X)’
     Returns the ordinal value of its argument.  For example, the
     ordinal value of a character is its ASCII value (on machines
     supporting the ASCII character set).  The argument X must be of an
     ordered type, which include integral, character and enumerated
     types.

‘SIZE(X)’
     Returns the size of its argument.  The argument X can be a variable
     or a type.

‘TRUNC(R)’
     Returns the integral part of R.

‘TSIZE(X)’
     Returns the size of its argument.  The argument X can be a variable
     or a type.

‘VAL(T,I)’
     Returns the member of the type T whose ordinal value is I.

     _Warning:_ Sets and their operations are not yet supported, so GDB
     treats the use of procedures ‘INCL’ and ‘EXCL’ as an error.


File: gdb.info,  Node: M2 Constants,  Next: M2 Types,  Prev: Built-In Func/Proc,  Up: Modula-2

15.4.9.3 Constants
..................

GDB allows you to express the constants of Modula-2 in the following
ways:

   • Integer constants are simply a sequence of digits.  When used in an
     expression, a constant is interpreted to be type-compatible with
     the rest of the expression.  Hexadecimal integers are specified by
     a trailing ‘H’, and octal integers by a trailing ‘B’.

   • Floating point constants appear as a sequence of digits, followed
     by a decimal point and another sequence of digits.  An optional
     exponent can then be specified, in the form ‘E[+|-]NNN’, where
     ‘[+|-]NNN’ is the desired exponent.  All of the digits of the
     floating point constant must be valid decimal (base 10) digits.

   • Character constants consist of a single character enclosed by a
     pair of like quotes, either single (‘'’) or double (‘"’).  They may
     also be expressed by their ordinal value (their ASCII value,
     usually) followed by a ‘C’.

   • String constants consist of a sequence of characters enclosed by a
     pair of like quotes, either single (‘'’) or double (‘"’).  Escape
     sequences in the style of C are also allowed.  *Note C and C++
     Constants: C Constants, for a brief explanation of escape
     sequences.

   • Enumerated constants consist of an enumerated identifier.

   • Boolean constants consist of the identifiers ‘TRUE’ and ‘FALSE’.

   • Pointer constants consist of integral values only.

   • Set constants are not yet supported.


File: gdb.info,  Node: M2 Types,  Next: M2 Defaults,  Prev: M2 Constants,  Up: Modula-2

15.4.9.4 Modula-2 Types
.......................

Currently GDB can print the following data types in Modula-2 syntax:
array types, record types, set types, pointer types, procedure types,
enumerated types, subrange types and base types.  You can also print the
contents of variables declared using these type.  This section gives a
number of simple source code examples together with sample GDB sessions.

   The first example contains the following section of code:

     VAR
        s: SET OF CHAR ;
        r: [20..40] ;

and you can request GDB to interrogate the type and value of ‘r’ and
‘s’.

     (gdb) print s
     {'A'..'C', 'Z'}
     (gdb) ptype s
     SET OF CHAR
     (gdb) print r
     21
     (gdb) ptype r
     [20..40]

Likewise if your source code declares ‘s’ as:

     VAR
        s: SET ['A'..'Z'] ;

then you may query the type of ‘s’ by:

     (gdb) ptype s
     type = SET ['A'..'Z']

Note that at present you cannot interactively manipulate set expressions
using the debugger.

   The following example shows how you might declare an array in
Modula-2 and how you can interact with GDB to print its type and
contents:

     VAR
        s: ARRAY [-10..10] OF CHAR ;

     (gdb) ptype s
     ARRAY [-10..10] OF CHAR

   Note that the array handling is not yet complete and although the
type is printed correctly, expression handling still assumes that all
arrays have a lower bound of zero and not ‘-10’ as in the example above.

   Here are some more type related Modula-2 examples:

     TYPE
        colour = (blue, red, yellow, green) ;
        t = [blue..yellow] ;
     VAR
        s: t ;
     BEGIN
        s := blue ;

The GDB interaction shows how you can query the data type and value of a
variable.

     (gdb) print s
     $1 = blue
     (gdb) ptype t
     type = [blue..yellow]

In this example a Modula-2 array is declared and its contents displayed.
Observe that the contents are written in the same way as their ‘C’
counterparts.

     VAR
        s: ARRAY [1..5] OF CARDINAL ;
     BEGIN
        s[1] := 1 ;

     (gdb) print s
     $1 = {1, 0, 0, 0, 0}
     (gdb) ptype s
     type = ARRAY [1..5] OF CARDINAL

   The Modula-2 language interface to GDB also understands pointer types
as shown in this example:

     VAR
        s: POINTER TO ARRAY [1..5] OF CARDINAL ;
     BEGIN
        NEW(s) ;
        s^[1] := 1 ;

and you can request that GDB describes the type of ‘s’.

     (gdb) ptype s
     type = POINTER TO ARRAY [1..5] OF CARDINAL

   GDB handles compound types as we can see in this example.  Here we
combine array types, record types, pointer types and subrange types:

     TYPE
        foo = RECORD
                 f1: CARDINAL ;
                 f2: CHAR ;
                 f3: myarray ;
              END ;

        myarray = ARRAY myrange OF CARDINAL ;
        myrange = [-2..2] ;
     VAR
        s: POINTER TO ARRAY myrange OF foo ;

and you can ask GDB to describe the type of ‘s’ as shown below.

     (gdb) ptype s
     type = POINTER TO ARRAY [-2..2] OF foo = RECORD
         f1 : CARDINAL;
         f2 : CHAR;
         f3 : ARRAY [-2..2] OF CARDINAL;
     END


File: gdb.info,  Node: M2 Defaults,  Next: Deviations,  Prev: M2 Types,  Up: Modula-2

15.4.9.5 Modula-2 Defaults
..........................

If type and range checking are set automatically by GDB, they both
default to ‘on’ whenever the working language changes to Modula-2.  This
happens regardless of whether you or GDB selected the working language.

   If you allow GDB to set the language automatically, then entering
code compiled from a file whose name ends with ‘.mod’ sets the working
language to Modula-2.  *Note Having GDB Infer the Source Language:
Automatically, for further details.


File: gdb.info,  Node: Deviations,  Next: M2 Checks,  Prev: M2 Defaults,  Up: Modula-2

15.4.9.6 Deviations from Standard Modula-2
..........................................

A few changes have been made to make Modula-2 programs easier to debug.
This is done primarily via loosening its type strictness:

   • Unlike in standard Modula-2, pointer constants can be formed by
     integers.  This allows you to modify pointer variables during
     debugging.  (In standard Modula-2, the actual address contained in
     a pointer variable is hidden from you; it can only be modified
     through direct assignment to another pointer variable or expression
     that returned a pointer.)

   • C escape sequences can be used in strings and characters to
     represent non-printable characters.  GDB prints out strings with
     these escape sequences embedded.  Single non-printable characters
     are printed using the ‘CHR(NNN)’ format.

   • The assignment operator (‘:=’) returns the value of its right-hand
     argument.

   • All built-in procedures both modify _and_ return their argument.


File: gdb.info,  Node: M2 Checks,  Next: M2 Scope,  Prev: Deviations,  Up: Modula-2

15.4.9.7 Modula-2 Type and Range Checks
.......................................

     _Warning:_ in this release, GDB does not yet perform type or range
     checking.

   GDB considers two Modula-2 variables type equivalent if:

   • They are of types that have been declared equivalent via a ‘TYPE T1
     = T2’ statement

   • They have been declared on the same line.  (Note: This is true of
     the GNU Modula-2 compiler, but it may not be true of other
     compilers.)

   As long as type checking is enabled, any attempt to combine variables
whose types are not equivalent is an error.

   Range checking is done on all mathematical operations, assignment,
array index bounds, and all built-in functions and procedures.


File: gdb.info,  Node: M2 Scope,  Next: GDB/M2,  Prev: M2 Checks,  Up: Modula-2

15.4.9.8 The Scope Operators ‘::’ and ‘.’
.........................................

There are a few subtle differences between the Modula-2 scope operator
(‘.’) and the GDB scope operator (‘::’).  The two have similar syntax:


     MODULE . ID
     SCOPE :: ID

where SCOPE is the name of a module or a procedure, MODULE the name of a
module, and ID is any declared identifier within your program, except
another module.

   Using the ‘::’ operator makes GDB search the scope specified by SCOPE
for the identifier ID.  If it is not found in the specified scope, then
GDB searches all scopes enclosing the one specified by SCOPE.

   Using the ‘.’ operator makes GDB search the current scope for the
identifier specified by ID that was imported from the definition module
specified by MODULE.  With this operator, it is an error if the
identifier ID was not imported from definition module MODULE, or if ID
is not an identifier in MODULE.


File: gdb.info,  Node: GDB/M2,  Prev: M2 Scope,  Up: Modula-2

15.4.9.9 GDB and Modula-2
.........................

Some GDB commands have little use when debugging Modula-2 programs.
Five subcommands of ‘set print’ and ‘show print’ apply specifically to C
and C++: ‘vtbl’, ‘demangle’, ‘asm-demangle’, ‘object’, and ‘union’.  The
first four apply to C++, and the last to the C ‘union’ type, which has
no direct analogue in Modula-2.

   The ‘@’ operator (*note Expressions: Expressions.), while available
with any language, is not useful with Modula-2.  Its intent is to aid
the debugging of “dynamic arrays”, which cannot be created in Modula-2
as they can in C or C++.  However, because an address can be specified
by an integral constant, the construct ‘{TYPE}ADREXP’ is still useful.

   In GDB scripts, the Modula-2 inequality operator ‘#’ is interpreted
as the beginning of a comment.  Use ‘<>’ instead.


File: gdb.info,  Node: Ada,  Prev: Modula-2,  Up: Supported Languages

15.4.10 Ada
-----------

The extensions made to GDB for Ada only support output from the GNU Ada
(GNAT) compiler.  Other Ada compilers are not currently supported, and
attempting to debug executables produced by them is most likely to be
difficult.

* Menu:

* Ada Mode Intro::              General remarks on the Ada syntax
                                   and semantics supported by Ada mode
                                   in GDB.
* Omissions from Ada::          Restrictions on the Ada expression syntax.
* Additions to Ada::            Extensions of the Ada expression syntax.
* Overloading support for Ada:: Support for expressions involving overloaded
                                   subprograms.
* Stopping Before Main Program:: Debugging the program during elaboration.
* Ada Exceptions::              Ada Exceptions
* Ada Tasks::                   Listing and setting breakpoints in tasks.
* Ada Tasks and Core Files::    Tasking Support when Debugging Core Files
* Ravenscar Profile::           Tasking Support when using the Ravenscar
                                   Profile
* Ada Source Character Set::    Character set of Ada source files.
* Ada Glitches::                Known peculiarities of Ada mode.


File: gdb.info,  Node: Ada Mode Intro,  Next: Omissions from Ada,  Up: Ada

15.4.10.1 Introduction
......................

The Ada mode of GDB supports a fairly large subset of Ada expression
syntax, with some extensions.  The philosophy behind the design of this
subset is

   • That GDB should provide basic literals and access to operations for
     arithmetic, dereferencing, field selection, indexing, and
     subprogram calls, leaving more sophisticated computations to
     subprograms written into the program (which therefore may be called
     from GDB).

   • That type safety and strict adherence to Ada language restrictions
     are not particularly important to the GDB user.

   • That brevity is important to the GDB user.

   Thus, for brevity, the debugger acts as if all names declared in
user-written packages are directly visible, even if they are not visible
according to Ada rules, thus making it unnecessary to fully qualify most
names with their packages, regardless of context.  Where this causes
ambiguity, GDB asks the user's intent.

   The debugger will start in Ada mode if it detects an Ada main
program.  As for other languages, it will enter Ada mode when stopped in
a program that was translated from an Ada source file.

   While in Ada mode, you may use '--' for comments.  This is useful
mostly for documenting command files.  The standard GDB comment (‘#’)
still works at the beginning of a line in Ada mode, but not in the
middle (to allow based literals).


File: gdb.info,  Node: Omissions from Ada,  Next: Additions to Ada,  Prev: Ada Mode Intro,  Up: Ada

15.4.10.2 Omissions from Ada
............................

Here are the notable omissions from the subset:

   • Only a subset of the attributes are supported:

        − 'First, 'Last, and 'Length on array objects (not on types and
          subtypes).

        − 'Min and 'Max.

        − 'Pos and 'Val.

        − 'Tag.

        − 'Range on array objects (not subtypes), but only as the right
          operand of the membership (‘in’) operator.

        − 'Access, 'Unchecked_Access, and 'Unrestricted_Access (a GNAT
          extension).

        − 'Address.

        − 'Size is available for objects (not types).

        − 'Object_Size is available, but not for indefinite types.

   • The names in ‘Characters.Latin_1’ are not available.

   • Equality tests (‘=’ and ‘/=’) on arrays test for bitwise equality
     of representations.  They will generally work correctly for strings
     and arrays whose elements have integer or enumeration types.  They
     may not work correctly for arrays whose element types have
     user-defined equality, for arrays of real values (in particular,
     IEEE-conformant floating point, because of negative zeroes and
     NaNs), and for arrays whose elements contain unused bits with
     indeterminate values.

   • The other component-by-component array operations (‘and’, ‘or’,
     ‘xor’, ‘not’, and relational tests other than equality) are not
     implemented.

   • There is limited support for array and record aggregates.  They are
     permitted only on the right sides of assignments, as in these
     examples:

          (gdb) set An_Array := (1, 2, 3, 4, 5, 6)
          (gdb) set An_Array := (1, others => 0)
          (gdb) set An_Array := (0|4 => 1, 1..3 => 2, 5 => 6)
          (gdb) set A_2D_Array := ((1, 2, 3), (4, 5, 6), (7, 8, 9))
          (gdb) set A_Record := (1, "Peter", True);
          (gdb) set A_Record := (Name => "Peter", Id => 1, Alive => True)

     Changing a discriminant's value by assigning an aggregate has an
     undefined effect if that discriminant is used within the record.
     However, you can first modify discriminants by directly assigning
     to them (which normally would not be allowed in Ada), and then
     performing an aggregate assignment.  For example, given a variable
     ‘A_Rec’ declared to have a type such as:

          type Rec (Len : Small_Integer := 0) is record
              Id : Integer;
              Vals : IntArray (1 .. Len);
          end record;

     you can assign a value with a different size of ‘Vals’ with two
     assignments:

          (gdb) set A_Rec.Len := 4
          (gdb) set A_Rec := (Id => 42, Vals => (1, 2, 3, 4))

     As this example also illustrates, GDB is very loose about the usual
     rules concerning aggregates.  You may leave out some of the
     components of an array or record aggregate (such as the ‘Len’
     component in the assignment to ‘A_Rec’ above); they will retain
     their original values upon assignment.  You may freely use dynamic
     values as indices in component associations.  You may even use
     overlapping or redundant component associations, although which
     component values are assigned in such cases is not defined.

   • Calls to dispatching subprograms are not implemented.

   • The overloading algorithm is much more limited (i.e., less
     selective) than that of real Ada.  It makes only limited use of the
     context in which a subexpression appears to resolve its meaning,
     and it is much looser in its rules for allowing type matches.  As a
     result, some function calls will be ambiguous, and the user will be
     asked to choose the proper resolution.

   • The ‘new’ operator is not implemented.

   • Entry calls are not implemented.

   • Aside from printing, arithmetic operations on the native VAX
     floating-point formats are not supported.

   • It is not possible to slice a packed array.

   • The names ‘True’ and ‘False’, when not part of a qualified name,
     are interpreted as if implicitly prefixed by ‘Standard’, regardless
     of context.  Should your program redefine these names in a package
     or procedure (at best a dubious practice), you will have to use
     fully qualified names to access their new definitions.

   • Based real literals are not implemented.


File: gdb.info,  Node: Additions to Ada,  Next: Overloading support for Ada,  Prev: Omissions from Ada,  Up: Ada

15.4.10.3 Additions to Ada
..........................

As it does for other languages, GDB makes certain generic extensions to
Ada (*note Expressions::):

   • If the expression E is a variable residing in memory (typically a
     local variable or array element) and N is a positive integer, then
     ‘E@N’ displays the values of E and the N-1 adjacent variables
     following it in memory as an array.  In Ada, this operator is
     generally not necessary, since its prime use is in displaying parts
     of an array, and slicing will usually do this in Ada.  However,
     there are occasional uses when debugging programs in which certain
     debugging information has been optimized away.

   • ‘B::VAR’ means "the variable named VAR that appears in function or
     file B."  When B is a file name, you must typically surround it in
     single quotes.

   • The expression ‘{TYPE} ADDR’ means "the variable of type TYPE that
     appears at address ADDR."

   • A name starting with ‘$’ is a convenience variable (*note
     Convenience Vars::) or a machine register (*note Registers::).

   In addition, GDB provides a few other shortcuts and outright
additions specific to Ada:

   • The assignment statement is allowed as an expression, returning its
     right-hand operand as its value.  Thus, you may enter

          (gdb) set x := y + 3
          (gdb) print A(tmp := y + 1)

   • The semicolon is allowed as an "operator," returning as its value
     the value of its right-hand operand.  This allows, for example,
     complex conditional breaks:

          (gdb) break f
          (gdb) condition 1 (report(i); k += 1; A(k) > 100)

   • An extension to based literals can be used to specify the exact
     byte contents of a floating-point literal.  After the base, you can
     use from zero to two ‘l’ characters, followed by an ‘f’.  The
     number of ‘l’ characters controls the width of the resulting real
     constant: zero means ‘Float’ is used, one means ‘Long_Float’, and
     two means ‘Long_Long_Float’.

          (gdb) print 16f#41b80000#
          $1 = 23.0

   • Rather than use catenation and symbolic character names to
     introduce special characters into strings, one may instead use a
     special bracket notation, which is also used to print strings.  A
     sequence of characters of the form ‘["XX"]’ within a string or
     character literal denotes the (single) character whose numeric
     encoding is XX in hexadecimal.  The sequence of characters ‘["""]’
     also denotes a single quotation mark in strings.  For example,
             "One line.["0a"]Next line.["0a"]"
     contains an ASCII newline character (‘Ada.Characters.Latin_1.LF’)
     after each period.

   • The subtype used as a prefix for the attributes 'Pos, 'Min, and
     'Max is optional (and is ignored in any case).  For example, it is
     valid to write

          (gdb) print 'max(x, y)

   • When printing arrays, GDB uses positional notation when the array
     has a lower bound of 1, and uses a modified named notation
     otherwise.  For example, a one-dimensional array of three integers
     with a lower bound of 3 might print as

          (3 => 10, 17, 1)

     That is, in contrast to valid Ada, only the first component has a
     ‘=>’ clause.

   • You may abbreviate attributes in expressions with any unique,
     multi-character subsequence of their names (an exact match gets
     preference).  For example, you may use a'len, a'gth, or a'lh in
     place of a'length.

   • Since Ada is case-insensitive, the debugger normally maps
     identifiers you type to lower case.  The GNAT compiler uses
     upper-case characters for some of its internal identifiers, which
     are normally of no interest to users.  For the rare occasions when
     you actually have to look at them, enclose them in angle brackets
     to avoid the lower-case mapping.  For example,
          (gdb) print <JMPBUF_SAVE>[0]

   • Printing an object of class-wide type or dereferencing an
     access-to-class-wide value will display all the components of the
     object's specific type (as indicated by its run-time tag).
     Likewise, component selection on such a value will operate on the
     specific type of the object.


File: gdb.info,  Node: Overloading support for Ada,  Next: Stopping Before Main Program,  Prev: Additions to Ada,  Up: Ada

15.4.10.4 Overloading support for Ada
.....................................

The debugger supports limited overloading.  Given a subprogram call in
which the function symbol has multiple definitions, it will use the
number of actual parameters and some information about their types to
attempt to narrow the set of definitions.  It also makes very limited
use of context, preferring procedures to functions in the context of the
‘call’ command, and functions to procedures elsewhere.

   If, after narrowing, the set of matching definitions still contains
more than one definition, GDB will display a menu to query which one it
should use, for instance:

     (gdb) print f(1)
     Multiple matches for f
     [0] cancel
     [1] foo.f (integer) return boolean at foo.adb:23
     [2] foo.f (foo.new_integer) return boolean at foo.adb:28
     >

   In this case, just select one menu entry either to cancel expression
evaluation (type ‘0’ and press <RET>) or to continue evaluation with a
specific instance (type the corresponding number and press <RET>).

   Here are a couple of commands to customize GDB's behavior in this
case:

‘set ada print-signatures’
     Control whether parameter types and return types are displayed in
     overloads selection menus.  It is ‘on’ by default.  *Note
     Overloading support for Ada::.

‘show ada print-signatures’
     Show the current setting for displaying parameter types and return
     types in overloads selection menu.  *Note Overloading support for
     Ada::.


File: gdb.info,  Node: Stopping Before Main Program,  Next: Ada Exceptions,  Prev: Overloading support for Ada,  Up: Ada

15.4.10.5 Stopping at the Very Beginning
........................................

It is sometimes necessary to debug the program during elaboration, and
before reaching the main procedure.  As defined in the Ada Reference
Manual, the elaboration code is invoked from a procedure called
‘adainit’.  To run your program up to the beginning of elaboration,
simply use the following two commands: ‘tbreak adainit’ and ‘run’.


File: gdb.info,  Node: Ada Exceptions,  Next: Ada Tasks,  Prev: Stopping Before Main Program,  Up: Ada

15.4.10.6 Ada Exceptions
........................

A command is provided to list all Ada exceptions:

‘info exceptions’
‘info exceptions REGEXP’
     The ‘info exceptions’ command allows you to list all Ada exceptions
     defined within the program being debugged, as well as their
     addresses.  With a regular expression, REGEXP, as argument, only
     those exceptions whose names match REGEXP are listed.

   Below is a small example, showing how the command can be used, first
without argument, and next with a regular expression passed as an
argument.

     (gdb) info exceptions
     All defined Ada exceptions:
     constraint_error: 0x613da0
     program_error: 0x613d20
     storage_error: 0x613ce0
     tasking_error: 0x613ca0
     const.aint_global_e: 0x613b00
     (gdb) info exceptions const.aint
     All Ada exceptions matching regular expression "const.aint":
     constraint_error: 0x613da0
     const.aint_global_e: 0x613b00

   It is also possible to ask GDB to stop your program's execution when
an exception is raised.  For more details, see *note Set Catchpoints::.


File: gdb.info,  Node: Ada Tasks,  Next: Ada Tasks and Core Files,  Prev: Ada Exceptions,  Up: Ada

15.4.10.7 Extensions for Ada Tasks
..................................

Support for Ada tasks is analogous to that for threads (*note
Threads::).  GDB provides the following task-related commands:

‘info tasks’
     This command shows a list of current Ada tasks, as in the following
     example:

          (gdb) info tasks
            ID       TID P-ID Pri State                 Name
             1   8088000   0   15 Child Activation Wait main_task
             2   80a4000   1   15 Accept Statement      b
             3   809a800   1   15 Child Activation Wait a
          *  4   80ae800   3   15 Runnable              c


     In this listing, the asterisk before the last task indicates it to
     be the task currently being inspected.

     ID
          Represents GDB's internal task number.

     TID
          The Ada task ID.

     P-ID
          The parent's task ID (GDB's internal task number).

     Pri
          The base priority of the task.

     State
          Current state of the task.

          ‘Unactivated’
               The task has been created but has not been activated.  It
               cannot be executing.

          ‘Runnable’
               The task is not blocked for any reason known to Ada.  (It
               may be waiting for a mutex, though.)  It is conceptually
               "executing" in normal mode.

          ‘Terminated’
               The task is terminated, in the sense of ARM 9.3 (5).  Any
               dependents that were waiting on terminate alternatives
               have been awakened and have terminated themselves.

          ‘Child Activation Wait’
               The task is waiting for created tasks to complete
               activation.

          ‘Accept or Select Term’
               The task is waiting on an accept or selective wait
               statement.

          ‘Waiting on entry call’
               The task is waiting on an entry call.

          ‘Async Select Wait’
               The task is waiting to start the abortable part of an
               asynchronous select statement.

          ‘Delay Sleep’
               The task is waiting on a select statement with only a
               delay alternative open.

          ‘Child Termination Wait’
               The task is sleeping having completed a master within
               itself, and is waiting for the tasks dependent on that
               master to become terminated or waiting on a terminate
               Phase.

          ‘Wait Child in Term Alt’
               The task is sleeping waiting for tasks on terminate
               alternatives to finish terminating.

          ‘Asynchronous Hold’
               The task has been held by
               ‘Ada.Asynchronous_Task_Control.Hold_Task’.

          ‘Activating’
               The task has been created and is being made runnable.

          ‘Selective Wait’
               The task is waiting in a selective wait statement.

          ‘Suspension object sleep’
               The task is waiting in a call to ‘Suspend_Until_True’ on
               a suspension object.

          ‘Accepting RV with TASKNO’
               The task is accepting a rendez-vous with the task TASKNO.

          ‘Waiting on RV with TASKNO’
               The task is waiting for a rendez-vous with the task
               TASKNO.

     Name
          Name of the task in the program.

‘info task TASKNO’
     This command shows detailed information on the specified task, as
     in the following example:
          (gdb) info tasks
            ID       TID P-ID Pri State                  Name
             1   8077880    0  15 Child Activation Wait  main_task
          *  2   807c468    1  15 Runnable               task_1
          (gdb) info task 2
          Ada Task: 0x807c468
          Name: "task_1"
          Thread: 0
          LWP: 0x1fac
          Parent: 1 ("main_task")
          Base Priority: 15
          State: Runnable

‘task’
     This command prints the ID and name of the current task.

          (gdb) info tasks
            ID       TID P-ID Pri State                  Name
             1   8077870    0  15 Child Activation Wait  main_task
          *  2   807c458    1  15 Runnable               some_task
          (gdb) task
          [Current task is 2 "some_task"]

‘task TASKNO’
     This command is like the ‘thread THREAD-ID’ command (*note
     Threads::).  It switches the context of debugging from the current
     task to the given task.

          (gdb) info tasks
            ID       TID P-ID Pri State                  Name
             1   8077870    0  15 Child Activation Wait  main_task
          *  2   807c458    1  15 Runnable               some_task
          (gdb) task 1
          [Switching to task 1 "main_task"]
          #0  0x8067726 in pthread_cond_wait ()
          (gdb) bt
          #0  0x8067726 in pthread_cond_wait ()
          #1  0x8056714 in system.os_interface.pthread_cond_wait ()
          #2  0x805cb63 in system.task_primitives.operations.sleep ()
          #3  0x806153e in system.tasking.stages.activate_tasks ()
          #4  0x804aacc in un () at un.adb:5

‘task apply [TASK-ID-LIST | all] [FLAG]... COMMAND’
     The ‘task apply’ command is the Ada tasking analogue of ‘thread
     apply’ (*note Threads::).  It allows you to apply the named COMMAND
     to one or more tasks.  Specify the tasks that you want affected
     using a list of task IDs, or specify ‘all’ to apply to all tasks.

     The FLAG arguments control what output to produce and how to handle
     errors raised when applying COMMAND to a task.  FLAG must start
     with a ‘-’ directly followed by one letter in ‘qcs’.  If several
     flags are provided, they must be given individually, such as ‘-c
     -q’.

     By default, GDB displays some task information before the output
     produced by COMMAND, and an error raised during the execution of a
     COMMAND will abort ‘task apply’.  The following flags can be used
     to fine-tune this behavior:

     ‘-c’
          The flag ‘-c’, which stands for ‘continue’, causes any errors
          in COMMAND to be displayed, and the execution of ‘task apply’
          then continues.
     ‘-s’
          The flag ‘-s’, which stands for ‘silent’, causes any errors or
          empty output produced by a COMMAND to be silently ignored.
          That is, the execution continues, but the task information and
          errors are not printed.
     ‘-q’
          The flag ‘-q’ (‘quiet’) disables printing the task
          information.

     Flags ‘-c’ and ‘-s’ cannot be used together.

‘break LOCSPEC task TASKNO’
‘break LOCSPEC task TASKNO if ...’
     These commands are like the ‘break ... thread ...’ command (*note
     Thread Stops::).  *Note Location Specifications::, for the various
     forms of LOCSPEC.

     Use the qualifier ‘task TASKNO’ with a breakpoint command to
     specify that you only want GDB to stop the program when a
     particular Ada task reaches this breakpoint.  The TASKNO is one of
     the numeric task identifiers assigned by GDB, shown in the first
     column of the ‘info tasks’ display.

     If you do not specify ‘task TASKNO’ when you set a breakpoint, the
     breakpoint applies to _all_ tasks of your program.

     You can use the ‘task’ qualifier on conditional breakpoints as
     well; in this case, place ‘task TASKNO’ before the breakpoint
     condition (before the ‘if’).

     For example,

          (gdb) info tasks
            ID       TID P-ID Pri State                 Name
             1 140022020   0   15 Child Activation Wait main_task
             2 140045060   1   15 Accept/Select Wait    t2
             3 140044840   1   15 Runnable              t1
          *  4 140056040   1   15 Runnable              t3
          (gdb) b 15 task 2
          Breakpoint 5 at 0x120044cb0: file test_task_debug.adb, line 15.
          (gdb) cont
          Continuing.
          task # 1 running
          task # 2 running

          Breakpoint 5, test_task_debug () at test_task_debug.adb:15
          15               flush;
          (gdb) info tasks
            ID       TID P-ID Pri State                 Name
             1 140022020   0   15 Child Activation Wait main_task
          *  2 140045060   1   15 Runnable              t2
             3 140044840   1   15 Runnable              t1
             4 140056040   1   15 Delay Sleep           t3


File: gdb.info,  Node: Ada Tasks and Core Files,  Next: Ravenscar Profile,  Prev: Ada Tasks,  Up: Ada

15.4.10.8 Tasking Support when Debugging Core Files
...................................................

When inspecting a core file, as opposed to debugging a live program,
tasking support may be limited or even unavailable, depending on the
platform being used.  For instance, on x86-linux, the list of tasks is
available, but task switching is not supported.

   On certain platforms, the debugger needs to perform some memory
writes in order to provide Ada tasking support.  When inspecting a core
file, this means that the core file must be opened with read-write
privileges, using the command ‘"set write on"’ (*note Patching::).
Under these circumstances, you should make a backup copy of the core
file before inspecting it with GDB.


File: gdb.info,  Node: Ravenscar Profile,  Next: Ada Source Character Set,  Prev: Ada Tasks and Core Files,  Up: Ada

15.4.10.9 Tasking Support when using the Ravenscar Profile
..........................................................

The “Ravenscar Profile” is a subset of the Ada tasking features,
specifically designed for systems with safety-critical real-time
requirements.

‘set ravenscar task-switching on’
     Allows task switching when debugging a program that uses the
     Ravenscar Profile.  This is the default.

‘set ravenscar task-switching off’
     Turn off task switching when debugging a program that uses the
     Ravenscar Profile.  This is mostly intended to disable the code
     that adds support for the Ravenscar Profile, in case a bug in
     either GDB or in the Ravenscar runtime is preventing GDB from
     working properly.  To be effective, this command should be run
     before the program is started.

‘show ravenscar task-switching’
     Show whether it is possible to switch from task to task in a
     program using the Ravenscar Profile.

   When Ravenscar task-switching is enabled, Ravenscar tasks are
announced by GDB as if they were threads:

     (gdb) continue
     [New Ravenscar Thread 0x2b8f0]

   Both Ravenscar tasks and the underlying CPU threads will show up in
the output of ‘info threads’:

     (gdb) info threads
       Id   Target Id                  Frame
       1    Thread 1 (CPU#0 [running]) simple () at simple.adb:10
       2    Thread 2 (CPU#1 [running]) 0x0000000000003d34 in __gnat_initialize_cpu_devices ()
       3    Thread 3 (CPU#2 [running]) 0x0000000000003d28 in __gnat_initialize_cpu_devices ()
       4    Thread 4 (CPU#3 [halted ]) 0x000000000000c6ec in system.task_primitives.operations.idle ()
     * 5    Ravenscar Thread 0x2b8f0   simple () at simple.adb:10
       6    Ravenscar Thread 0x2f150   0x000000000000c6ec in system.task_primitives.operations.idle ()

   One known limitation of the Ravenscar support in GDB is that it isn't
currently possible to single-step through the runtime initialization
sequence.  If you need to debug this code, you should use ‘set ravenscar
task-switching off’.


File: gdb.info,  Node: Ada Source Character Set,  Next: Ada Glitches,  Prev: Ravenscar Profile,  Up: Ada

15.4.10.10 Ada Source Character Set
...................................

The GNAT compiler supports a number of character sets for source files.
*Note Character Set Control: (gnat_ugn)Character Set Control.  GDB
includes support for this as well.

‘set ada source-charset CHARSET’
     Set the source character set for Ada.  The character set must be
     supported by GNAT. Because this setting affects the decoding of
     symbols coming from the debug information in your program, the
     setting should be set as early as possible.  The default is
     ‘ISO-8859-1’, because that is also GNAT's default.

‘show ada source-charset’
     Show the current source character set for Ada.


File: gdb.info,  Node: Ada Glitches,  Prev: Ada Source Character Set,  Up: Ada

15.4.10.11 Known Peculiarities of Ada Mode
..........................................

Besides the omissions listed previously (*note Omissions from Ada::), we
know of several problems with and limitations of Ada mode in GDB, some
of which will be fixed with planned future releases of the debugger and
the GNU Ada compiler.

   • Static constants that the compiler chooses not to materialize as
     objects in storage are invisible to the debugger.

   • Named parameter associations in function argument lists are ignored
     (the argument lists are treated as positional).

   • Many useful library packages are currently invisible to the
     debugger.

   • Fixed-point arithmetic, conversions, input, and output is carried
     out using floating-point arithmetic, and may give results that only
     approximate those on the host machine.

   • The GNAT compiler never generates the prefix ‘Standard’ for any of
     the standard symbols defined by the Ada language.  GDB knows about
     this: it will strip the prefix from names when you use it, and will
     never look for a name you have so qualified among local symbols,
     nor match against symbols in other packages or subprograms.  If you
     have defined entities anywhere in your program other than
     parameters and local variables whose simple names match names in
     ‘Standard’, GNAT's lack of qualification here can cause confusion.
     When this happens, you can usually resolve the confusion by
     qualifying the problematic names with package ‘Standard’
     explicitly.

   Older versions of the compiler sometimes generate erroneous debugging
information, resulting in the debugger incorrectly printing the value of
affected entities.  In some cases, the debugger is able to work around
an issue automatically.  In other cases, the debugger is able to work
around the issue, but the work-around has to be specifically enabled.

‘set ada trust-PAD-over-XVS on’
     Configure GDB to strictly follow the GNAT encoding when computing
     the value of Ada entities, particularly when ‘PAD’ and ‘PAD___XVS’
     types are involved (see ‘ada/exp_dbug.ads’ in the GCC sources for a
     complete description of the encoding used by the GNAT compiler).
     This is the default.

‘set ada trust-PAD-over-XVS off’
     This is related to the encoding using by the GNAT compiler.  If GDB
     sometimes prints the wrong value for certain entities, changing
     ‘ada trust-PAD-over-XVS’ to ‘off’ activates a work-around which may
     fix the issue.  It is always safe to set ‘ada trust-PAD-over-XVS’
     to ‘off’, but this incurs a slight performance penalty, so it is
     recommended to leave this setting to ‘on’ unless necessary.

   Internally, the debugger also relies on the compiler following a
number of conventions known as the ‘GNAT Encoding’, all documented in
‘gcc/ada/exp_dbug.ads’ in the GCC sources.  This encoding describes how
the debugging information should be generated for certain types.  In
particular, this convention makes use of “descriptive types”, which are
artificial types generated purely to help the debugger.

   These encodings were defined at a time when the debugging information
format used was not powerful enough to describe some of the more complex
types available in Ada.  Since DWARF allows us to express nearly all Ada
features, the long-term goal is to slowly replace these descriptive
types by their pure DWARF equivalent.  To facilitate that transition, a
new maintenance option is available to force the debugger to ignore
those descriptive types.  It allows the user to quickly evaluate how
well GDB works without them.

‘maintenance ada set ignore-descriptive-types [on|off]’
     Control whether the debugger should ignore descriptive types.  The
     default is not to ignore descriptives types (‘off’).

‘maintenance ada show ignore-descriptive-types’
     Show if descriptive types are ignored by GDB.


File: gdb.info,  Node: Unsupported Languages,  Prev: Supported Languages,  Up: Languages

15.5 Unsupported Languages
==========================

In addition to the other fully-supported programming languages, GDB also
provides a pseudo-language, called ‘minimal’.  It does not represent a
real programming language, but provides a set of capabilities close to
what the C or assembly languages provide.  This should allow most simple
operations to be performed while debugging an application that uses a
language currently not supported by GDB.

   If the language is set to ‘auto’, GDB will automatically select this
language if the current frame corresponds to an unsupported language.


File: gdb.info,  Node: Symbols,  Next: Altering,  Prev: Languages,  Up: Top

16 Examining the Symbol Table
*****************************

The commands described in this chapter allow you to inquire about the
symbols (names of variables, functions and types) defined in your
program.  This information is inherent in the text of your program and
does not change as your program executes.  GDB finds it in your
program's symbol table, in the file indicated when you started GDB
(*note Choosing Files: File Options.), or by one of the file-management
commands (*note Commands to Specify Files: Files.).

   Occasionally, you may need to refer to symbols that contain unusual
characters, which GDB ordinarily treats as word delimiters.  The most
frequent case is in referring to static variables in other source files
(*note Program Variables: Variables.).  File names are recorded in
object files as debugging symbols, but GDB would ordinarily parse a
typical file name, like ‘foo.c’, as the three words ‘foo’ ‘.’ ‘c’.  To
allow GDB to recognize ‘foo.c’ as a single symbol, enclose it in single
quotes; for example,

     p 'foo.c'::x

looks up the value of ‘x’ in the scope of the file ‘foo.c’.

‘set case-sensitive on’
‘set case-sensitive off’
‘set case-sensitive auto’
     Normally, when GDB looks up symbols, it matches their names with
     case sensitivity determined by the current source language.
     Occasionally, you may wish to control that.  The command ‘set
     case-sensitive’ lets you do that by specifying ‘on’ for
     case-sensitive matches or ‘off’ for case-insensitive ones.  If you
     specify ‘auto’, case sensitivity is reset to the default suitable
     for the source language.  The default is case-sensitive matches for
     all languages except for Fortran, for which the default is
     case-insensitive matches.

‘show case-sensitive’
     This command shows the current setting of case sensitivity for
     symbols lookups.

‘set print type methods’
‘set print type methods on’
‘set print type methods off’
     Normally, when GDB prints a class, it displays any methods declared
     in that class.  You can control this behavior either by passing the
     appropriate flag to ‘ptype’, or using ‘set print type methods’.
     Specifying ‘on’ will cause GDB to display the methods; this is the
     default.  Specifying ‘off’ will cause GDB to omit the methods.

‘show print type methods’
     This command shows the current setting of method display when
     printing classes.

‘set print type nested-type-limit LIMIT’
‘set print type nested-type-limit unlimited’
     Set the limit of displayed nested types that the type printer will
     show.  A LIMIT of ‘unlimited’ or ‘-1’ will show all nested
     definitions.  By default, the type printer will not show any nested
     types defined in classes.

‘show print type nested-type-limit’
     This command shows the current display limit of nested types when
     printing classes.

‘set print type typedefs’
‘set print type typedefs on’
‘set print type typedefs off’

     Normally, when GDB prints a class, it displays any typedefs defined
     in that class.  You can control this behavior either by passing the
     appropriate flag to ‘ptype’, or using ‘set print type typedefs’.
     Specifying ‘on’ will cause GDB to display the typedef definitions;
     this is the default.  Specifying ‘off’ will cause GDB to omit the
     typedef definitions.  Note that this controls whether the typedef
     definition itself is printed, not whether typedef names are
     substituted when printing other types.

‘show print type typedefs’
     This command shows the current setting of typedef display when
     printing classes.

‘set print type hex’
‘set print type hex on’
‘set print type hex off’

     When GDB prints sizes and offsets of struct members, it can use
     either the decimal or hexadecimal notation.  You can select one or
     the other either by passing the appropriate flag to ‘ptype’, or by
     using the ‘set print type hex’ command.

‘show print type hex’
     This command shows whether the sizes and offsets of struct members
     are printed in decimal or hexadecimal notation.

‘info address SYMBOL’
     Describe where the data for SYMBOL is stored.  For a register
     variable, this says which register it is kept in.  For a
     non-register local variable, this prints the stack-frame offset at
     which the variable is always stored.

     Note the contrast with ‘print &SYMBOL’, which does not work at all
     for a register variable, and for a stack local variable prints the
     exact address of the current instantiation of the variable.

‘info symbol ADDR’
     Print the name of a symbol which is stored at the address ADDR.  If
     no symbol is stored exactly at ADDR, GDB prints the nearest symbol
     and an offset from it:

          (gdb) info symbol 0x54320
          _initialize_vx + 396 in section .text

     This is the opposite of the ‘info address’ command.  You can use it
     to find out the name of a variable or a function given its address.

     For dynamically linked executables, the name of executable or
     shared library containing the symbol is also printed:

          (gdb) info symbol 0x400225
          _start + 5 in section .text of /tmp/a.out
          (gdb) info symbol 0x2aaaac2811cf
          __read_nocancel + 6 in section .text of /usr/lib64/libc.so.6

‘demangle [-l LANGUAGE] [--] NAME’
     Demangle NAME.  If LANGUAGE is provided it is the name of the
     language to demangle NAME in.  Otherwise NAME is demangled in the
     current language.

     The ‘--’ option specifies the end of options, and is useful when
     NAME begins with a dash.

     The parameter ‘demangle-style’ specifies how to interpret the kind
     of mangling used.  *Note Print Settings::.

‘whatis[/FLAGS] [ARG]’
     Print the data type of ARG, which can be either an expression or a
     name of a data type.  With no argument, print the data type of ‘$’,
     the last value in the value history.

     If ARG is an expression (*note Expressions: Expressions.), it is
     not actually evaluated, and any side-effecting operations (such as
     assignments or function calls) inside it do not take place.

     If ARG is a variable or an expression, ‘whatis’ prints its literal
     type as it is used in the source code.  If the type was defined
     using a ‘typedef’, ‘whatis’ will _not_ print the data type
     underlying the ‘typedef’.  If the type of the variable or the
     expression is a compound data type, such as ‘struct’ or ‘class’,
     ‘whatis’ never prints their fields or methods.  It just prints the
     ‘struct’/‘class’ name (a.k.a. its “tag”).  If you want to see the
     members of such a compound data type, use ‘ptype’.

     If ARG is a type name that was defined using ‘typedef’, ‘whatis’
     “unrolls” only one level of that ‘typedef’.  Unrolling means that
     ‘whatis’ will show the underlying type used in the ‘typedef’
     declaration of ARG.  However, if that underlying type is also a
     ‘typedef’, ‘whatis’ will not unroll it.

     For C code, the type names may also have the form ‘class
     CLASS-NAME’, ‘struct STRUCT-TAG’, ‘union UNION-TAG’ or ‘enum
     ENUM-TAG’.

     FLAGS can be used to modify how the type is displayed.  Available
     flags are:

     ‘r’
          Display in "raw" form.  Normally, GDB substitutes template
          parameters and typedefs defined in a class when printing the
          class' members.  The ‘/r’ flag disables this.

     ‘m’
          Do not print methods defined in the class.

     ‘M’
          Print methods defined in the class.  This is the default, but
          the flag exists in case you change the default with ‘set print
          type methods’.

     ‘t’
          Do not print typedefs defined in the class.  Note that this
          controls whether the typedef definition itself is printed, not
          whether typedef names are substituted when printing other
          types.

     ‘T’
          Print typedefs defined in the class.  This is the default, but
          the flag exists in case you change the default with ‘set print
          type typedefs’.

     ‘o’
          Print the offsets and sizes of fields in a struct, similar to
          what the ‘pahole’ tool does.  This option implies the ‘/tm’
          flags.

     ‘x’
          Use hexadecimal notation when printing offsets and sizes of
          fields in a struct.

     ‘d’
          Use decimal notation when printing offsets and sizes of fields
          in a struct.

          For example, given the following declarations:

               struct tuv
               {
                 int a1;
                 char *a2;
                 int a3;
               };

               struct xyz
               {
                 int f1;
                 char f2;
                 void *f3;
                 struct tuv f4;
               };

               union qwe
               {
                 struct tuv fff1;
                 struct xyz fff2;
               };

               struct tyu
               {
                 int a1 : 1;
                 int a2 : 3;
                 int a3 : 23;
                 char a4 : 2;
                 int64_t a5;
                 int a6 : 5;
                 int64_t a7 : 3;
               };

          Issuing a ‘ptype /o struct tuv’ command would print:

               (gdb) ptype /o struct tuv
               /* offset      |    size */  type = struct tuv {
               /*      0      |       4 */    int a1;
               /* XXX  4-byte hole      */
               /*      8      |       8 */    char *a2;
               /*     16      |       4 */    int a3;

                                              /* total size (bytes):   24 */
                                            }

          Notice the format of the first column of comments.  There, you
          can find two parts separated by the ‘|’ character: the
          _offset_, which indicates where the field is located inside
          the struct, in bytes, and the _size_ of the field.  Another
          interesting line is the marker of a _hole_ in the struct,
          indicating that it may be possible to pack the struct and make
          it use less space by reorganizing its fields.

          It is also possible to print offsets inside an union:

               (gdb) ptype /o union qwe
               /* offset      |    size */  type = union qwe {
               /*                    24 */    struct tuv {
               /*      0      |       4 */        int a1;
               /* XXX  4-byte hole      */
               /*      8      |       8 */        char *a2;
               /*     16      |       4 */        int a3;

                                                  /* total size (bytes):   24 */
                                              } fff1;
               /*                    40 */    struct xyz {
               /*      0      |       4 */        int f1;
               /*      4      |       1 */        char f2;
               /* XXX  3-byte hole      */
               /*      8      |       8 */        void *f3;
               /*     16      |      24 */        struct tuv {
               /*     16      |       4 */            int a1;
               /* XXX  4-byte hole      */
               /*     24      |       8 */            char *a2;
               /*     32      |       4 */            int a3;

                                                      /* total size (bytes):   24 */
                                                  } f4;

                                                  /* total size (bytes):   40 */
                                              } fff2;

                                              /* total size (bytes):   40 */
                                            }

          In this case, since ‘struct tuv’ and ‘struct xyz’ occupy the
          same space (because we are dealing with an union), the offset
          is not printed for them.  However, you can still examine the
          offset of each of these structures' fields.

          Another useful scenario is printing the offsets of a struct
          containing bitfields:

               (gdb) ptype /o struct tyu
               /* offset      |    size */  type = struct tyu {
               /*      0:31   |       4 */    int a1 : 1;
               /*      0:28   |       4 */    int a2 : 3;
               /*      0: 5   |       4 */    int a3 : 23;
               /*      3: 3   |       1 */    signed char a4 : 2;
               /* XXX  3-bit hole       */
               /* XXX  4-byte hole      */
               /*      8      |       8 */    int64_t a5;
               /*     16: 0   |       4 */    int a6 : 5;
               /*     16: 5   |       8 */    int64_t a7 : 3;
               /* XXX  7-byte padding   */

                                              /* total size (bytes):   24 */
                                            }

          Note how the offset information is now extended to also
          include the first bit of the bitfield.

‘ptype[/FLAGS] [ARG]’
     ‘ptype’ accepts the same arguments as ‘whatis’, but prints a
     detailed description of the type, instead of just the name of the
     type.  *Note Expressions: Expressions.

     Contrary to ‘whatis’, ‘ptype’ always unrolls any ‘typedef’s in its
     argument declaration, whether the argument is a variable,
     expression, or a data type.  This means that ‘ptype’ of a variable
     or an expression will not print literally its type as present in
     the source code--use ‘whatis’ for that.  ‘typedef’s at the pointer
     or reference targets are also unrolled.  Only ‘typedef’s of fields,
     methods and inner ‘class typedef’s of ‘struct’s, ‘class’es and
     ‘union’s are not unrolled even with ‘ptype’.

     For example, for this variable declaration:

          typedef double real_t;
          struct complex { real_t real; double imag; };
          typedef struct complex complex_t;
          complex_t var;
          real_t *real_pointer_var;

     the two commands give this output:

          (gdb) whatis var
          type = complex_t
          (gdb) ptype var
          type = struct complex {
              real_t real;
              double imag;
          }
          (gdb) whatis complex_t
          type = struct complex
          (gdb) whatis struct complex
          type = struct complex
          (gdb) ptype struct complex
          type = struct complex {
              real_t real;
              double imag;
          }
          (gdb) whatis real_pointer_var
          type = real_t *
          (gdb) ptype real_pointer_var
          type = double *

     As with ‘whatis’, using ‘ptype’ without an argument refers to the
     type of ‘$’, the last value in the value history.

     Sometimes, programs use opaque data types or incomplete
     specifications of complex data structure.  If the debug information
     included in the program does not allow GDB to display a full
     declaration of the data type, it will say ‘<incomplete type>’.  For
     example, given these declarations:

              struct foo;
              struct foo *fooptr;

     but no definition for ‘struct foo’ itself, GDB will say:

            (gdb) ptype foo
            $1 = <incomplete type>

     "Incomplete type" is C terminology for data types that are not
     completely specified.

     Othertimes, information about a variable's type is completely
     absent from the debug information included in the program.  This
     most often happens when the program or library where the variable
     is defined includes no debug information at all.  GDB knows the
     variable exists from inspecting the linker/loader symbol table
     (e.g., the ELF dynamic symbol table), but such symbols do not
     contain type information.  Inspecting the type of a (global)
     variable for which GDB has no type information shows:

            (gdb) ptype var
            type = <data variable, no debug info>

     *Note no debug info variables: Variables, for how to print the
     values of such variables.

‘info types [-q] [REGEXP]’
     Print a brief description of all types whose names match the
     regular expression REGEXP (or all types in your program, if you
     supply no argument).  Each complete typename is matched as though
     it were a complete line; thus, ‘i type value’ gives information on
     all types in your program whose names include the string ‘value’,
     but ‘i type ^value$’ gives information only on types whose complete
     name is ‘value’.

     In programs using different languages, GDB chooses the syntax to
     print the type description according to the ‘set language’ value:
     using ‘set language auto’ (see *note Set Language Automatically:
     Automatically.) means to use the language of the type, other values
     mean to use the manually specified language (see *note Set Language
     Manually: Manually.).

     This command differs from ‘ptype’ in two ways: first, like
     ‘whatis’, it does not print a detailed description; second, it
     lists all source files and line numbers where a type is defined.

     The output from ‘into types’ is proceeded with a header line
     describing what types are being listed.  The optional flag ‘-q’,
     which stands for ‘quiet’, disables printing this header
     information.

‘info type-printers’
     Versions of GDB that ship with Python scripting enabled may have
     "type printers" available.  When using ‘ptype’ or ‘whatis’, these
     printers are consulted when the name of a type is needed.  *Note
     Type Printing API::, for more information on writing type printers.

     ‘info type-printers’ displays all the available type printers.

‘enable type-printer NAME...’
‘disable type-printer NAME...’
     These commands can be used to enable or disable type printers.

‘info scope LOCSPEC’
     List all the variables local to the lexical scope of the code
     location that results from resolving LOCSPEC.  *Note Location
     Specifications::, for details about supported forms of LOCSPEC.
     For example:

          (gdb) info scope command_line_handler
          Scope for command_line_handler:
          Symbol rl is an argument at stack/frame offset 8, length 4.
          Symbol linebuffer is in static storage at address 0x150a18, length 4.
          Symbol linelength is in static storage at address 0x150a1c, length 4.
          Symbol p is a local variable in register $esi, length 4.
          Symbol p1 is a local variable in register $ebx, length 4.
          Symbol nline is a local variable in register $edx, length 4.
          Symbol repeat is a local variable at frame offset -8, length 4.

     This command is especially useful for determining what data to
     collect during a “trace experiment”, see *note collect: Tracepoint
     Actions.

‘info source’
     Show information about the current source file--that is, the source
     file for the function containing the current point of execution:
        • the name of the source file, and the directory containing it,
        • the directory it was compiled in,
        • its length, in lines,
        • which programming language it is written in,
        • if the debug information provides it, the program that
          compiled the file (which may include, e.g., the compiler
          version and command line arguments),
        • whether the executable includes debugging information for that
          file, and if so, what format the information is in (e.g.,
          Dwarf 2), and
        • whether the debugging information includes information about
          preprocessor macros.

‘info sources [-dirname | -basename] [--] [REGEXP]’

     With no options ‘info sources’ prints the names of all source files
     in your program for which there is debugging information.  The
     source files are presented based on a list of object files
     (executables and libraries) currently loaded into GDB.  For each
     object file all of the associated source files are listed.

     Each source file will only be printed once for each object file,
     but a single source file can be repeated in the output if it is
     part of multiple object files.

     If the optional REGEXP is provided, then only source files that
     match the regular expression will be printed.  The matching is
     case-sensitive, except on operating systems that have
     case-insensitive filesystem (e.g., MS-Windows).  ‘--’ can be used
     before REGEXP to prevent GDB interpreting REGEXP as a command
     option (e.g.  if REGEXP starts with ‘-’).

     By default, the REGEXP is used to match anywhere in the filename.
     If ‘-dirname’, only files having a dirname matching REGEXP are
     shown.  If ‘-basename’, only files having a basename matching
     REGEXP are shown.

     It is possible that an object file may be printed in the list with
     no associated source files.  This can happen when either no source
     files match REGEXP, or, the object file was compiled without debug
     information and so GDB is unable to find any source file names.

‘info functions [-q] [-n]’
     Print the names and data types of all defined functions.  Similarly
     to ‘info types’, this command groups its output by source files and
     annotates each function definition with its source line number.

     In programs using different languages, GDB chooses the syntax to
     print the function name and type according to the ‘set language’
     value: using ‘set language auto’ (see *note Set Language
     Automatically: Automatically.) means to use the language of the
     function, other values mean to use the manually specified language
     (see *note Set Language Manually: Manually.).

     The ‘-n’ flag excludes “non-debugging symbols” from the results.  A
     non-debugging symbol is a symbol that comes from the executable's
     symbol table, not from the debug information (for example, DWARF)
     associated with the executable.

     The optional flag ‘-q’, which stands for ‘quiet’, disables printing
     header information and messages explaining why no functions have
     been printed.

‘info functions [-q] [-n] [-t TYPE_REGEXP] [REGEXP]’
     Like ‘info functions’, but only print the names and data types of
     the functions selected with the provided regexp(s).

     If REGEXP is provided, print only the functions whose names match
     the regular expression REGEXP.  Thus, ‘info fun step’ finds all
     functions whose names include ‘step’; ‘info fun ^step’ finds those
     whose names start with ‘step’.  If a function name contains
     characters that conflict with the regular expression language (e.g.
     ‘operator*()’), they may be quoted with a backslash.

     If TYPE_REGEXP is provided, print only the functions whose types,
     as printed by the ‘whatis’ command, match the regular expression
     TYPE_REGEXP.  If TYPE_REGEXP contains space(s), it should be
     enclosed in quote characters.  If needed, use backslash to escape
     the meaning of special characters or quotes.  Thus, ‘info fun -t
     '^int ('’ finds the functions that return an integer; ‘info fun -t
     '(.*int.*'’ finds the functions that have an argument type
     containing int; ‘info fun -t '^int (' ^step’ finds the functions
     whose names start with ‘step’ and that return int.

     If both REGEXP and TYPE_REGEXP are provided, a function is printed
     only if its name matches REGEXP and its type matches TYPE_REGEXP.

‘info variables [-q] [-n]’
     Print the names and data types of all variables that are defined
     outside of functions (i.e. excluding local variables).  The printed
     variables are grouped by source files and annotated with their
     respective source line numbers.

     In programs using different languages, GDB chooses the syntax to
     print the variable name and type according to the ‘set language’
     value: using ‘set language auto’ (see *note Set Language
     Automatically: Automatically.) means to use the language of the
     variable, other values mean to use the manually specified language
     (see *note Set Language Manually: Manually.).

     The ‘-n’ flag excludes non-debugging symbols from the results.

     The optional flag ‘-q’, which stands for ‘quiet’, disables printing
     header information and messages explaining why no variables have
     been printed.

‘info variables [-q] [-n] [-t TYPE_REGEXP] [REGEXP]’
     Like ‘info variables’, but only print the variables selected with
     the provided regexp(s).

     If REGEXP is provided, print only the variables whose names match
     the regular expression REGEXP.

     If TYPE_REGEXP is provided, print only the variables whose types,
     as printed by the ‘whatis’ command, match the regular expression
     TYPE_REGEXP.  If TYPE_REGEXP contains space(s), it should be
     enclosed in quote characters.  If needed, use backslash to escape
     the meaning of special characters or quotes.

     If both REGEXP and TYPE_REGEXP are provided, an argument is printed
     only if its name matches REGEXP and its type matches TYPE_REGEXP.

‘info modules [-q] [REGEXP]’
     List all Fortran modules in the program, or all modules matching
     the optional regular expression REGEXP.

     The optional flag ‘-q’, which stands for ‘quiet’, disables printing
     header information and messages explaining why no modules have been
     printed.

‘info module functions [-q] [-m MODULE-REGEXP] [-t TYPE-REGEXP] [REGEXP]’
‘info module variables [-q] [-m MODULE-REGEXP] [-t TYPE-REGEXP] [REGEXP]’
     List all functions or variables within all Fortran modules.  The
     set of functions or variables listed can be limited by providing
     some or all of the optional regular expressions.  If MODULE-REGEXP
     is provided, then only Fortran modules matching MODULE-REGEXP will
     be searched.  Only functions or variables whose type matches the
     optional regular expression TYPE-REGEXP will be listed.  And only
     functions or variables whose name matches the optional regular
     expression REGEXP will be listed.

     The optional flag ‘-q’, which stands for ‘quiet’, disables printing
     header information and messages explaining why no functions or
     variables have been printed.

‘info main’
     Print the name of the starting function of the program.  This
     serves primarily Fortran programs, which have a user-supplied name
     for the main subroutine.

‘info classes’
‘info classes REGEXP’
     Display all Objective-C classes in your program, or (with the
     REGEXP argument) all those matching a particular regular
     expression.

‘info selectors’
‘info selectors REGEXP’
     Display all Objective-C selectors in your program, or (with the
     REGEXP argument) all those matching a particular regular
     expression.

‘set opaque-type-resolution on’
     Tell GDB to resolve opaque types.  An opaque type is a type
     declared as a pointer to a ‘struct’, ‘class’, or ‘union’--for
     example, ‘struct MyType *’--that is used in one source file
     although the full declaration of ‘struct MyType’ is in another
     source file.  The default is on.

     A change in the setting of this subcommand may not take effect
     until the next time symbols for a file are loaded.

‘set opaque-type-resolution off’
     Tell GDB not to resolve opaque types.  In this case, the type is
     printed as follows:
          {<no data fields>}

‘show opaque-type-resolution’
     Show whether opaque types are resolved or not.

‘set print symbol-loading’
‘set print symbol-loading full’
‘set print symbol-loading brief’
‘set print symbol-loading off’
     The ‘set print symbol-loading’ command allows you to control the
     printing of messages when GDB loads symbol information.  By default
     a message is printed for the executable and one for each shared
     library, and normally this is what you want.  However, when
     debugging apps with large numbers of shared libraries these
     messages can be annoying.  When set to ‘brief’ a message is printed
     for each executable, and when GDB loads a collection of shared
     libraries at once it will only print one message regardless of the
     number of shared libraries.  When set to ‘off’ no messages are
     printed.

‘show print symbol-loading’
     Show whether messages will be printed when a GDB command entered
     from the keyboard causes symbol information to be loaded.

‘maint print symbols [-pc ADDRESS] [FILENAME]’
‘maint print symbols [-objfile OBJFILE] [-source SOURCE] [--] [FILENAME]’
‘maint print msymbols [-objfile OBJFILE] [--] [FILENAME]’
     Write a dump of debugging symbol data into the file FILENAME or the
     terminal if FILENAME is unspecified.  If ‘-objfile OBJFILE’ is
     specified, only dump symbols for that objfile.  If ‘-pc ADDRESS’ is
     specified, only dump symbols for the file with code at that
     address.  Note that ADDRESS may be a symbol like ‘main’.  If
     ‘-source SOURCE’ is specified, only dump symbols for that source
     file.

     These commands are used to debug the GDB symbol-reading code.
     These commands do not modify internal GDB state, therefore ‘maint
     print symbols’ will only print symbols for already expanded symbol
     tables.  You can use the command ‘info sources’ to find out which
     files these are.  ‘maint print msymbols’ dumps "minimal symbols",
     e.g., "ELF symbols".

     *Note Commands to Specify Files: Files, for a discussion of how GDB
     reads symbols (in the description of ‘symbol-file’).

‘maint info symtabs [ REGEXP ]’

     List the ‘struct symtab’ structures whose names match REGEXP.  If
     REGEXP is not given, list them all.  The output includes
     expressions which you can copy into a GDB debugging this one to
     examine a particular structure in more detail.  For example:

          (gdb) maint info symtabs
          { objfile /home/gnu/build/binutils-gdb/gdb/a.out ((struct objfile *) 0x7d48d1c92b80)
            { ((struct compunit_symtab *) 0x7cd8d1be44b0)
              debugformat DWARF 5
              producer GNU C23 15.2.1 20260209 -mtune=generic -march=x86-64 -g3 -O0
              name test.c
              dirname /home/gnu/build/binutils-gdb/gdb
              blockvector ((struct blockvector *) 0x7c48d1c1efc0)
              user ((struct compunit_symtab *) (null))
                  { symtab test.c ((struct symtab *) 0x7df8d1d588a0)
                    fullname /home/gnu/build/binutils-gdb/gdb/test.c
                    linetable ((struct linetable *) 0x7df8d1d58cc0)
                  }
                  { symtab /usr/include/stdc-predef.h ((struct symtab *) 0x7df8d1d588e0)
                    fullname (void)
                    linetable ((struct linetable *) 0x0)
                  }
            }
          }

‘maint info line-table [ REGEXP ]’

     List the ‘struct linetable’ from all ‘struct symtab’ instances
     whose name matches REGEXP.  If REGEXP is not given, list the
     ‘struct linetable’ from all ‘struct symtab’.  For example:

          (gdb) maint info line-table
          objfile: /home/gnu/build/a.out ((struct objfile *) 0x6120000e0d40)
          compunit_symtab: simple.cpp ((struct compunit_symtab *) 0x6210000ff450)
          symtab: /home/gnu/src/simple.cpp ((struct symtab *) 0x6210000ff4d0)
          linetable: ((struct linetable *) 0x62100012b760):
          INDEX  LINE   ADDRESS            IS-STMT PROLOGUE-END EPILOGUE-BEGIN
          0      3      0x0000000000401110 Y
          1      4      0x0000000000401114 Y       Y            Y
          2      9      0x0000000000401120 Y
          3      10     0x0000000000401124 Y       Y
          4      10     0x0000000000401129 Y                    Y
          5      15     0x0000000000401130 Y
          6      16     0x0000000000401134 Y       Y
          7      16     0x0000000000401139
          8      21     0x0000000000401140 Y                    Y
          9      22     0x000000000040114f Y       Y
          10     22     0x0000000000401154                      Y
          11     END    0x000000000040115a Y
     The ‘IS-STMT’ column indicates if the address is a recommended
     breakpoint location to represent a line or a statement.  The
     ‘PROLOGUE-END’ column indicates that a given address is an adequate
     place to set a breakpoint at the first instruction following a
     function prologue.  The ‘EPILOGUE-BEGIN’ column indicates that a
     given address marks the point where a block's frame is destroyed,
     making local variables hard or impossible to find.

‘set always-read-ctf [on|off]’
‘show always-read-ctf’

     When off, CTF debug info is only read if DWARF debug info is not
     present.  When on, CTF debug info is read regardless of whether
     DWARF debug info is present.  The default value is off.

‘maint set symbol-cache-size SIZE’
     Set the size of the symbol cache to SIZE.  The default size is
     intended to be good enough for debugging most applications.  This
     option exists to allow for experimenting with different sizes.

‘maint show symbol-cache-size’
     Show the size of the symbol cache.

‘maint print symbol-cache’
     Print the contents of the symbol cache.  This is useful when
     debugging symbol cache issues.

‘maint print symbol-cache-statistics’
     Print symbol cache usage statistics.  This helps determine how well
     the cache is being utilized.

‘maint flush symbol-cache’
‘maint flush-symbol-cache’
     Flush the contents of the symbol cache, all entries are removed.
     This command is useful when debugging the symbol cache.  It is also
     useful when collecting performance data.  The command ‘maint
     flush-symbol-cache’ is deprecated in favor of ‘maint flush
     symbol-cache’..

‘maint set ignore-prologue-end-flag [on|off]’
     Enable or disable the use of the ‘PROLOGUE-END’ flag from the
     line-table.  When ‘off’ (the default), GDB uses the ‘PROLOGUE-END’
     flag to place breakpoints past the end of a function prologue.
     When ‘on’, GDB ignores the flag and relies on prologue analyzers to
     skip function prologues.

‘maint show ignore-prologue-end-flag’
     Show whether GDB will ignore the ‘PROLOGUE-END’ flag.


File: gdb.info,  Node: Altering,  Next: GDB Files,  Prev: Symbols,  Up: Top

17 Altering Execution
*********************

Once you think you have found an error in your program, you might want
to find out for certain whether correcting the apparent error would lead
to correct results in the rest of the run.  You can find the answer by
experiment, using the GDB features for altering execution of the
program.

   For example, you can store new values into variables or memory
locations, give your program a signal, restart it at a different
address, or even return prematurely from a function.

* Menu:

* Assignment::                  Assignment to variables
* Jumping::                     Continuing at a different address
* Signaling::                   Giving your program a signal
* Returning::                   Returning from a function
* Calling::                     Calling your program's functions
* Patching::                    Patching your program
* Compiling and Injecting Code:: Compiling and injecting code in GDB


File: gdb.info,  Node: Assignment,  Next: Jumping,  Up: Altering

17.1 Assignment to Variables
============================

To alter the value of a variable, evaluate an assignment expression.
*Note Expressions: Expressions.  For example,

     print x=4

stores the value 4 into the variable ‘x’, and then prints the value of
the assignment expression (which is 4).  *Note Using GDB with Different
Languages: Languages, for more information on operators in supported
languages.

   If you are not interested in seeing the value of the assignment, use
the ‘set’ command instead of the ‘print’ command.  ‘set’ is really the
same as ‘print’ except that the expression's value is not printed and is
not put in the value history (*note Value History: Value History.).  The
expression is evaluated only for its effects.

   If the beginning of the argument string of the ‘set’ command appears
identical to a ‘set’ subcommand, use the ‘set variable’ command instead
of just ‘set’.  This command is identical to ‘set’ except for its lack
of subcommands.  For example, if your program has a variable ‘width’,
you get an error if you try to set a new value with just ‘set width=13’,
because GDB has the command ‘set width’:

     (gdb) whatis width
     type = double
     (gdb) p width
     $4 = 13
     (gdb) set width=47
     Invalid syntax in expression.

The invalid expression, of course, is ‘=47’.  In order to actually set
the program's variable ‘width’, use

     (gdb) set var width=47

   Because the ‘set’ command has many subcommands that can conflict with
the names of program variables, it is a good idea to use the ‘set
variable’ command instead of just ‘set’.  For example, if your program
has a variable ‘g’, you run into problems if you try to set a new value
with just ‘set g=4’, because GDB has the command ‘set gnutarget’,
abbreviated ‘set g’:

     (gdb) whatis g
     type = double
     (gdb) p g
     $1 = 1
     (gdb) set g=4
     (gdb) p g
     $2 = 1
     (gdb) r
     The program being debugged has been started already.
     Start it from the beginning? (y or n) y
     Starting program: /home/smith/cc_progs/a.out
     "/home/smith/cc_progs/a.out": can't open to read symbols:
                                      Invalid bfd target.
     (gdb) show g
     The current BFD target is "=4".

The program variable ‘g’ did not change, and you silently set the
‘gnutarget’ to an invalid value.  In order to set the variable ‘g’, use

     (gdb) set var g=4

   GDB allows more implicit conversions in assignments than C; you can
freely store an integer value into a pointer variable or vice versa, and
you can convert any structure to any other structure that is the same
length or shorter.

   To store values into arbitrary places in memory, use the ‘{...}’
construct to generate a value of specified type at a specified address
(*note Expressions: Expressions.).  For example, ‘{int}0x83040’ refers
to memory location ‘0x83040’ as an integer (which implies a certain size
and representation in memory), and

     set {int}0x83040 = 4

stores the value 4 into that memory location.


File: gdb.info,  Node: Jumping,  Next: Signaling,  Prev: Assignment,  Up: Altering

17.2 Continuing at a Different Address
======================================

Ordinarily, when you continue your program, you do so at the place where
it stopped, with the ‘continue’ command.  You can instead continue at an
address of your own choosing, with the following commands:

‘jump LOCSPEC’
‘j LOCSPEC’
     Resume execution at the address of the code location that results
     from resolving LOCSPEC.  *Note Location Specifications::, for a
     description of the different forms of LOCSPEC.  If LOCSPEC resolves
     to more than one address, those outside the current compilation
     unit are ignored.  If considering just the addresses in the current
     compilation unit still doesn't yield a unique address, the command
     aborts before jumping.  Execution stops again immediately if there
     is a breakpoint there.  It is common practice to use the ‘tbreak’
     command in conjunction with ‘jump’.  *Note Setting Breakpoints: Set
     Breaks.

     The ‘jump’ command does not change the current stack frame, or the
     stack pointer, or the contents of any memory location or any
     register other than the program counter.  If LOCSPEC resolves to an
     address in a different function from the one currently executing,
     the results may be bizarre if the two functions expect different
     patterns of arguments or of local variables.  For this reason, the
     ‘jump’ command requests confirmation if the jump address is not in
     the function currently executing.  However, even bizarre results
     are predictable if you are well acquainted with the
     machine-language code of your program.

   On many systems, you can get much the same effect as the ‘jump’
command by storing a new value into the register ‘$pc’.  The difference
is that this does not start your program running; it only changes the
address of where it _will_ run when you continue.  For example,

     set $pc = 0x485

makes the next ‘continue’ command or stepping command execute at address
‘0x485’, rather than at the address where your program stopped.  *Note
Continuing and Stepping: Continuing and Stepping.

   However, writing directly to ‘$pc’ will only change the value of the
program-counter register, while using ‘jump’ will ensure that any
additional auxiliary state is also updated.  For example, on SPARC,
‘jump’ will update both ‘$pc’ and ‘$npc’ registers prior to resuming
execution.  When using the approach of writing directly to ‘$pc’ it is
your job to also update the ‘$npc’ register.

   The most common occasion to use the ‘jump’ command is to back
up--perhaps with more breakpoints set--over a portion of a program that
has already executed, in order to examine its execution in more detail.


File: gdb.info,  Node: Signaling,  Next: Returning,  Prev: Jumping,  Up: Altering

17.3 Giving your Program a Signal
=================================

‘signal SIGNAL’
     Resume execution where your program is stopped, but immediately
     give it the signal SIGNAL.  The SIGNAL can be the name or the
     number of a signal.  For example, on many systems ‘signal 2’ and
     ‘signal SIGINT’ are both ways of sending an interrupt signal.

     Alternatively, if SIGNAL is zero, continue execution without giving
     a signal.  This is useful when your program stopped on account of a
     signal and would ordinarily see the signal when resumed with the
     ‘continue’ command; ‘signal 0’ causes it to resume without a
     signal.

     _Note:_ When resuming a multi-threaded program, SIGNAL is delivered
     to the currently selected thread, not the thread that last reported
     a stop.  This includes the situation where a thread was stopped due
     to a signal.  So if you want to continue execution suppressing the
     signal that stopped a thread, you should select that same thread
     before issuing the ‘signal 0’ command.  If you issue the ‘signal 0’
     command with another thread as the selected one, GDB detects that
     and asks for confirmation.

     Invoking the ‘signal’ command is not the same as invoking the
     ‘kill’ utility from the shell.  Sending a signal with ‘kill’ causes
     GDB to decide what to do with the signal depending on the signal
     handling tables (*note Signals::).  The ‘signal’ command passes the
     signal directly to your program.

     ‘signal’ does not repeat when you press <RET> a second time after
     executing the command.

‘queue-signal SIGNAL’
     Queue SIGNAL to be delivered immediately to the current thread when
     execution of the thread resumes.  The SIGNAL can be the name or the
     number of a signal.  For example, on many systems ‘signal 2’ and
     ‘signal SIGINT’ are both ways of sending an interrupt signal.  The
     handling of the signal must be set to pass the signal to the
     program, otherwise GDB will report an error.  You can control the
     handling of signals from GDB with the ‘handle’ command (*note
     Signals::).

     Alternatively, if SIGNAL is zero, any currently queued signal for
     the current thread is discarded and when execution resumes no
     signal will be delivered.  This is useful when your program stopped
     on account of a signal and would ordinarily see the signal when
     resumed with the ‘continue’ command.

     This command differs from the ‘signal’ command in that the signal
     is just queued, execution is not resumed.  And ‘queue-signal’
     cannot be used to pass a signal whose handling state has been set
     to ‘nopass’ (*note Signals::).

   *Note stepping into signal handlers::, for information on how
stepping commands behave when the thread has a signal queued.


File: gdb.info,  Node: Returning,  Next: Calling,  Prev: Signaling,  Up: Altering

17.4 Returning from a Function
==============================

‘return’
‘return EXPRESSION’
     You can cancel execution of a function call with the ‘return’
     command.  If you give an EXPRESSION argument, its value is used as
     the function's return value.

   When you use ‘return’, GDB discards the selected stack frame (and all
frames within it).  You can think of this as making the discarded frame
return prematurely.  If you wish to specify a value to be returned, give
that value as the argument to ‘return’.

   This pops the selected stack frame (*note Selecting a Frame:
Selection.), and any other frames inside of it, leaving its caller as
the innermost remaining frame.  That frame becomes selected.  The
specified value is stored in the registers used for returning values of
functions.

   The ‘return’ command does not resume execution; it leaves the program
stopped in the state that would exist if the function had just returned.
In contrast, the ‘finish’ command (*note Continuing and Stepping:
Continuing and Stepping.) resumes execution until the selected stack
frame returns naturally.

   GDB needs to know how the EXPRESSION argument should be set for the
inferior.  The concrete registers assignment depends on the OS ABI and
the type being returned by the selected stack frame.  For example it is
common for OS ABI to return floating point values in FPU registers while
integer values in CPU registers.  Still some ABIs return even floating
point values in CPU registers.  Larger integer widths (such as ‘long
long int’) also have specific placement rules.  GDB already knows the OS
ABI from its current target so it needs to find out also the type being
returned to make the assignment into the right register(s).

   Normally, the selected stack frame has debug info.  GDB will always
use the debug info instead of the implicit type of EXPRESSION when the
debug info is available.  For example, if you type ‘return -1’, and the
function in the current stack frame is declared to return a ‘long long
int’, GDB transparently converts the implicit ‘int’ value of -1 into a
‘long long int’:

     Breakpoint 1, func () at gdb.base/return-nodebug.c:29
     29        return 31;
     (gdb) return -1
     Make func return now? (y or n) y
     #0  0x004004f6 in main () at gdb.base/return-nodebug.c:43
     43        printf ("result=%lld\n", func ());
     (gdb)

   However, if the selected stack frame does not have a debug info,
e.g., if the function was compiled without debug info, GDB has to find
out the type to return from user.  Specifying a different type by
mistake may set the value in different inferior registers than the
caller code expects.  For example, typing ‘return -1’ with its implicit
type ‘int’ would set only a part of a ‘long long int’ result for a debug
info less function (on 32-bit architectures).  Therefore the user is
required to specify the return type by an appropriate cast explicitly:

     Breakpoint 2, 0x0040050b in func ()
     (gdb) return -1
     Return value type not available for selected stack frame.
     Please use an explicit cast of the value to return.
     (gdb) return (long long int) -1
     Make selected stack frame return now? (y or n) y
     #0  0x00400526 in main ()
     (gdb)


File: gdb.info,  Node: Calling,  Next: Patching,  Prev: Returning,  Up: Altering

17.5 Calling Program Functions
==============================

‘print EXPR’
     Evaluate the expression EXPR and display the resulting value.  The
     expression may include calls to functions in the program being
     debugged.

‘call EXPR’
     Evaluate the expression EXPR without displaying ‘void’ returned
     values.

     You can use this variant of the ‘print’ command if you want to
     execute a function from your program that does not return anything
     (a.k.a. “a void function”), but without cluttering the output with
     ‘void’ returned values that GDB will otherwise print.  If the
     result is not void, it is printed and saved in the value history.

   It is possible for the function you call via the ‘print’ or ‘call’
command to generate a signal (e.g., if there's a bug in the function, or
if you passed it incorrect arguments).  What happens in that case is
controlled by the ‘set unwind-on-signal’ command.

   Similarly, with a C++ program it is possible for the function you
call via the ‘print’ or ‘call’ command to generate an exception that is
not handled due to the constraints of the dummy frame.  In this case,
any exception that is raised in the frame, but has an out-of-frame
exception handler will not be found.  GDB builds a dummy-frame for the
inferior function call, and the unwinder cannot seek for exception
handlers outside of this dummy-frame.  What happens in that case is
controlled by the ‘set unwind-on-terminating-exception’ command.

‘set unwind-on-signal’
     Set unwinding of the stack if a signal is received while in a
     function that GDB called in the program being debugged.  If set to
     on, GDB unwinds the stack it created for the call and restores the
     context to what it was before the call.  If set to off (the
     default), GDB stops in the frame where the signal was received.

     The command ‘set unwindonsignal’ is an alias for this command, and
     is maintained for backward compatibility.

‘show unwind-on-signal’
     Show the current setting of stack unwinding in the functions called
     by GDB.

     The command ‘show unwindonsignal’ is an alias for this command, and
     is maintained for backward compatibility.

‘set unwind-on-terminating-exception’
     Set unwinding of the stack if a C++ exception is raised, but left
     unhandled while in a function that GDB called in the program being
     debugged.  If set to on (the default), GDB unwinds the stack it
     created for the call and restores the context to what it was before
     the call.  If set to off, GDB the exception is delivered to the
     default C++ exception handler and the inferior terminated.

‘show unwind-on-terminating-exception’
     Show the current setting of stack unwinding in the functions called
     by GDB.

‘set unwind-on-timeout’
     Set unwinding of the stack if a function called from GDB times out.
     If set to ‘off’ (the default), GDB stops in the frame where the
     timeout occurred.  If set to ‘on’, GDB unwinds the stack it created
     for the call and restores the context to what it was before the
     call.

‘show unwind-on-timeout’
     Show whether GDB will unwind the stack if a function called from
     GDB times out.

‘set may-call-functions’
     Set permission to call functions in the program.  This controls
     whether GDB will attempt to call functions in the program, such as
     with expressions in the ‘print’ command.  It defaults to ‘on’.

     To call a function in the program, GDB has to temporarily modify
     the state of the inferior.  This has potentially undesired side
     effects.  Also, having GDB call nested functions is likely to be
     erroneous and may even crash the program being debugged.  You can
     avoid such hazards by forbidding GDB from calling functions in the
     program being debugged.  If calling functions in the program is
     forbidden, GDB will throw an error when a command (such as printing
     an expression) starts a function call in the program.

‘show may-call-functions’
     Show permission to call functions in the program.

   When calling a function within a program, it is possible that the
program could enter a state from which the called function may never
return.  If this happens then it is possible to interrupt the function
call by typing the interrupt character (often ‘Ctrl-c’).

   If a called function is interrupted for any reason, including hitting
a breakpoint, or triggering a watchpoint, and the stack is not unwound
due to ‘set unwind-on-terminating-exception on’, ‘set unwind-on-timeout
on’, or ‘set unwind-on-signal on’ (*note stack unwind settings::), then
the dummy-frame, created by GDB to facilitate the call to the program
function, will be visible in the backtrace, for example frame ‘#3’ in
the following backtrace:

     (gdb) backtrace
     #0  0x00007ffff7b3d1e7 in nanosleep () from /lib64/libc.so.6
     #1  0x00007ffff7b3d11e in sleep () from /lib64/libc.so.6
     #2  0x000000000040113f in deadlock () at test.cc:13
     #3  <function called from gdb>
     #4  breakpt () at test.cc:20
     #5  0x0000000000401151 in main () at test.cc:25

   At this point it is possible to examine the state of the inferior
just like any other stop.

   Depending on why the function was interrupted then it may be possible
to resume the inferior (using commands like ‘continue’, ‘step’, etc).
In this case, when the inferior finally returns to the dummy-frame, GDB
will once again halt the inferior.

   On targets that support asynchronous execution (*note Background
Execution::) GDB can place a timeout on any functions called from GDB.
If the timeout expires and the function call is still ongoing, then GDB
will interrupt the program.

   If a function called from GDB is interrupted by a timeout, then by
default the inferior is left in the frame where the timeout occurred,
this behavior can be adjusted with ‘set unwind-on-timeout’ (*note set
unwind-on-timeout::).

   For targets that don't support asynchronous execution (*note
Background Execution::) then timeouts for functions called from GDB are
not supported, the timeout settings described below will be treated as
‘unlimited’, meaning GDB will wait indefinitely for function call to
complete, unless interrupted by the user using ‘Ctrl-C’.

‘set direct-call-timeout SECONDS’
     Set the timeout used when calling functions in the program to
     SECONDS, which should be an integer greater than zero, or the
     special value ‘unlimited’, which indicates no timeout should be
     used.  The default for this setting is ‘unlimited’.

     This setting is used when the user calls a function directly from
     the command prompt, for example with a ‘call’ or ‘print’ command.

     This setting only works for targets that support asynchronous
     execution (*note Background Execution::), for any other target the
     setting is treated as ‘unlimited’.

‘show direct-call-timeout’
     Show the timeout used when calling functions in the program with a
     ‘call’ or ‘print’ command.

   It is also possible to call functions within the program from the
condition of a conditional breakpoint (*note Break Conditions:
Conditions.).  A different setting controls the timeout used for
function calls made from a breakpoint condition.

‘set indirect-call-timeout SECONDS’
     Set the timeout used when calling functions in the program from a
     breakpoint or watchpoint condition to SECONDS, which should be an
     integer greater than zero, or the special value ‘unlimited’, which
     indicates no timeout should be used.  The default for this setting
     is ‘30’ seconds.

     This setting only works for targets that support asynchronous
     execution (*note Background Execution::), for any other target the
     setting is treated as ‘unlimited’.

     If a function called from a breakpoint or watchpoint condition
     times out, then GDB will stop at the point where the timeout
     occurred.  The breakpoint condition evaluation will be abandoned.

‘show indirect-call-timeout’
     Show the timeout used when calling functions in the program from a
     breakpoint or watchpoint condition.

17.5.1 Calling functions with no debug info
-------------------------------------------

Sometimes, a function you wish to call is missing debug information.  In
such case, GDB does not know the type of the function, including the
types of the function's parameters.  To avoid calling the inferior
function incorrectly, which could result in the called function
functioning erroneously and even crash, GDB refuses to call the function
unless you tell it the type of the function.

   For prototyped (i.e. ANSI/ISO style) functions, there are two ways to
do that.  The simplest is to cast the call to the function's declared
return type.  For example:

     (gdb) p getenv ("PATH")
     'getenv' has unknown return type; cast the call to its declared return type
     (gdb) p (char *) getenv ("PATH")
     $1 = 0x7fffffffe7ba "/usr/local/bin:/"...

   Casting the return type of a no-debug function is equivalent to
casting the function to a pointer to a prototyped function that has a
prototype that matches the types of the passed-in arguments, and calling
that.  I.e., the call above is equivalent to:

     (gdb) p ((char * (*) (const char *)) getenv) ("PATH")

and given this prototyped C or C++ function with float parameters:

     float multiply (float v1, float v2) { return v1 * v2; }

these calls are equivalent:

     (gdb) p (float) multiply (2.0f, 3.0f)
     (gdb) p ((float (*) (float, float)) multiply) (2.0f, 3.0f)

   If the function you wish to call is declared as unprototyped (i.e.
old K&R style), you must use the cast-to-function-pointer syntax, so
that GDB knows that it needs to apply default argument promotions
(promote float arguments to double).  *Note float promotion: ABI. For
example, given this unprototyped C function with float parameters, and
no debug info:

     float
     multiply_noproto (v1, v2)
       float v1, v2;
     {
       return v1 * v2;
     }

you call it like this:

       (gdb) p ((float (*) ()) multiply_noproto) (2.0f, 3.0f)


File: gdb.info,  Node: Patching,  Next: Compiling and Injecting Code,  Prev: Calling,  Up: Altering

17.6 Patching Programs
======================

By default, GDB opens the file containing your program's executable code
(or the corefile) read-only.  This prevents accidental alterations to
machine code; but it also prevents you from intentionally patching your
program's binary.

   If you'd like to be able to patch the binary, you can specify that
explicitly with the ‘set write’ command.  For example, you might want to
turn on internal debugging flags, or even to make emergency repairs.

‘set write on’
‘set write off’
     If you specify ‘set write on’, GDB opens executable and core files
     for both reading and writing; if you specify ‘set write off’ (the
     default), GDB opens them read-only.

     If you have already loaded a file, you must load it again (using
     the ‘exec-file’ or ‘core-file’ command) after changing ‘set write’,
     for your new setting to take effect.

‘show write’
     Display whether executable files and core files are opened for
     writing as well as reading.


File: gdb.info,  Node: Compiling and Injecting Code,  Prev: Patching,  Up: Altering

17.7 Compiling and injecting code in GDB
========================================

GDB supports on-demand compilation and code injection into programs
running under GDB.  GCC 5.0 or higher built with ‘libcc1.so’ must be
installed for this functionality to be enabled.  This functionality is
implemented with the following commands.

‘compile code SOURCE-CODE’
‘compile code -raw -- SOURCE-CODE’
     Compile SOURCE-CODE with the compiler language found as the current
     language in GDB (*note Languages::).  If compilation and injection
     is not supported with the current language specified in GDB, or the
     compiler does not support this feature, an error message will be
     printed.  If SOURCE-CODE compiles and links successfully, GDB will
     load the object-code emitted, and execute it within the context of
     the currently selected inferior.  It is important to note that the
     compiled code is executed immediately.  After execution, the
     compiled code is removed from GDB and any new types or variables
     you have defined will be deleted.

     The command allows you to specify SOURCE-CODE in two ways.  The
     simplest method is to provide a single line of code to the command.
     E.g.:

          compile code printf ("hello world\n");

     If you specify options on the command line as well as source code,
     they may conflict.  The ‘--’ delimiter can be used to separate
     options from actual source code.  E.g.:

          compile code -r -- printf ("hello world\n");

     Alternatively you can enter source code as multiple lines of text.
     To enter this mode, invoke the ‘compile code’ command without any
     text following the command.  This will start the multiple-line
     editor and allow you to type as many lines of source code as
     required.  When you have completed typing, enter ‘end’ on its own
     line to exit the editor.

          compile code
          >printf ("hello\n");
          >printf ("world\n");
          >end

     Specifying ‘-raw’, prohibits GDB from wrapping the provided
     SOURCE-CODE in a callable scope.  In this case, you must specify
     the entry point of the code by defining a function named
     ‘_gdb_expr_’.  The ‘-raw’ code cannot access variables of the
     inferior.  Using ‘-raw’ option may be needed for example when
     SOURCE-CODE requires ‘#include’ lines which may conflict with
     inferior symbols otherwise.

‘compile file FILENAME’
‘compile file -raw FILENAME’
     Like ‘compile code’, but take the source code from FILENAME.

          compile file /home/user/example.c

     The FILENAME argument supports escaping and quoting, see *note
     Filenames As Command Arguments: Filename Arguments.

‘compile print [[OPTIONS] --] EXPR’
‘compile print [[OPTIONS] --] /F EXPR’
     Compile and execute EXPR with the compiler language found as the
     current language in GDB (*note Languages::).  By default the value
     of EXPR is printed in a format appropriate to its data type; you
     can choose a different format by specifying ‘/F’, where F is a
     letter specifying the format; see *note Output Formats: Output
     Formats.  The ‘compile print’ command accepts the same options as
     the ‘print’ command; see *note print options::.

‘compile print [[OPTIONS] --]’
‘compile print [[OPTIONS] --] /F’
     Alternatively you can enter the expression (source code producing
     it) as multiple lines of text.  To enter this mode, invoke the
     ‘compile print’ command without any text following the command.
     This will start the multiple-line editor.

The process of compiling and injecting the code can be inspected using:

‘set debug compile’
     Turns on or off display of GDB process of compiling and injecting
     the code.  The default is off.

‘show debug compile’
     Displays the current state of displaying GDB process of compiling
     and injecting the code.

‘set debug compile-cplus-types’
     Turns on or off the display of C++ type conversion debugging
     information.  The default is off.

‘show debug compile-cplus-types’
     Displays the current state of displaying debugging information for
     C++ type conversion.

17.7.1 Compilation options for the ‘compile’ command
----------------------------------------------------

GDB needs to specify the right compilation options for the code to be
injected, in part to make its ABI compatible with the inferior and in
part to make the injected code compatible with GDB's injecting process.

The options used, in increasing precedence:

target architecture and OS options (‘gdbarch’)
     These options depend on target processor type and target operating
     system, usually they specify at least 32-bit (‘-m32’) or 64-bit
     (‘-m64’) compilation option.

compilation options recorded in the target
     GCC (since version 4.7) stores the options used for compilation
     into ‘DW_AT_producer’ part of DWARF debugging information according
     to the GCC option ‘-grecord-gcc-switches’.  One has to explicitly
     specify ‘-g’ during inferior compilation otherwise GCC produces no
     DWARF. This feature is only relevant for platforms where ‘-g’
     produces DWARF by default, otherwise one may try to enforce DWARF
     by using ‘-gdwarf-4’.

compilation options set by ‘set compile-args’

You can override compilation options using the following command:

‘set compile-args’
     Set compilation options used for compiling and injecting code with
     the ‘compile’ commands.  These options override any conflicting
     ones from the target architecture and/or options stored during
     inferior compilation.

‘show compile-args’
     Displays the current state of compilation options override.  This
     does not show all the options actually used during compilation, use
     *note set debug compile:: for that.

17.7.2 Caveats when using the ‘compile’ command
-----------------------------------------------

There are a few caveats to keep in mind when using the ‘compile’
command.  As the caveats are different per language, the table below
highlights specific issues on a per language basis.

C code examples and caveats
     When the language in GDB is set to ‘C’, the compiler will attempt
     to compile the source code with a ‘C’ compiler.  The source code
     provided to the ‘compile’ command will have much the same access to
     variables and types as it normally would if it were part of the
     program currently being debugged in GDB.

     Below is a sample program that forms the basis of the examples that
     follow.  This program has been compiled and loaded into GDB, much
     like any other normal debugging session.

          void function1 (void)
          {
             int i = 42;
             printf ("function 1\n");
          }

          void function2 (void)
          {
             int j = 12;
             function1 ();
          }

          int main(void)
          {
             int k = 6;
             int *p;
             function2 ();
             return 0;
          }

     For the purposes of the examples in this section, the program above
     has been compiled, loaded into GDB, stopped at the function ‘main’,
     and GDB is awaiting input from the user.

     To access variables and types for any program in GDB, the program
     must be compiled and packaged with debug information.  The
     ‘compile’ command is not an exception to this rule.  Without debug
     information, you can still use the ‘compile’ command, but you will
     be very limited in what variables and types you can access.

     So with that in mind, the example above has been compiled with
     debug information enabled.  The ‘compile’ command will have access
     to all variables and types (except those that may have been
     optimized out).  Currently, as GDB has stopped the program in the
     ‘main’ function, the ‘compile’ command would have access to the
     variable ‘k’.  You could invoke the ‘compile’ command and type some
     source code to set the value of ‘k’.  You can also read it, or do
     anything with that variable you would normally do in ‘C’.  Be aware
     that changes to inferior variables in the ‘compile’ command are
     persistent.  In the following example:

          compile code k = 3;

     the variable ‘k’ is now 3.  It will retain that value until
     something else in the example program changes it, or another
     ‘compile’ command changes it.

     Normal scope and access rules apply to source code compiled and
     injected by the ‘compile’ command.  In the example, the variables
     ‘j’ and ‘k’ are not accessible yet, because the program is
     currently stopped in the ‘main’ function, where these variables are
     not in scope.  Therefore, the following command

          compile code j = 3;

     will result in a compilation error message.

     Once the program is continued, execution will bring these variables
     in scope, and they will become accessible; then the code you
     specify via the ‘compile’ command will be able to access them.

     You can create variables and types with the ‘compile’ command as
     part of your source code.  Variables and types that are created as
     part of the ‘compile’ command are not visible to the rest of the
     program for the duration of its run.  This example is valid:

          compile code int ff = 5; printf ("ff is %d\n", ff);

     However, if you were to type the following into GDB after that
     command has completed:

          compile code printf ("ff is %d\n'', ff);

     a compiler error would be raised as the variable ‘ff’ no longer
     exists.  Object code generated and injected by the ‘compile’
     command is removed when its execution ends.  Caution is advised
     when assigning to program variables values of variables created by
     the code submitted to the ‘compile’ command.  This example is
     valid:

          compile code int ff = 5; k = ff;

     The value of the variable ‘ff’ is assigned to ‘k’.  The variable
     ‘k’ does not require the existence of ‘ff’ to maintain the value it
     has been assigned.  However, pointers require particular care in
     assignment.  If the source code compiled with the ‘compile’ command
     changed the address of a pointer in the example program, perhaps to
     a variable created in the ‘compile’ command, that pointer would
     point to an invalid location when the command exits.  The following
     example would likely cause issues with your debugged program:

          compile code int ff = 5; p = &ff;

     In this example, ‘p’ would point to ‘ff’ when the ‘compile’ command
     is executing the source code provided to it.  However, as variables
     in the (example) program persist with their assigned values, the
     variable ‘p’ would point to an invalid location when the command
     exists.  A general rule should be followed in that you should
     either assign ‘NULL’ to any assigned pointers, or restore a valid
     location to the pointer before the command exits.

     Similar caution must be exercised with any structs, unions, and
     typedefs defined in ‘compile’ command.  Types defined in the
     ‘compile’ command will no longer be available in the next ‘compile’
     command.  Therefore, if you cast a variable to a type defined in
     the ‘compile’ command, care must be taken to ensure that any future
     need to resolve the type can be achieved.

          (gdb) compile code static struct a { int a; } v = { 42 }; argv = &v;
          (gdb) compile code printf ("%d\n", ((struct a *) argv)->a);
          gdb command line:1:36: error: dereferencing pointer to incomplete type ‘struct a’
          Compilation failed.
          (gdb) compile code struct a { int a; }; printf ("%d\n", ((struct a *) argv)->a);
          42

     Variables that have been optimized away by the compiler are not
     accessible to the code submitted to the ‘compile’ command.  Access
     to those variables will generate a compiler error which GDB will
     print to the console.

17.7.3 Compiler search for the ‘compile’ command
------------------------------------------------

GDB needs to find GCC for the inferior being debugged which may not be
obvious for remote targets of different architecture than where GDB is
running.  Environment variable ‘PATH’ on GDB host is searched for GCC
binary matching the target architecture and operating system.  This
search can be overridden by ‘set compile-gcc’ GDB command below.  ‘PATH’
is taken from shell that executed GDB, it is not the value set by GDB
command ‘set environment’).  *Note Environment::.

   Specifically ‘PATH’ is searched for binaries matching regular
expression ‘ARCH(-[^-]*)?-OS-gcc’ according to the inferior target being
debugged.  ARCH is processor name -- multiarch is supported, so for
example both ‘i386’ and ‘x86_64’ targets look for pattern
‘(x86_64|i.86)’ and both ‘s390’ and ‘s390x’ targets look for pattern
‘s390x?’.  OS is currently supported only for pattern ‘linux(-gnu)?’.

   On Posix hosts the compiler driver GDB needs to find also shared
library ‘libcc1.so’ from the compiler.  It is searched in default shared
library search path (overridable with usual environment variable
‘LD_LIBRARY_PATH’), unrelated to ‘PATH’ or ‘set compile-gcc’ settings.
Contrary to it ‘libcc1plugin.so’ is found according to the installation
of the found compiler -- as possibly specified by the ‘set compile-gcc’
command.

‘set compile-gcc’
     Set compilation command used for compiling and injecting code with
     the ‘compile’ commands.  If this option is not set (it is set to an
     empty string), the search described above will occur -- that is the
     default.

‘show compile-gcc’
     Displays the current compile command GCC driver filename.  If set,
     it is the main command ‘gcc’, found usually for example under name
     ‘x86_64-linux-gnu-gcc’.


File: gdb.info,  Node: GDB Files,  Next: Targets,  Prev: Altering,  Up: Top

18 GDB Files
************

GDB needs to know the file name of the program to be debugged, both in
order to read its symbol table and in order to start your program.  To
debug a core dump of a previous run, you must also tell GDB the name of
the core dump file.

* Menu:

* Files::                       Commands to specify files
* File Caching::                Information about GDB's file caching
* Separate Debug Files::        Debugging information in separate files
* MiniDebugInfo::               Debugging information in a special section
* Index Files::                 Index files speed up GDB
* Debug Names::                 Extensions to .debug_names
* Symbol Errors::               Errors reading symbol files
* Data Files::                  GDB data files


File: gdb.info,  Node: Files,  Next: File Caching,  Up: GDB Files

18.1 Commands to Specify Files
==============================

You may want to specify executable and core dump file names.  The usual
way to do this is at start-up time, using the arguments to GDB's
start-up commands (*note Getting In and Out of GDB: Invocation.).

   Occasionally it is necessary to change to a different file during a
GDB session.  Or you may run GDB and forget to specify a file you want
to use.  Or you are debugging a remote target via ‘gdbserver’ (*note
file: Server.).  In these situations the GDB commands to specify new
files are useful.

‘file FILENAME’
     Use FILENAME as the program to be debugged.  It is read for its
     symbols and for the contents of pure memory.  It is also the
     program executed when you use the ‘run’ command.  If you do not
     specify a directory and the file is not found in the GDB working
     directory, GDB uses the environment variable ‘PATH’ as a list of
     directories to search, just as the shell does when looking for a
     program to run.  You can change the value of this variable, for
     both GDB and your program, using the ‘path’ command.

     The FILENAME argument supports escaping and quoting, see *note
     Filenames As Command Arguments: Filename Arguments.

     You can load unlinked object ‘.o’ files into GDB using the ‘file’
     command.  You will not be able to "run" an object file, but you can
     disassemble functions and inspect variables.  Also, if the
     underlying BFD functionality supports it, you could use ‘gdb
     -write’ to patch object files using this technique.  Note that GDB
     can neither interpret nor modify relocations in this case, so
     branches and some initialized variables will appear to go to the
     wrong place.  But this feature is still handy from time to time.

‘file’
     ‘file’ with no argument makes GDB discard any information it has on
     both executable file and the symbol table.

‘exec-file [ FILENAME ]’
     Specify that the program to be run (but not the symbol table) is
     found in FILENAME.  GDB searches the environment variable ‘PATH’ if
     necessary to locate your program.  Omitting FILENAME means to
     discard information on the executable file.

     The FILENAME argument supports escaping and quoting, see *note
     Filenames As Command Arguments: Filename Arguments.

‘symbol-file [ FILENAME [ -o OFFSET ]]’
     Read symbol table information from file FILENAME.  ‘PATH’ is
     searched when necessary.  Use the ‘file’ command to get both symbol
     table and program to run from the same file.

     If an optional OFFSET is specified, it is added to the start
     address of each section in the symbol file.  This is useful if the
     program is relocated at runtime, such as the Linux kernel with
     kASLR enabled.

     ‘symbol-file’ with no argument clears out GDB information on your
     program's symbol table.

     The ‘symbol-file’ command causes GDB to forget the contents of some
     breakpoints and auto-display expressions.  This is because they may
     contain pointers to the internal data recording symbols and data
     types, which are part of the old symbol table data being discarded
     inside GDB.

     ‘symbol-file’ does not repeat if you press <RET> again after
     executing it once.

     The FILENAME argument supports escaping and quoting, see *note
     Filenames As Command Arguments: Filename Arguments.

     When GDB is configured for a particular environment, it understands
     debugging information in whatever format is the standard generated
     for that environment; you may use either a GNU compiler, or other
     compilers that adhere to the local conventions.  Best results are
     usually obtained from GNU compilers; for example, using ‘GCC’ you
     can generate debugging information for optimized code.

     For most kinds of object files, with the exception of old SVR3
     systems using COFF, the ‘symbol-file’ command does not normally
     read the symbol table in full right away.  Instead, it scans the
     symbol table quickly to find which source files and which symbols
     are present.  The details are read later, one source file at a
     time, as they are needed.

     The purpose of this two-stage reading strategy is to make GDB start
     up faster.  For the most part, it is invisible except for
     occasional pauses while the symbol table details for a particular
     source file are being read.  (The ‘set verbose’ command can turn
     these pauses into messages if desired.  *Note Optional Warnings and
     Messages: Messages/Warnings.)

     We have not implemented the two-stage strategy for COFF yet.  When
     the symbol table is stored in COFF format, ‘symbol-file’ reads the
     symbol table data in full right away.

‘symbol-file [ -readnow ] FILENAME’
‘file [ -readnow ] FILENAME’
     You can override the GDB two-stage strategy for reading symbol
     tables by using the ‘-readnow’ option with any of the commands that
     load symbol table information, if you want to be sure GDB has the
     entire symbol table available.

‘symbol-file [ -readnever ] FILENAME’
‘file [ -readnever ] FILENAME’
     You can instruct GDB to never read the symbolic information
     contained in FILENAME by using the ‘-readnever’ option.  *Note
     --readnever::.

‘core-file [FILENAME]’
‘core’
     Specify the whereabouts of a core dump file to be used as the
     "contents of memory".  Traditionally, core files contain only some
     parts of the address space of the process that generated them; GDB
     can access the executable file itself for other parts.

     ‘core-file’ with no argument specifies that no core file is to be
     used.

     Note that the core file is ignored when your program is actually
     running under GDB.  So, if you have been running your program and
     you wish to debug a core file instead, you must kill the subprocess
     in which the program is running.  To do this, use the ‘kill’
     command (*note Killing the Child Process: Kill Process.).

     To see which core files are loaded into which inferiors, use the
     ‘info inferiors’ command (*note info inferiors command::).

‘add-symbol-file FILENAME [ -readnow | -readnever ] [ -o OFFSET ] [ TEXTADDRESS ] [ -s SECTION ADDRESS ... ]’
     The ‘add-symbol-file’ command reads additional symbol table
     information from the file FILENAME.  You would use this command
     when FILENAME has been dynamically loaded (by some other means)
     into the program that is running.  The TEXTADDRESS parameter gives
     the memory address at which the file's text section has been
     loaded.  You can additionally specify the base address of other
     sections using an arbitrary number of ‘-s SECTION ADDRESS’ pairs.
     If a section is omitted, GDB will use its default addresses as
     found in FILENAME.  Any ADDRESS or TEXTADDRESS can be given as an
     expression.

     If an optional OFFSET is specified, it is added to the start
     address of each section, except those for which the address was
     specified explicitly.

     The symbol table of the file FILENAME is added to the symbol table
     originally read with the ‘symbol-file’ command.  You can use the
     ‘add-symbol-file’ command any number of times; the new symbol data
     thus read is kept in addition to the old.

     The FILENAME argument supports escaping and quoting, see *note
     Filenames As Command Arguments: Filename Arguments.

     Changes can be reverted using the command ‘remove-symbol-file’.

     Although FILENAME is typically a shared library file, an executable
     file, or some other object file which has been fully relocated for
     loading into a process, you can also load symbolic information from
     relocatable ‘.o’ files, as long as:

        • the file's symbolic information refers only to linker symbols
          defined in that file, not to symbols defined by other object
          files,
        • every section the file's symbolic information refers to has
          actually been loaded into the inferior, as it appears in the
          file, and
        • you can determine the address at which every section was
          loaded, and provide these to the ‘add-symbol-file’ command.

     Some embedded operating systems, like Sun Chorus and VxWorks, can
     load relocatable files into an already running program; such
     systems typically make the requirements above easy to meet.
     However, it's important to recognize that many native systems use
     complex link procedures (‘.linkonce’ section factoring and C++
     constructor table assembly, for example) that make the requirements
     difficult to meet.  In general, one cannot assume that using
     ‘add-symbol-file’ to read a relocatable object file's symbolic
     information will have the same effect as linking the relocatable
     object file into the program in the normal way.

     ‘add-symbol-file’ does not repeat if you press <RET> after using
     it.

‘remove-symbol-file FILENAME’
‘remove-symbol-file -a ADDRESS’
     Remove a symbol file added via the ‘add-symbol-file’ command.  The
     file to remove can be identified by its FILENAME or by an ADDRESS
     that lies within the boundaries of this symbol file in memory.
     Example:

          (gdb) add-symbol-file /home/user/gdb/mylib.so 0x7ffff7ff9480
          add symbol table from file "/home/user/gdb/mylib.so" at
              .text_addr = 0x7ffff7ff9480
          (y or n) y
          Reading symbols from /home/user/gdb/mylib.so...
          (gdb) remove-symbol-file -a 0x7ffff7ff9480
          Remove symbol table from file "/home/user/gdb/mylib.so"? (y or n) y
          (gdb)

     The ADDRESS can be any expression which evaluates to an address.

     ‘remove-symbol-file’ does not repeat if you press <RET> after using
     it.

     The FILENAME argument supports escaping and quoting, see *note
     Filenames As Command Arguments: Filename Arguments.

‘add-symbol-file-from-memory ADDRESS’
     Load symbols from the given ADDRESS in a dynamically loaded object
     file whose image is mapped directly into the inferior's memory.
     For example, the Linux kernel maps a ‘syscall DSO’ into each
     process's address space; this DSO provides kernel-specific code for
     some system calls.  The argument can be any expression whose
     evaluation yields the address of the file's shared object file
     header.  For this command to work, you must have used ‘symbol-file’
     or ‘exec-file’ commands in advance.

‘section SECTION ADDR’
     The ‘section’ command changes the base address of the named SECTION
     of the exec file to ADDR.  This can be used if the exec file does
     not contain section addresses, (such as in the ‘a.out’ format), or
     when the addresses specified in the file itself are wrong.  Each
     section must be changed separately.  The ‘info files’ command,
     described below, lists all the sections and their addresses.

‘info files’
‘info target’
     ‘info files’ and ‘info target’ are synonymous; both print the
     current target (*note Specifying a Debugging Target: Targets.),
     including the names of the executable and core dump files currently
     in use by GDB, and the files from which symbols were loaded.  The
     command ‘help target’ lists all possible targets rather than
     current ones.

‘maint info sections [-all-objects] [FILTER-LIST]’
     Another command that can give you extra information about program
     sections is ‘maint info sections’.  In addition to the section
     information displayed by ‘info files’, this command displays the
     flags and file offset of each section in the executable and core
     dump files.

     When ‘-all-objects’ is passed then sections from all loaded object
     files, including shared libraries, are printed.

     The optional FILTER-LIST is a space separated list of filter
     keywords.  Sections that match any one of the filter criteria will
     be printed.  There are two types of filter:

     ‘SECTION-NAME’
          Display information about any section named SECTION-NAME.
     ‘SECTION-FLAG’
          Display information for any section with SECTION-FLAG.  The
          section flags that GDB currently knows about are:
          ‘ALLOC’
               Section will have space allocated in the process when
               loaded.  Set for all sections except those containing
               debug information.
          ‘LOAD’
               Section will be loaded from the file into the child
               process memory.  Set for pre-initialized code and data,
               clear for ‘.bss’ sections.
          ‘RELOC’
               Section needs to be relocated before loading.
          ‘READONLY’
               Section cannot be modified by the child process.
          ‘CODE’
               Section contains executable code only.
          ‘DATA’
               Section contains data only (no executable code).
          ‘ROM’
               Section will reside in ROM.
          ‘CONSTRUCTOR’
               Section contains data for constructor/destructor lists.
          ‘HAS_CONTENTS’
               Section is not empty.
          ‘NEVER_LOAD’
               An instruction to the linker to not output the section.
          ‘COFF_SHARED_LIBRARY’
               A notification to the linker that the section contains
               COFF shared library information.
          ‘IS_COMMON’
               Section contains common symbols.

‘maint info target-sections’
     This command prints GDB's internal section table.  For each target
     GDB maintains a table containing the allocatable sections from all
     currently mapped objects, along with information about where the
     section is mapped.

‘set trust-readonly-sections on’
     Tell GDB that readonly sections in your object file really are
     read-only (i.e. that their contents will not change).  In that
     case, GDB can fetch values from these sections out of the object
     file, rather than from the target program.  For some targets
     (notably embedded ones), this can be a significant enhancement to
     debugging performance.

     The default is off.

‘set trust-readonly-sections off’
     Tell GDB not to trust readonly sections.  This means that the
     contents of the section might change while the program is running,
     and must therefore be fetched from the target when needed.

‘show trust-readonly-sections’
     Show the current setting of trusting readonly sections.

   All file-specifying commands allow both absolute and relative file
names as arguments.  GDB always converts the file name to an absolute
file name and remembers it that way.

   GDB supports GNU/Linux, MS-Windows, SunOS, Darwin/Mach-O, SVr4, IBM
RS/6000 AIX, FDPIC (FR-V), and DSBT (TIC6X) shared libraries.

   On MS-Windows GDB must be linked with the Expat library to support
shared libraries.  *Note Expat::.

   GDB automatically loads symbol definitions from shared libraries when
you use the ‘run’ command, or when you examine a core file.  (Before you
issue the ‘run’ command, GDB does not understand references to a
function in a shared library, however--unless you are debugging a core
file).

   There are times, however, when you may wish to not automatically load
symbol definitions from shared libraries, such as when they are
particularly large or there are many of them.

   To control the automatic loading of shared library symbols, use the
commands:

‘set auto-solib-add MODE’
     If MODE is ‘on’, symbols from all shared object libraries will be
     loaded automatically when the inferior begins execution, you attach
     to an independently started inferior, or when the dynamic linker
     informs GDB that a new library has been loaded.  If MODE is ‘off’,
     symbols must be loaded manually, using the ‘sharedlibrary’ command.
     The default value is ‘on’.

     If your program uses lots of shared libraries with debug info that
     takes large amounts of memory, you can decrease the GDB memory
     footprint by preventing it from automatically loading the symbols
     from shared libraries.  To that end, type ‘set auto-solib-add off’
     before running the inferior, then load each library whose debug
     symbols you do need with ‘sharedlibrary REGEXP’, where REGEXP is a
     regular expression that matches the libraries whose symbols you
     want to be loaded.

‘show auto-solib-add’
     Display the current autoloading mode.

   To explicitly load shared library symbols, use the ‘sharedlibrary’
command:

‘info share REGEX’
‘info sharedlibrary REGEX’
     Print the names of the shared libraries which are currently loaded
     that match REGEX.  If REGEX is omitted then print all shared
     libraries that are loaded.

     For each library, GDB also lists the address range allocated to
     that library if it can be determined.  If the address range cannot
     be determined then the address range for the ‘.text’ section from
     the library will be listed.  If the ‘.text’ section cannot be found
     then no addresses will be listed.

     On systems that support linker namespaces, the output includes an
     additional column ‘NS’ if the inferior has more than one active
     namespace when the command is used.  This column the linker
     namespace that the shared library was loaded into.

     “Linker namespaces” are a feature of some standard libraries, that
     allow shared objects to be loaded in isolated environment,
     eliminating the possibility that those objects may cross-talk.
     Each set of isolated shared objects is said to belong to a
     "namespace", and linker related actions such as relocations do not
     cross namespace boundaries.

‘info dll REGEX’
     This is an alias of ‘info sharedlibrary’.

‘sharedlibrary REGEX’
‘share REGEX’
     Load shared object library symbols for files matching a Unix
     regular expression.  As with files loaded automatically, it only
     loads shared libraries required by your program for a core file or
     after typing ‘run’.  If REGEX is omitted all shared libraries
     required by your program are loaded.

‘nosharedlibrary’
     Unload all shared object library symbols.  This discards all
     symbols that have been loaded from all shared libraries.  Symbols
     from shared libraries that were loaded by explicit user requests
     are not discarded.

   Sometimes you may wish that GDB stops and gives you control when any
of shared library events happen.  The best way to do this is to use
‘catch load’ and ‘catch unload’ (*note Set Catchpoints::).

   GDB also supports the ‘set stop-on-solib-events’ command for this.
This command exists for historical reasons.  It is less useful than
setting a catchpoint, because it does not allow for conditions or
commands as a catchpoint does.

‘info linker-namespaces’
‘info linker-namespaces [[N]]’

     With no argument, print the number of linker namespaces which are
     currently active -- that is, namespaces that have libraries loaded
     into them.  Then, it prints the number of libraries loaded into
     each namespace, and all the libraries loaded into them, in the same
     way as ‘info sharedlibrary’.

     If an argument ‘[[N]]’ is provided, only prints the library count
     and the libraried for the provided namespace N.  The surrounding
     square brackets are optional.

‘set stop-on-solib-events’
     This command controls whether GDB should give you control when the
     dynamic linker notifies it about some shared library event.  The
     most common event of interest is loading or unloading of a new
     shared library.

‘show stop-on-solib-events’
     Show whether GDB stops and gives you control when shared library
     events happen.

   Shared libraries are also supported in many cross or remote debugging
configurations.  GDB needs to have access to the target's libraries;
this can be accomplished either by providing copies of the libraries on
the host system, or by asking GDB to automatically retrieve the
libraries from the target.  If copies of the target libraries are
provided, they need to be the same as the target libraries, although the
copies on the target can be stripped as long as the copies on the host
are not.

   For remote debugging, you need to tell GDB where the target libraries
are, so that it can load the correct copies--otherwise, it may try to
load the host's libraries.  GDB has two variables to specify the search
directories for target libraries.

‘set sysroot PATH’
     Use PATH as the system root for the program being debugged.  Any
     absolute shared library paths will be prefixed with PATH; many
     runtime loaders store the absolute paths to the shared library in
     the target program's memory.  When starting processes remotely, and
     when attaching to already-running processes (local or remote),
     their executable filenames will be prefixed with PATH if reported
     to GDB as absolute by the operating system.  If you use ‘set
     sysroot’ to find executables and shared libraries, they need to be
     laid out in the same way that they are on the target, with e.g. a
     ‘/bin’, ‘/lib’ and ‘/usr/lib’ hierarchy under PATH.

     If PATH starts with the sequence ‘target:’ and the target system is
     remote then GDB will retrieve the target binaries from the remote
     system.  This is only supported when using a remote target that
     supports the ‘remote get’ command (*note Sending files to a remote
     system: File Transfer.).  The part of PATH following the initial
     ‘target:’ (if present) is used as system root prefix on the remote
     file system.  If PATH starts with the sequence ‘remote:’ this is
     converted to the sequence ‘target:’ by ‘set sysroot’(1).  If you
     want to specify a local system root using a directory that happens
     to be named ‘target:’ or ‘remote:’, you need to use some equivalent
     variant of the name like ‘./target:’.

     For targets with an MS-DOS based filesystem, such as MS-Windows,
     GDB tries prefixing a few variants of the target absolute file name
     with PATH.  But first, on Unix hosts, GDB converts all backslash
     directory separators into forward slashes, because the backslash is
     not a directory separator on Unix:

            c:\foo\bar.dll ⇒ c:/foo/bar.dll

     Then, GDB attempts prefixing the target file name with PATH, and
     looks for the resulting file name in the host file system:

            c:/foo/bar.dll ⇒ /path/to/sysroot/c:/foo/bar.dll

     If that does not find the binary, GDB tries removing the ‘:’
     character from the drive spec, both for convenience, and, for the
     case of the host file system not supporting file names with colons:

            c:/foo/bar.dll ⇒ /path/to/sysroot/c/foo/bar.dll

     This makes it possible to have a system root that mirrors a target
     with more than one drive.  E.g., you may want to setup your local
     copies of the target system shared libraries like so (note ‘c’ vs
     ‘z’):

           /path/to/sysroot/c/sys/bin/foo.dll
           /path/to/sysroot/c/sys/bin/bar.dll
           /path/to/sysroot/z/sys/bin/bar.dll

     and point the system root at ‘/path/to/sysroot’, so that GDB can
     find the correct copies of both ‘c:\sys\bin\foo.dll’, and
     ‘z:\sys\bin\bar.dll’.

     If that still does not find the binary, GDB tries removing the
     whole drive spec from the target file name:

            c:/foo/bar.dll ⇒ /path/to/sysroot/foo/bar.dll

     This last lookup makes it possible to not care about the drive
     name, if you don't want or need to.

     The ‘set solib-absolute-prefix’ command is an alias for ‘set
     sysroot’.

     You can set the default system root by using the configure-time
     ‘--with-sysroot’ option.  If the system root is inside GDB's
     configured binary prefix (set with ‘--prefix’ or ‘--exec-prefix’),
     then the default system root will be updated automatically if the
     installed GDB is moved to a new location.

‘show sysroot’
     Display the current executable and shared library prefix.

‘set solib-search-path PATH’
     If this variable is set, PATH is a colon-separated list of
     directories to search for shared libraries.  ‘solib-search-path’ is
     used after ‘sysroot’ fails to locate the library, or if the path to
     the library is relative instead of absolute.  If you want to use
     ‘solib-search-path’ instead of ‘sysroot’, be sure to set ‘sysroot’
     to a nonexistent directory to prevent GDB from finding your host's
     libraries.  ‘sysroot’ is preferred; setting it to a nonexistent
     directory may interfere with automatic loading of shared library
     symbols.

‘show solib-search-path’
     Display the current shared library search path.

‘set target-file-system-kind KIND’
     Set assumed file system kind for target reported file names.

     Shared library file names as reported by the target system may not
     make sense as is on the system GDB is running on.  For example,
     when remote debugging a target that has MS-DOS based file system
     semantics, from a Unix host, the target may be reporting to GDB a
     list of loaded shared libraries with file names such as
     ‘c:\Windows\kernel32.dll’.  On Unix hosts, there's no concept of
     drive letters, so the ‘c:\’ prefix is not normally understood as
     indicating an absolute file name, and neither is the backslash
     normally considered a directory separator character.  In that case,
     the native file system would interpret this whole absolute file
     name as a relative file name with no directory components.  This
     would make it impossible to point GDB at a copy of the remote
     target's shared libraries on the host using ‘set sysroot’, and
     impractical with ‘set solib-search-path’.  Setting
     ‘target-file-system-kind’ to ‘dos-based’ tells GDB to interpret
     such file names similarly to how the target would, and to map them
     to file names valid on GDB's native file system semantics.  The
     value of KIND can be ‘"auto"’, in addition to one of the supported
     file system kinds.  In that case, GDB tries to determine the
     appropriate file system variant based on the current target's
     operating system (*note Configuring the Current ABI: ABI.). The
     supported file system settings are:

     ‘unix’
          Instruct GDB to assume the target file system is of Unix kind.
          Only file names starting the forward slash (‘/’) character are
          considered absolute, and the directory separator character is
          also the forward slash.

     ‘dos-based’
          Instruct GDB to assume the target file system is DOS based.
          File names starting with either a forward slash, or a drive
          letter followed by a colon (e.g., ‘c:’), are considered
          absolute, and both the slash (‘/’) and the backslash (‘\\’)
          characters are considered directory separators.

     ‘auto’
          Instruct GDB to use the file system kind associated with the
          target operating system (*note Configuring the Current ABI:
          ABI.). This is the default.

   When processing file names provided by the user, GDB frequently needs
to compare them to the file names recorded in the program's debug info.
Normally, GDB compares just the “base names” of the files as strings,
which is reasonably fast even for very large programs.  (The base name
of a file is the last portion of its name, after stripping all the
leading directories.)  This shortcut in comparison is based upon the
assumption that files cannot have more than one base name.  This is
usually true, but references to files that use symlinks or similar
filesystem facilities violate that assumption.  If your program records
files using such facilities, or if you provide file names to GDB using
symlinks etc., you can set ‘basenames-may-differ’ to ‘true’ to instruct
GDB to completely canonicalize each pair of file names it needs to
compare.  This will make file-name comparisons accurate, but at a price
of a significant slowdown.

‘set basenames-may-differ’
     Set whether a source file may have multiple base names.

‘show basenames-may-differ’
     Show whether a source file may have multiple base names.

   ---------- Footnotes ----------

   (1) Historically the functionality to retrieve binaries from the
remote system was provided by prefixing PATH with ‘remote:’


File: gdb.info,  Node: File Caching,  Next: Separate Debug Files,  Prev: Files,  Up: GDB Files

18.2 File Caching
=================

To speed up file loading, and reduce memory usage, GDB will reuse the
‘bfd’ objects used to track open files.  *Note BFD: (bfd)Top.  The
following commands allow visibility and control of the caching behavior.

‘maint info bfds’
     This prints information about each ‘bfd’ object that is known to
     GDB.

‘maint set bfd-sharing’
‘maint show bfd-sharing’
     Control whether ‘bfd’ objects can be shared.  When sharing is
     enabled GDB reuses already open ‘bfd’ objects rather than reopening
     the same file.  Turning sharing off does not cause already shared
     ‘bfd’ objects to be unshared, but all future files that are opened
     will create a new ‘bfd’ object.  Similarly, re-enabling sharing
     does not cause multiple existing ‘bfd’ objects to be collapsed into
     a single shared ‘bfd’ object.

‘set debug bfd-cache LEVEL’
     Turns on debugging of the bfd cache, setting the level to LEVEL.

‘show debug bfd-cache’
     Show the current debugging level of the bfd cache.


File: gdb.info,  Node: Separate Debug Files,  Next: MiniDebugInfo,  Prev: File Caching,  Up: GDB Files

18.3 Debugging Information in Separate Files
============================================

GDB allows you to put a program's debugging information in a file
separate from the executable itself, in a way that allows GDB to find
and load the debugging information automatically.  Since debugging
information can be very large--sometimes larger than the executable code
itself--some systems distribute debugging information for their
executables in separate files, which users can install only when they
need to debug a problem.

   GDB supports two ways of specifying the separate debug info file:

   • The executable contains a “debug link” that specifies the name of
     the separate debug info file.  The separate debug file's name is
     usually ‘EXECUTABLE.debug’, where EXECUTABLE is the name of the
     corresponding executable file without leading directories (e.g.,
     ‘ls.debug’ for ‘/usr/bin/ls’).  In addition, the debug link
     specifies a 32-bit “Cyclic Redundancy Check” (CRC) checksum for the
     debug file, which GDB uses to validate that the executable and the
     debug file came from the same build.

   • The executable contains a “build ID”, a unique bit string that is
     also present in the corresponding debug info file.  (This is
     supported only on some operating systems, when using the ELF or PE
     file formats for binary files and the GNU Binutils.)  For more
     details about this feature, see the description of the ‘--build-id’
     command-line option in *note Command Line Options: (ld)Options.
     The debug info file's name is not specified explicitly by the build
     ID, but can be computed from the build ID, see below.

   Depending on the way the debug info file is specified, GDB uses two
different methods of looking for the debug file:

   • For the "debug link" method, GDB looks up the named file in the
     directory of the executable file, then in a subdirectory of that
     directory named ‘.debug’, and finally under each one of the global
     debug directories, in a subdirectory whose name is identical to the
     leading directories of the executable's absolute file name.  (On
     MS-Windows/MS-DOS, the drive letter of the executable's leading
     directories is converted to a one-letter subdirectory, i.e.
     ‘d:/usr/bin/’ is converted to ‘/d/usr/bin/’, because Windows
     filesystems disallow colons in file names.)

   • For the "build ID" method, GDB looks in the ‘.build-id’
     subdirectory of each one of the global debug directories for a file
     named ‘NN/NNNNNNNN.debug’, where NN are the first 2 hex characters
     of the build ID bit string, and NNNNNNNN are the rest of the bit
     string.  (Real build ID strings are 32 or more hex characters, not
     10.)  GDB can automatically query ‘debuginfod’ servers using build
     IDs in order to download separate debug files that cannot be found
     locally.  For more information see *note Debuginfod::.

   So, for example, suppose you ask GDB to debug ‘/usr/bin/ls’, which
has a debug link that specifies the file ‘ls.debug’, and a build ID
whose value in hex is ‘abcdef1234’.  If the list of the global debug
directories includes ‘/usr/lib/debug’, then GDB will look for the
following debug information files, in the indicated order:

   − ‘/usr/lib/debug/.build-id/ab/cdef1234.debug’
   − ‘/usr/bin/ls.debug’
   − ‘/usr/bin/.debug/ls.debug’
   − ‘/usr/lib/debug/usr/bin/ls.debug’.

   If the debug file still has not been found and ‘debuginfod’ (*note
Debuginfod::) is enabled, GDB will attempt to download the file from
‘debuginfod’ servers.

   Global debugging info directories default to what is set by GDB
configure option ‘--with-separate-debug-dir’ and augmented by the
colon-separated list of directories provided via GDB configure option
‘--additional-debug-dirs’.  During GDB run you can also set the global
debugging info directories, and view the list GDB is currently using.

‘set debug-file-directory DIRECTORIES’
     Set the directories which GDB searches for separate debugging
     information files to DIRECTORY.  Multiple path components can be
     set concatenating them by a path separator.

‘show debug-file-directory’
     Show the directories GDB searches for separate debugging
     information files.

   A debug link is a special section of the executable file named
‘.gnu_debuglink’.  The section must contain:

   • A filename, with any leading directory components removed, followed
     by a zero byte,
   • zero to three bytes of padding, as needed to reach the next
     four-byte boundary within the section, and
   • a four-byte CRC checksum, stored in the same endianness used for
     the executable file itself.  The checksum is computed on the
     debugging information file's full contents by the function given
     below, passing zero as the CRC argument.

   Any executable file format can carry a debug link, as long as it can
contain a section named ‘.gnu_debuglink’ with the contents described
above.

   The build ID is a special section in the executable file (and in
other ELF binary files that GDB may consider).  This section is often
named ‘.note.gnu.build-id’, but that name is not mandatory.  It contains
unique identification for the built files--the ID remains the same
across multiple builds of the same build tree.  The default algorithm
SHA1 produces 160 bits (40 hexadecimal characters) of the content for
the build ID string.  The same section with an identical value is
present in the original built binary with symbols, in its stripped
variant, and in the separate debugging information file.

   The debugging information file itself should be an ordinary
executable, containing a full set of linker symbols, sections, and
debugging information.  The sections of the debugging information file
should have the same names, addresses, and sizes as the original file,
but they need not contain any data--much like a ‘.bss’ section in an
ordinary executable.

   The GNU binary utilities (Binutils) package includes the ‘objcopy’
utility that can produce the separated executable / debugging
information file pairs using the following commands:

     objcopy --only-keep-debug foo foo.debug
     strip -g foo

These commands remove the debugging information from the executable file
‘foo’ and place it in the file ‘foo.debug’.  You can use the first,
second or both methods to link the two files:

   • The debug link method needs the following additional command to
     also leave behind a debug link in ‘foo’:

          objcopy --add-gnu-debuglink=foo.debug foo

     Ulrich Drepper's ‘elfutils’ package, starting with version 0.53,
     contains a version of the ‘strip’ command such that the command
     ‘strip foo -f foo.debug’ has the same functionality as the two
     ‘objcopy’ commands and the ‘ln -s’ command above, together.

   • Build ID gets embedded into the main executable using ‘ld
     --build-id’ or the GCC counterpart ‘gcc -Wl,--build-id’.  Build ID
     support plus compatibility fixes for debug files separation are
     present in GNU binary utilities (Binutils) package since version
     2.18.

   The CRC used in ‘.gnu_debuglink’ is the CRC-32 defined in IEEE 802.3
using the polynomial:

      x^{32} + x^{26} + x^{23} + x^{22} + x^{16} + x^{12} + x^{11}
      + x^{10} + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1

   The function is computed byte at a time, taking the least significant
bit of each byte first.  The initial pattern ‘0xffffffff’ is used, to
ensure leading zeros affect the CRC and the final result is inverted to
ensure trailing zeros also affect the CRC.

   _Note:_ This is the same CRC polynomial as used in handling the
“Remote Serial Protocol” ‘qCRC’ packet (*note qCRC packet::).  However
in the case of the Remote Serial Protocol, the CRC is computed _most_
significant bit first, and the result is not inverted, so trailing zeros
have no effect on the CRC value.

   To complete the description, we show below the code of the function
which produces the CRC used in ‘.gnu_debuglink’.  Inverting the
initially supplied ‘crc’ argument means that an initial call to this
function passing in zero will start computing the CRC using
‘0xffffffff’.

     unsigned long
     gnu_debuglink_crc32 (unsigned long crc,
                          unsigned char *buf, size_t len)
     {
       static const unsigned long crc32_table[256] =
         {
           0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
           0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
           0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
           0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
           0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
           0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
           0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
           0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
           0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
           0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
           0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
           0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
           0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
           0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
           0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
           0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
           0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
           0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
           0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
           0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
           0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
           0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
           0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
           0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
           0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
           0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
           0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
           0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
           0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
           0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
           0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
           0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
           0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
           0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
           0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
           0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
           0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
           0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
           0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
           0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
           0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
           0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
           0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
           0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
           0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
           0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
           0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
           0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
           0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
           0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
           0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
           0x2d02ef8d
         };
       unsigned char *end;

       crc = ~crc & 0xffffffff;
       for (end = buf + len; buf < end; ++buf)
         crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
       return ~crc & 0xffffffff;
     }

This computation does not apply to the "build ID" method.


File: gdb.info,  Node: MiniDebugInfo,  Next: Index Files,  Prev: Separate Debug Files,  Up: GDB Files

18.4 Debugging information in a special section
===============================================

Some systems ship pre-built executables and libraries that have a
special ‘.gnu_debugdata’ section.  This feature is called
“MiniDebugInfo”.  This section holds an LZMA-compressed object and is
used to supply extra symbols for backtraces.

   The intent of this section is to provide extra minimal debugging
information for use in simple backtraces.  It is not intended to be a
replacement for full separate debugging information (*note Separate
Debug Files::).  The example below shows the intended use; however, GDB
does not currently put restrictions on what sort of debugging
information might be included in the section.

   GDB has support for this extension.  If the section exists, then it
is used provided that no other source of debugging information can be
found, and that GDB was configured with LZMA support.

   This section can be easily created using ‘objcopy’ and other standard
utilities:

     # Extract the dynamic symbols from the main binary, there is no need
     # to also have these in the normal symbol table.
     nm -D BINARY --format=posix --defined-only \
       | awk '{ print $1 }' | sort > dynsyms

     # Extract all the text (i.e. function) symbols from the debuginfo.
     # (Note that we actually also accept "D" symbols, for the benefit
     # of platforms like PowerPC64 that use function descriptors.)
     nm BINARY --format=posix --defined-only \
       | awk '{ if ($2 == "T" || $2 == "t" || $2 == "D") print $1 }' \
       | sort > funcsyms

     # Keep all the function symbols not already in the dynamic symbol
     # table.
     comm -13 dynsyms funcsyms > keep_symbols

     # Separate full debug info into debug binary.
     objcopy --only-keep-debug BINARY debug

     # Copy the full debuginfo, keeping only a minimal set of symbols and
     # removing some unnecessary sections.
     objcopy -S --remove-section .gdb_index --remove-section .comment \
       --keep-symbols=keep_symbols debug mini_debuginfo

     # Drop the full debug info from the original binary.
     strip --strip-all -R .comment BINARY

     # Inject the compressed data into the .gnu_debugdata section of the
     # original binary.
     xz mini_debuginfo
     objcopy --add-section .gnu_debugdata=mini_debuginfo.xz BINARY


File: gdb.info,  Node: Index Files,  Next: Debug Names,  Prev: MiniDebugInfo,  Up: GDB Files

18.5 Index Files Speed Up GDB
=============================

When GDB finds a symbol file, it scans the symbols in the file in order
to construct an internal symbol table.  This lets most GDB operations
work quickly--at the cost of a delay early on.  For large programs, this
delay can be quite lengthy, so GDB provides a way to build an index,
which speeds up startup.

   For convenience, GDB comes with a program, ‘gdb-add-index’, which can
be used to add the index to a symbol file.  It takes the symbol file as
its only argument:

     $ gdb-add-index symfile

   *Note gdb-add-index::.

   It is also possible to do the work manually.  Here is what
‘gdb-add-index’ does behind the curtains.

   The index is stored as a section in the symbol file.  GDB can write
the index to a file, then you can put it into the symbol file using
‘objcopy’.

   To create an index file, use the ‘save gdb-index’ command:

‘save gdb-index [-dwarf-5] DIRECTORY’
     Create index files for all symbol files currently known by GDB.
     For each known SYMBOL-FILE, this command by default produces a
     single file ‘SYMBOL-FILE.gdb-index’.  If you invoke this command
     with the ‘-dwarf-5’ option, it produces 2 files:
     ‘SYMBOL-FILE.debug_names’ and ‘SYMBOL-FILE.debug_str’.  The files
     are created in the given DIRECTORY.

     The DIRECTORY argument supports escaping and quoting, see *note
     Filenames As Command Arguments: Filename Arguments.

   Once you have created an index file you can merge it into your symbol
file, here named ‘symfile’, using ‘objcopy’:

     $ objcopy --add-section .gdb_index=symfile.gdb-index \
         --set-section-flags .gdb_index=readonly symfile symfile

   Or for ‘-dwarf-5’:

     $ objcopy --dump-section .debug_str=symfile.debug_str.new symfile
     $ cat symfile.debug_str >>symfile.debug_str.new
     $ objcopy --add-section .debug_names=symfile.gdb-index \
         --set-section-flags .debug_names=readonly \
         --update-section .debug_str=symfile.debug_str.new symfile symfile

   GDB will normally ignore older versions of ‘.gdb_index’ sections that
have been deprecated.  Usually they are deprecated because they are
missing a new feature or have performance issues.  To tell GDB to use a
deprecated index section anyway specify ‘set
use-deprecated-index-sections on’.  The default is ‘off’.  This can
speed up startup, but may result in some functionality being lost.
*Note Index Section Format::.

   _Warning:_ Setting ‘use-deprecated-index-sections’ to ‘on’ must be
done before gdb reads the file.  The following will not work:

     $ gdb -ex "set use-deprecated-index-sections on" <program>

   Instead you must do, for example,

     $ gdb -iex "set use-deprecated-index-sections on" <program>

   Indices only work when using DWARF debugging information.

18.5.1 Automatic symbol index cache
-----------------------------------

It is possible for GDB to automatically save a copy of this index in a
cache on disk and retrieve it from there when loading the same binary in
the future.  This feature can be turned on with ‘set index-cache enabled
on’.  Note that the cache will use the ELF build ID to identify the
cached indices, so files without build ID will not have their index
cached.  The following commands can be used to tweak the behavior of the
index cache.

‘set index-cache enabled on’
‘set index-cache enabled off’
     Enable or disable the use of the symbol index cache.

‘set index-cache directory DIRECTORY’
‘show index-cache directory’
     Set/show the directory where index files will be saved.

     The default value for this directory depends on the host platform.
     On most systems, the index is cached in the ‘gdb’ subdirectory of
     the directory pointed to by the ‘XDG_CACHE_HOME’ environment
     variable, if it is defined, else in the ‘.cache/gdb’ subdirectory
     of your home directory.  However, on some systems, the default may
     differ according to local convention.

     There is no limit on the disk space used by index cache.  It is
     perfectly safe to delete the content of that directory to free up
     disk space.

‘show index-cache stats’
     Print the number of cache hits and misses since the launch of GDB.

18.5.2 Building the index with GNU ‘gold’
-----------------------------------------

The GNU ‘gold’ linker can write the index at link time into the
resulting ELF file, by passing the ‘--gdb-index’ flag to ‘gold’.

   This is especially helpful if you intend to build a program and
immediately run it under GDB.  You may find that it is faster overall
for the linker to write the index while it has the relevant information
in memory anyways, rather than re-reloading the data from disk with
‘gdb-add-index’, or debugging the program without an index at all,
especially for large programs.


File: gdb.info,  Node: Debug Names,  Next: Symbol Errors,  Prev: Index Files,  Up: GDB Files

18.6 Extensions to ‘.debug_names’
=================================

The DWARF specification documents an optional index section called
‘.debug_names’.  GDB can both read and create this section.  However, in
order to work with GDB, some extensions were necessary.

   GDB uses an augmentation string to specify which extensions are in
use and to allow support of backwards-incompatible changes in this
functionality.  The augmentation string has the form ‘GDBN’, where N is
an integral version number of the extensions, which is incremented when
the extensions are added or modified.  The smallest N is 2; earlier
versions of GDB used just ‘GDB’ with no version number, but these
versions of the index are no longer supported.

   Here is a list of augmentation string versions along with the changes
introduced with each version, compared to the previous version.

‘GDB2’
     Specifies the use of attributes ‘DW_IDX_GNU_internal’,
     ‘DW_IDX_GNU_main’, ‘DW_IDX_GNU_language’ and
     ‘DW_IDX_GNU_linkage_name’, described below.

‘GDB3’
     Changes the semantic of the ‘DW_IDX_parent’ attribute.  With
     ‘GDB2’, ‘DW_IDX_parent’ provided an offset into the name table.
     With ‘GDB3’, it now provides an offset to the index entry of the
     parent, relative to the start of the entry pool region.

   GDB produces indexes with the augmentation string ‘GDB3’.

   GDB can read indexes with augmentation strings ‘GDB2’ or ‘GDB3’.  GDB
does not support reading indexes with any other augmentation strings.

   GDB does not use the specified hash table.  Therefore, because this
hash table is optional, GDB also does not write it.

   GDB generates and uses the following non-standard index attributes:

‘DW_IDX_GNU_internal’
     This has the value ‘0x2000’.  It is a flag that, when set,
     indicates that the associated entry has ‘static’ linkage.

‘DW_IDX_GNU_main’
     This has the value ‘0x2002’.  It is a flag that, when set,
     indicates that the associated entry is the program's ‘main’.

‘DW_IDX_GNU_language’
     This has the value ‘0x2003’.  It is a ‘DW_LANG_’ constant,
     indicating the language of the associated entry.

‘DW_IDX_GNU_linkage_name’
     This has the value ‘0x2004’.  It is a flag that, when set,
     indicates that the associated entry is a linkage name, and not a
     source name.

   GDB also has some special handling for cases not considered in the
DWARF specification.

   • The ‘DW_IDX_parent’ for a C-style enumerator does not point at the
     entry for ‘enum’ itself, but rather the parent of the type.  The
     reason for this is that C-style enumerators are injected into the
     containing scope, and so their name is not qualified by the ‘enum’;
     and furthermore there is no way to distinguish between C-style
     enumerators and ‘enum class’-style enumerators in ‘.debug_names’.

   • Similarly, ‘DW_IDX_parent’ is omitted for any linkage name entries
     that are written.

   • Definitions in partial units are handled differently.  These most
     typically are seen in the output of ‘dwz’.

     In general, a DWARF partial unit cannot be read in isolation, but
     only by reading it in the context of some other unit that
     references it via ‘DW_TAG_imported_unit’.

     Therefore, an ordinary definition in a partial unit is attributed
     to one of the outermost containing units.  This is done by
     referencing this containing CU in the ‘DW_IDX_compile_unit’
     attribute.

     A further special case applies to ‘DW_TAG_inlined_subroutine’
     entries.  An inlined subroutine appearing in a partial unit may be
     inlined in all of the outermost compilation units that directly or
     indirectly include the partial unit.  Therefore, in this case, GDB
     will emit a separate index entry for the entry, once for each such
     containing unit.


File: gdb.info,  Node: Symbol Errors,  Next: Data Files,  Prev: Debug Names,  Up: GDB Files

18.7 Errors Reading Symbol Files
================================

While reading a symbol file, GDB occasionally encounters problems, such
as symbol types it does not recognize, or known bugs in compiler output.
By default, GDB does not notify you of such problems, since they are
relatively common and primarily of interest to people debugging
compilers.  If you are interested in seeing information about
ill-constructed symbol tables, you can either ask GDB to print only one
message about each such type of problem, no matter how many times the
problem occurs; or you can ask GDB to print more messages, to see how
many times the problems occur, with the ‘set complaints’ command (*note
Optional Warnings and Messages: Messages/Warnings.).

   The messages currently printed, and their meanings, include:

‘inner block not inside outer block in SYMBOL’

     The symbol information shows where symbol scopes begin and end
     (such as at the start of a function or a block of statements).
     This error indicates that an inner scope block is not fully
     contained in its outer scope blocks.

     GDB circumvents the problem by treating the inner block as if it
     had the same scope as the outer block.  In the error message,
     SYMBOL may be shown as "‘(don't know)’" if the outer block is not a
     function.

‘block at ADDRESS out of order’

     The symbol information for symbol scope blocks should occur in
     order of increasing addresses.  This error indicates that it does
     not do so.

     GDB does not circumvent this problem, and has trouble locating
     symbols in the source file whose symbols it is reading.  (You can
     often determine what source file is affected by specifying ‘set
     verbose on’.  *Note Optional Warnings and Messages:
     Messages/Warnings.)

‘bad block start address patched’

     The symbol information for a symbol scope block has a start address
     smaller than the address of the preceding source line.  This is
     known to occur in the SunOS 4.1.1 (and earlier) C compiler.

     GDB circumvents the problem by treating the symbol scope block as
     starting on the previous source line.

‘bad string table offset in symbol N’

     Symbol number N contains a pointer into the string table which is
     larger than the size of the string table.

     GDB circumvents the problem by considering the symbol to have the
     name ‘foo’, which may cause other problems if many symbols end up
     with this name.

‘unknown symbol type 0xNN’

     The symbol information contains new data types that GDB does not
     yet know how to read.  ‘0xNN’ is the symbol type of the
     uncomprehended information, in hexadecimal.

     GDB circumvents the error by ignoring this symbol information.
     This usually allows you to debug your program, though certain
     symbols are not accessible.  If you encounter such a problem and
     feel like debugging it, you can debug ‘gdb’ with itself, breakpoint
     on ‘complain’, then go up to the function ‘read_dbx_symtab’ and
     examine ‘*bufp’ to see the symbol.

‘stub type has NULL name’

     GDB could not find the full definition for a struct or class.

‘const/volatile indicator missing (ok if using g++ v1.x), got...’
     The symbol information for a C++ member function is missing some
     information that recent versions of the compiler should have output
     for it.

‘info mismatch between compiler and debugger’

     GDB could not parse a type specification output by the compiler.


File: gdb.info,  Node: Data Files,  Prev: Symbol Errors,  Up: GDB Files

18.8 GDB Data Files
===================

GDB will sometimes read an auxiliary data file.  These files are kept in
a directory known as the “data directory”.

   You can set the data directory's name, and view the name GDB is
currently using.

‘set data-directory DIRECTORY’
     Set the directory which GDB searches for auxiliary data files to
     DIRECTORY.

‘show data-directory’
     Show the directory GDB searches for auxiliary data files.

   You can set the default data directory by using the configure-time
‘--with-gdb-datadir’ option.  If the data directory is inside GDB's
configured binary prefix (set with ‘--prefix’ or ‘--exec-prefix’), then
the default data directory will be updated automatically if the
installed GDB is moved to a new location.

   The data directory may also be specified with the ‘--data-directory’
command line option.  *Note Mode Options::.


File: gdb.info,  Node: Targets,  Next: Remote Debugging,  Prev: GDB Files,  Up: Top

19 Specifying a Debugging Target
********************************

A “target” is the execution environment occupied by your program.

   Often, GDB runs in the same host environment as your program; in that
case, the debugging target is specified as a side effect when you use
the ‘file’ or ‘core’ commands.  When you need more flexibility--for
example, running GDB on a physically separate host, or controlling a
standalone system over a serial port or a realtime system over a TCP/IP
connection--you can use the ‘target’ command to specify one of the
target types configured for GDB (*note Commands for Managing Targets:
Target Commands.).

   It is possible to build GDB for several different “target
architectures”.  When GDB is built like that, you can choose one of the
available architectures with the ‘set architecture’ command.

‘set architecture ARCH’
     This command sets the current target architecture to ARCH.  The
     value of ARCH can be ‘"auto"’, in addition to one of the supported
     architectures.

‘show architecture’
     Show the current target architecture.

‘set processor’
‘processor’
     These are alias commands for, respectively, ‘set architecture’ and
     ‘show architecture’.

* Menu:

* Active Targets::              Active targets
* Target Commands::             Commands for managing targets
* Byte Order::                  Choosing target byte order


File: gdb.info,  Node: Active Targets,  Next: Target Commands,  Up: Targets

19.1 Active Targets
===================

There are multiple classes of targets such as: processes, executable
files or recording sessions.  Core files belong to the process class,
making core file and process mutually exclusive.  Otherwise, GDB can
work concurrently on multiple active targets, one in each class.  This
allows you to (for example) start a process and inspect its activity,
while still having access to the executable file after the process
finishes.  Or if you start process recording (*note Reverse Execution::)
and ‘reverse-step’ there, you are presented a virtual layer of the
recording target, while the process target remains stopped at the
chronologically last point of the process execution.

   Use the ‘core-file’ and ‘exec-file’ commands to select a new core
file or executable target (*note Commands to Specify Files: Files.).  To
specify as a target a process that is already running, use the ‘attach’
command (*note Debugging an Already-running Process: Attach.).


File: gdb.info,  Node: Target Commands,  Next: Byte Order,  Prev: Active Targets,  Up: Targets

19.2 Commands for Managing Targets
==================================

‘target TYPE PARAMETERS’
     Connects the GDB host environment to a target machine or process.
     A target is typically a protocol for talking to debugging
     facilities.  You use the argument TYPE to specify the type or
     protocol of the target machine.

     Further PARAMETERS are interpreted by the target protocol, but
     typically include things like device names or host names to connect
     with, process numbers, and baud rates.

     The ‘target’ command does not repeat if you press <RET> again after
     executing the command.

‘help target’
     Displays the names of all targets available.  To display targets
     currently selected, use either ‘info target’ or ‘info files’ (*note
     Commands to Specify Files: Files.).

‘help target NAME’
     Describe a particular target, including any parameters necessary to
     select it.

‘set gnutarget ARGS’
     GDB uses its own library BFD to read your files.  GDB knows whether
     it is reading an “executable”, a “core”, or a “.o” file; however,
     you can specify the file format with the ‘set gnutarget’ command.
     Unlike most ‘target’ commands, with ‘gnutarget’ the ‘target’ refers
     to a program, not a machine.

          _Warning:_ To specify a file format with ‘set gnutarget’, you
          must know the actual BFD name.

     *Note Commands to Specify Files: Files.

‘show gnutarget’
     Use the ‘show gnutarget’ command to display what file format
     ‘gnutarget’ is set to read.  If you have not set ‘gnutarget’, GDB
     will determine the file format for each file automatically, and
     ‘show gnutarget’ displays ‘The current BFD target is "auto"’.

   Here are some common targets (available, or not, depending on the GDB
configuration):

‘target exec PROGRAM’
     An executable file.  ‘target exec PROGRAM’ is the same as
     ‘exec-file PROGRAM’.

     The PROGRAM argument supports escaping and quoting, see *note
     Filenames As Command Arguments: Filename Arguments.

‘target core FILENAME’
     A core dump file.  ‘target core FILENAME’ is the same as ‘core-file
     FILENAME’.

     The FILENAME argument supports escaping and quoting, see *note
     Filenames As Command Arguments: Filename Arguments.

‘target remote MEDIUM’
     A remote system connected to GDB via a serial line or network
     connection.  This command tells GDB to use its own remote protocol
     over MEDIUM for debugging.  *Note Remote Debugging::.

     For example, if you have a board connected to ‘/dev/ttya’ on the
     machine running GDB, you could say:

          target remote /dev/ttya

     ‘target remote’ supports the ‘load’ command.  This is only useful
     if you have some other way of getting the stub to the target
     system, and you can put it somewhere in memory where it won't get
     clobbered by the download.

‘target sim [SIMARGS] ...’
     Builtin CPU simulator.  GDB includes simulators for most
     architectures.  In general,
                  target sim
                  load
                  run
     works; however, you cannot assume that a specific memory map,
     device drivers, or even basic I/O is available, although some
     simulators do provide these.  For info about any processor-specific
     simulator details, see the appropriate section in *note Embedded
     Processors: Embedded Processors.

‘target native’
     Setup for local/native process debugging.  Useful to make the ‘run’
     command spawn native processes (likewise ‘attach’, etc.) even when
     ‘set auto-connect-native-target’ is ‘off’ (*note set
     auto-connect-native-target::).

   Different targets are available on different configurations of GDB;
your configuration may have more or fewer targets.

   Many remote targets require you to download the executable's code
once you've successfully established a connection.  You may wish to
control various aspects of this process.

‘set hash’
     This command controls whether a hash mark ‘#’ is displayed while
     downloading a file to the remote monitor.  If on, a hash mark is
     displayed after each S-record is successfully downloaded to the
     monitor.

‘show hash’
     Show the current status of displaying the hash mark.

‘set debug monitor’
     Enable or disable display of communications messages between GDB
     and the remote monitor.

‘show debug monitor’
     Show the current status of displaying communications between GDB
     and the remote monitor.

‘load FILENAME OFFSET’
     Depending on what remote debugging facilities are configured into
     GDB, the ‘load’ command may be available.  Where it exists, it is
     meant to make FILENAME (an executable) available for debugging on
     the remote system--by downloading, or dynamic linking, for example.
     ‘load’ also records the FILENAME symbol table in GDB, like the
     ‘add-symbol-file’ command.

     If your GDB does not have a ‘load’ command, attempting to execute
     it gets the error message "‘You can't do that when your target is
     ...’"

     The file is loaded at whatever address is specified in the
     executable.  For some object file formats, you can specify the load
     address when you link the program; for other formats, like a.out,
     the object file format specifies a fixed address.

     It is also possible to tell GDB to load the executable file at a
     specific offset described by the optional argument OFFSET.  When
     OFFSET is provided, FILENAME must also be provided.

     Depending on the remote side capabilities, GDB may be able to load
     programs into flash memory.

     ‘load’ does not repeat if you press <RET> again after using it.

‘flash-erase’

     Erases all known flash memory regions on the target.


File: gdb.info,  Node: Byte Order,  Prev: Target Commands,  Up: Targets

19.3 Choosing Target Byte Order
===============================

Some types of processors, such as the MIPS, PowerPC, and Renesas SH,
offer the ability to run either big-endian or little-endian byte orders.
Usually the executable or symbol will include a bit to designate the
endian-ness, and you will not need to worry about which to use.
However, you may still find it useful to adjust GDB's idea of processor
endian-ness manually.

‘set endian big’
     Instruct GDB to assume the target is big-endian.

‘set endian little’
     Instruct GDB to assume the target is little-endian.

‘set endian auto’
     Instruct GDB to use the byte order associated with the executable.

‘show endian’
     Display GDB's current idea of the target byte order.

   If the ‘set endian auto’ mode is in effect and no executable has been
selected, then the endianness used is the last one chosen either by one
of the ‘set endian big’ and ‘set endian little’ commands or by inferring
from the last executable used.  If no endianness has been previously
chosen, then the default for this mode is inferred from the target GDB
has been built for, and is ‘little’ if the name of the target CPU has an
‘el’ suffix and ‘big’ otherwise.

   Note that these commands merely adjust interpretation of symbolic
data on the host, and that they have absolutely no effect on the target
system.


File: gdb.info,  Node: Remote Debugging,  Next: Configurations,  Prev: Targets,  Up: Top

20 Debugging Remote Programs
****************************

If you are trying to debug a program running on a machine that cannot
run GDB in the usual way, it is often useful to use remote debugging.
For example, you might use remote debugging on an operating system
kernel, or on a small system which does not have a general purpose
operating system powerful enough to run a full-featured debugger.

   Some configurations of GDB have special serial or TCP/IP interfaces
to make this work with particular debugging targets.  In addition, GDB
comes with a generic serial protocol (specific to GDB, but not specific
to any particular target system) which you can use if you write the
remote stubs--the code that runs on the remote system to communicate
with GDB.

   Other remote targets may be available in your configuration of GDB;
use ‘help target’ to list them.

* Menu:

* Connecting::                  Connecting to a remote target
* File Transfer::               Sending files to a remote system
* Server::                      Using the gdbserver program
* Remote Configuration::        Remote configuration
* Remote Stub::                 Implementing a remote stub


File: gdb.info,  Node: Connecting,  Next: File Transfer,  Up: Remote Debugging

20.1 Connecting to a Remote Target
==================================

This section describes how to connect to a remote target, including the
types of connections and their differences, how to set up executable and
symbol files on the host and target, and the commands used for
connecting to and disconnecting from the remote target.

20.1.1 Types of Remote Connections
----------------------------------

GDB supports two types of remote connections, ‘target remote’ mode and
‘target extended-remote’ mode.  Note that many remote targets support
only ‘target remote’ mode.  There are several major differences between
the two types of connections, enumerated here:

Result of detach or program exit
     *With target remote mode:* When the debugged program exits or you
     detach from it, GDB disconnects from the target.  When using
     ‘gdbserver’, ‘gdbserver’ will exit.

     *With target extended-remote mode:* When the debugged program exits
     or you detach from it, GDB remains connected to the target, even
     though no program is running.  You can rerun the program, attach to
     a running program, or use ‘monitor’ commands specific to the
     target.

     When using ‘gdbserver’ in this case, it does not exit unless it was
     invoked using the ‘--once’ option.  If the ‘--once’ option was not
     used, you can ask ‘gdbserver’ to exit using the ‘monitor exit’
     command (*note Monitor Commands for gdbserver::).

Specifying the program to debug
     For both connection types you use the ‘file’ command to specify the
     program on the host system.  If you are using ‘gdbserver’ there are
     some differences in how to specify the location of the program on
     the target.

     *With target remote mode:* You must either specify the program to
     debug on the ‘gdbserver’ command line or use the ‘--attach’ option
     (*note Attaching to a Running Program: Attaching to a program.).

     *With target extended-remote mode:* You may specify the program to
     debug on the ‘gdbserver’ command line, or you can load the program
     or attach to it using GDB commands after connecting to ‘gdbserver’.

     You can start ‘gdbserver’ without supplying an initial command to
     run or process ID to attach.  To do this, use the ‘--multi’ command
     line option.  Then you can connect using ‘target extended-remote’
     and start the program you want to debug (see below for details on
     using the ‘run’ command in this scenario).  Note that the
     conditions under which ‘gdbserver’ terminates depend on how GDB
     connects to it (‘target remote’ or ‘target extended-remote’).  The
     ‘--multi’ option to ‘gdbserver’ has no influence on that.

The ‘run’ command
     *With target remote mode:* The ‘run’ command is not supported.
     Once a connection has been established, you can use all the usual
     GDB commands to examine and change data.  The remote program is
     already running, so you can use commands like ‘step’ and
     ‘continue’.

     *With target extended-remote mode:* The ‘run’ command is supported.
     The ‘run’ command uses the value set by ‘set remote exec-file’
     (*note set remote exec-file::) to select the program to run.
     Command line arguments are supported, except for wildcard expansion
     and I/O redirection (*note Arguments::).

     If you specify the program to debug on the command line, then the
     ‘run’ command is not required to start execution, and you can
     resume using commands like ‘step’ and ‘continue’ as with ‘target
     remote’ mode.

Attaching
     *With target remote mode:* The GDB command ‘attach’ is not
     supported.  To attach to a running program using ‘gdbserver’, you
     must use the ‘--attach’ option (*note Running gdbserver::).

     *With target extended-remote mode:* To attach to a running program,
     you may use the ‘attach’ command after the connection has been
     established.  If you are using ‘gdbserver’, you may also invoke
     ‘gdbserver’ using the ‘--attach’ option (*note Running
     gdbserver::).

     Some remote targets allow GDB to determine the executable file
     running in the process the debugger is attaching to.  In such a
     case, GDB uses the value of ‘exec-file-mismatch’ to handle a
     possible mismatch between the executable file name running in the
     process and the name of the current exec-file loaded by GDB (*note
     set exec-file-mismatch::).

20.1.2 Host and Target Files
----------------------------

GDB, running on the host, needs access to symbol and debugging
information for your program running on the target.  This requires
access to an unstripped copy of your program, and possibly any
associated symbol files.  Note that this section applies equally to both
‘target remote’ mode and ‘target extended-remote’ mode.

   Some remote targets (*note qXfer executable filename read::, and
*note Host I/O Packets::) allow GDB to access program files over the
same connection used to communicate with GDB.  With such a target, if
the remote program is unstripped, the only command you need is ‘target
remote’ (or ‘target extended-remote’).

   If the remote program is stripped, or the target does not support
remote program file access, start up GDB using the name of the local
unstripped copy of your program as the first argument, or use the ‘file’
command.  Use ‘set sysroot’ to specify the location (on the host) of
target libraries (unless your GDB was compiled with the correct sysroot
using ‘--with-sysroot’).  Alternatively, you may use ‘set
solib-search-path’ to specify how GDB locates target libraries.

   The symbol file and target libraries must exactly match the
executable and libraries on the target, with one exception: the files on
the host system should not be stripped, even if the files on the target
system are.  Mismatched or missing files will lead to confusing results
during debugging.  On GNU/Linux targets, mismatched or missing files may
also prevent ‘gdbserver’ from debugging multi-threaded programs.

20.1.3 Remote Connection Commands
---------------------------------

GDB can communicate with the target over a serial line, a local Unix
domain socket, or over an IP network using TCP or UDP.  In each case,
GDB uses the same protocol for debugging your program; only the medium
carrying the debugging packets varies.  The ‘target remote’ and ‘target
extended-remote’ commands establish a connection to the target.  Both
commands accept the same arguments, which indicate the medium to use:

‘target remote SERIAL-DEVICE’
‘target extended-remote SERIAL-DEVICE’
     Use SERIAL-DEVICE to communicate with the target.  For example, to
     use a serial line connected to the device named ‘/dev/ttyb’:

          target remote /dev/ttyb

     If you're using a serial line, you may want to give GDB the
     ‘--baud’ option, or use the ‘set serial baud’ command (*note set
     serial baud: Remote Configuration.) before the ‘target’ command.

‘target remote LOCAL-SOCKET’
‘target extended-remote LOCAL-SOCKET’
     Use LOCAL-SOCKET to communicate with the target.  For example, to
     use a local Unix domain socket bound to the file system entry
     ‘/tmp/gdb-socket0’:

          target remote /tmp/gdb-socket0

     Note that this command has the same form as the command to connect
     to a serial line.  GDB will automatically determine which kind of
     file you have specified and will make the appropriate kind of
     connection.  This feature is not available if the host system does
     not support Unix domain sockets.

‘target remote HOST:PORT’
‘target remote [HOST]:PORT’
‘target remote tcp:HOST:PORT’
‘target remote tcp:[HOST]:PORT’
‘target remote tcp4:HOST:PORT’
‘target remote tcp6:HOST:PORT’
‘target remote tcp6:[HOST]:PORT’
‘target extended-remote HOST:PORT’
‘target extended-remote [HOST]:PORT’
‘target extended-remote tcp:HOST:PORT’
‘target extended-remote tcp:[HOST]:PORT’
‘target extended-remote tcp4:HOST:PORT’
‘target extended-remote tcp6:HOST:PORT’
‘target extended-remote tcp6:[HOST]:PORT’
     Debug using a TCP connection to PORT on HOST.  The HOST may be
     either a host name, a numeric IPv4 address, or a numeric IPv6
     address (with or without the square brackets to separate the
     address from the port); PORT must be a decimal number.  The HOST
     could be the target machine itself, if it is directly connected to
     the net, or it might be a terminal server which in turn has a
     serial line to the target.

     For example, to connect to port 2828 on a terminal server named
     ‘manyfarms’:

          target remote manyfarms:2828

     To connect to port 2828 on a terminal server whose address is
     ‘2001:0db8:85a3:0000:0000:8a2e:0370:7334’, you can either use the
     square bracket syntax:

          target remote [2001:0db8:85a3:0000:0000:8a2e:0370:7334]:2828

     or explicitly specify the IPv6 protocol:

          target remote tcp6:2001:0db8:85a3:0000:0000:8a2e:0370:7334:2828

     This last example may be confusing to the reader, because there is
     no visible separation between the hostname and the port number.
     Therefore, we recommend the user to provide IPv6 addresses using
     square brackets for clarity.  However, it is important to mention
     that for GDB there is no ambiguity: the number after the last colon
     is considered to be the port number.

     If your remote target is actually running on the same machine as
     your debugger session (e.g. a simulator for your target running on
     the same host), you can omit the hostname.  For example, to connect
     to port 1234 on your local machine:

          target remote :1234

     Note that the colon is still required here.

‘target remote udp:HOST:PORT’
‘target remote udp:[HOST]:PORT’
‘target remote udp4:HOST:PORT’
‘target remote udp6:[HOST]:PORT’
‘target extended-remote udp:HOST:PORT’
‘target extended-remote udp:HOST:PORT’
‘target extended-remote udp:[HOST]:PORT’
‘target extended-remote udp4:HOST:PORT’
‘target extended-remote udp6:HOST:PORT’
‘target extended-remote udp6:[HOST]:PORT’
     Debug using UDP packets to PORT on HOST.  For example, to connect
     to UDP port 2828 on a terminal server named ‘manyfarms’:

          target remote udp:manyfarms:2828

     When using a UDP connection for remote debugging, you should keep
     in mind that the 'U' stands for "Unreliable".  UDP can silently
     drop packets on busy or unreliable networks, which will cause havoc
     with your debugging session.

‘target remote | COMMAND’
‘target extended-remote | COMMAND’
     Run COMMAND in the background and communicate with it using a pipe.
     The COMMAND is a shell command, to be parsed and expanded by the
     system's command shell, ‘/bin/sh’; it should expect remote protocol
     packets on its standard input, and send replies on its standard
     output.  You could use this to run a stand-alone simulator that
     speaks the remote debugging protocol, to make net connections using
     programs like ‘ssh’, or for other similar tricks.

     If COMMAND closes its standard output (perhaps by exiting), GDB
     will try to send it a ‘SIGTERM’ signal.  (If the program has
     already exited, this will have no effect.)

   Whenever GDB is waiting for the remote program, if you type the
interrupt character (often ‘Ctrl-c’), GDB attempts to stop the program.
This may or may not succeed, depending in part on the hardware and the
serial drivers the remote system uses.  If you type the interrupt
character once again, GDB displays this prompt:

     Interrupted while waiting for the program.
     Give up (and stop debugging it)?  (y or n)

   In ‘target remote’ mode, if you type ‘y’, GDB abandons the remote
debugging session.  (If you decide you want to try again later, you can
use ‘target remote’ again to connect once more.)  If you type ‘n’, GDB
goes back to waiting.

   In ‘target extended-remote’ mode, typing ‘n’ will leave GDB connected
to the target.

‘detach’
     When you have finished debugging the remote program, you can use
     the ‘detach’ command to release it from GDB control.  Detaching
     from the target normally resumes its execution, but the results
     will depend on your particular remote stub.  After the ‘detach’
     command in ‘target remote’ mode, GDB is free to connect to another
     target.  In ‘target extended-remote’ mode, GDB is still connected
     to the target.

‘disconnect’
     The ‘disconnect’ command closes the connection to the target, and
     the target is generally not resumed.  It will wait for GDB (this
     instance or another one) to connect and continue debugging.  After
     the ‘disconnect’ command, GDB is again free to connect to another
     target.

‘monitor CMD’
     This command allows you to send arbitrary commands directly to the
     remote monitor.  Since GDB doesn't care about the commands it sends
     like this, this command is the way to extend GDB--you can add new
     commands that only the external monitor will understand and
     implement.


File: gdb.info,  Node: File Transfer,  Next: Server,  Prev: Connecting,  Up: Remote Debugging

20.2 Sending files to a remote system
=====================================

Some remote targets offer the ability to transfer files over the same
connection used to communicate with GDB.  This is convenient for targets
accessible through other means, e.g. GNU/Linux systems running
‘gdbserver’ over a network interface.  For other targets, e.g. embedded
devices with only a single serial port, this may be the only way to
upload or download files.

   Not all remote targets support these commands.

‘remote put HOSTFILE TARGETFILE’
     Copy file HOSTFILE from the host system (the machine running GDB)
     to TARGETFILE on the target system.

‘remote get TARGETFILE HOSTFILE’
     Copy file TARGETFILE from the target system to HOSTFILE on the host
     system.

‘remote delete TARGETFILE’
     Delete TARGETFILE from the target system.


File: gdb.info,  Node: Server,  Next: Remote Configuration,  Prev: File Transfer,  Up: Remote Debugging

20.3 Using the ‘gdbserver’ Program
==================================

‘gdbserver’ is a control program for Unix-like systems, which allows you
to connect your program with a remote GDB via ‘target remote’ or ‘target
extended-remote’--but without linking in the usual debugging stub.

   ‘gdbserver’ is not a complete replacement for the debugging stubs,
because it requires essentially the same operating-system facilities
that GDB itself does.  In fact, a system that can run ‘gdbserver’ to
connect to a remote GDB could also run GDB locally!  ‘gdbserver’ is
sometimes useful nevertheless, because it is a much smaller program than
GDB itself.  It is also easier to port than all of GDB, so you may be
able to get started more quickly on a new system by using ‘gdbserver’.
Finally, if you develop code for real-time systems, you may find that
the tradeoffs involved in real-time operation make it more convenient to
do as much development work as possible on another system, for example
by cross-compiling.  You can use ‘gdbserver’ to make a similar choice
for debugging.

   GDB and ‘gdbserver’ communicate via either a serial line or a TCP
connection, using the standard GDB remote serial protocol.

     _Warning:_ ‘gdbserver’ does not have any built-in security.  Do not
     run ‘gdbserver’ connected to any public network; a GDB connection
     to ‘gdbserver’ provides access to the target system with the same
     privileges as the user running ‘gdbserver’.

20.3.1 Running ‘gdbserver’
--------------------------

Run ‘gdbserver’ on the target system.  You need a copy of the program
you want to debug, including any libraries it requires.  ‘gdbserver’
does not need your program's symbol table, so you can strip the program
if necessary to save space.  GDB on the host system does all the symbol
handling.

   To use the server, you must tell it how to communicate with GDB; the
name of your program; and the arguments for your program.  The usual
syntax is:

     target> gdbserver COMM PROGRAM [ ARGS ... ]

   COMM is either a device name (to use a serial line), or a TCP
hostname and portnumber, or ‘-’ or ‘stdio’ to use stdin/stdout of
‘gdbserver’.  For example, to debug Emacs with the argument ‘foo.txt’
and communicate with GDB over the serial port ‘/dev/com1’:

     target> gdbserver /dev/com1 emacs foo.txt

   ‘gdbserver’ waits passively for the host GDB to communicate with it.

   To use a TCP connection instead of a serial line:

     target> gdbserver host:2345 emacs foo.txt

   The only difference from the previous example is the first argument,
specifying that you are communicating with the host GDB via TCP. The
‘host:2345’ argument means that ‘gdbserver’ is to expect a TCP
connection from machine ‘host’ to local TCP port 2345.  (Currently, the
‘host’ part is ignored.)  You can choose any number you want for the
port number as long as it does not conflict with any TCP ports already
in use on the target system (for example, ‘23’ is reserved for
‘telnet’).(1)  You must use the same port number with the host GDB
‘target remote’ command.

   The ‘stdio’ connection is useful when starting ‘gdbserver’ with ssh:

     (gdb) target remote | ssh -T hostname gdbserver - hello

   The ‘-T’ option to ssh is provided because we don't need a remote
pty, and we don't want escape-character handling.  Ssh does this by
default when a command is provided, the flag is provided to make it
explicit.  You could elide it if you want to.

   Programs started with stdio-connected gdbserver have ‘/dev/null’ for
‘stdin’, and ‘stdout’,‘stderr’ are sent back to gdb for display through
a pipe connected to gdbserver.  Both ‘stdout’ and ‘stderr’ use the same
pipe.

20.3.1.1 Attaching to a Running Program
.......................................

On some targets, ‘gdbserver’ can also attach to running programs.  This
is accomplished via the ‘--attach’ argument.  The syntax is:

     target> gdbserver --attach COMM PID

   PID is the process ID of a currently running process.  It isn't
necessary to point ‘gdbserver’ at a binary for the running process.

   In ‘target extended-remote’ mode, you can also attach using the GDB
attach command (*note Attaching in Types of Remote Connections::).

   You can debug processes by name instead of process ID if your target
has the ‘pidof’ utility:

     target> gdbserver --attach COMM `pidof PROGRAM`

   In case more than one copy of PROGRAM is running, or PROGRAM has
multiple threads, most versions of ‘pidof’ support the ‘-s’ option to
only return the first process ID.

20.3.1.2 TCP port allocation lifecycle of ‘gdbserver’
.....................................................

This section applies only when ‘gdbserver’ is run to listen on a TCP
port.

   ‘gdbserver’ normally terminates after all of its debugged processes
have terminated in ‘target remote’ mode.  On the other hand, for ‘target
extended-remote’, ‘gdbserver’ stays running even with no processes left.
GDB normally terminates the spawned debugged process on its exit, which
normally also terminates ‘gdbserver’ in the ‘target remote’ mode.
Therefore, when the connection drops unexpectedly, and GDB cannot ask
‘gdbserver’ to kill its debugged processes, ‘gdbserver’ stays running
even in the ‘target remote’ mode.

   When ‘gdbserver’ stays running, GDB can connect to it again later.
Such reconnecting is useful for features like *note disconnected
tracing::.  For completeness, at most one GDB can be connected at a
time.

   By default, ‘gdbserver’ keeps the listening TCP port open, so that
subsequent connections are possible.  However, if you start ‘gdbserver’
with the ‘--once’ option, it will stop listening for any further
connection attempts after connecting to the first GDB session.  This
means no further connections to ‘gdbserver’ will be possible after the
first one.  It also means ‘gdbserver’ will terminate after the first
connection with remote GDB has closed, even for unexpectedly closed
connections and even in the ‘target extended-remote’ mode.  The ‘--once’
option allows reusing the same port number for connecting to multiple
instances of ‘gdbserver’ running on the same host, since each instance
closes its port after the first connection.

20.3.1.3 Other Command-Line Arguments for ‘gdbserver’
.....................................................

You can use the ‘--multi’ option to start ‘gdbserver’ without specifying
a program to debug or a process to attach to.  Then you can attach in
‘target extended-remote’ mode and run or attach to a program.  For more
information, *note --multi Option in Types of Remote Connections::.

   The ‘--debug[=option1,option2,...]’ option tells ‘gdbserver’ to
display extra diagnostic information about the debugging process.  The
options (OPTION1, OPTION2, etc) control for which areas of ‘gdbserver’
additional information will be displayed, possible values are:

‘all’
     This enables all available diagnostic output.
‘threads’
     This enables diagnostic output related to threading.  Currently
     other general diagnostic output is included in this category, but
     this could change in future releases of ‘gdbserver’.
‘event-loop’
     This enables event-loop specific diagnostic output.
‘remote’
     This enables diagnostic output related to the transfer of remote
     protocol packets too and from the debugger.

If no options are passed to ‘--debug’ then this is treated as equivalent
to ‘--debug=threads’.  This could change in future releases of
‘gdbserver’.  The options passed to ‘--debug’ are processed left to
right, and individual options can be prefixed with the ‘-’ (minus)
character to disable diagnostic output from this area, so it is possible
to use:

       target> gdbserver --debug=all,-event-loop

In order to enable all diagnostic output except that for the event-loop.

   The ‘--debug-file=FILENAME’ option tells ‘gdbserver’ to write any
debug output to the given FILENAME.  These options are intended for
‘gdbserver’ development and for bug reports to the developers.

   The ‘--debug-format=option1[,option2,...]’ option tells ‘gdbserver’
to include additional information in each output.  Possible options are:

‘none’
     Turn off all extra information in debugging output.
‘all’
     Turn on all extra information in debugging output.
‘timestamps’
     Include a timestamp in each line of debugging output.

   Options are processed in order.  Thus, for example, if ‘none’ appears
last then no additional information is added to debugging output.

   The ‘--wrapper’ option specifies a wrapper to launch programs for
debugging.  The option should be followed by the name of the wrapper,
then any command-line arguments to pass to the wrapper, then ‘--’
indicating the end of the wrapper arguments.

   ‘gdbserver’ runs the specified wrapper program with a combined
command line including the wrapper arguments, then the name of the
program to debug, then any arguments to the program.  The wrapper runs
until it executes your program, and then GDB gains control.

   You can use any program that eventually calls ‘execve’ with its
arguments as a wrapper.  Several standard Unix utilities do this, e.g.
‘env’ and ‘nohup’.  Any Unix shell script ending with ‘exec "$@"’ will
also work.

   For example, you can use ‘env’ to pass an environment variable to the
debugged program, without setting the variable in ‘gdbserver’'s
environment:

     $ gdbserver --wrapper env LD_PRELOAD=libtest.so -- :2222 ./testprog

   The ‘--selftest’ option runs the self tests in ‘gdbserver’:

     $ gdbserver --selftest
     Ran 2 unit tests, 0 failed

   These tests are disabled in release.

20.3.2 Connecting to ‘gdbserver’
--------------------------------

The basic procedure for connecting to the remote target is:

   • Run GDB on the host system.

   • Make sure you have the necessary symbol files (*note Host and
     target files::).  Load symbols for your application using the
     ‘file’ command before you connect.  Use ‘set sysroot’ to locate
     target libraries (unless your GDB was compiled with the correct
     sysroot using ‘--with-sysroot’).

   • Connect to your target (*note Connecting to a Remote Target:
     Connecting.).  For TCP connections, you must start up ‘gdbserver’
     prior to using the ‘target’ command.  Otherwise you may get an
     error whose text depends on the host system, but which usually
     looks something like ‘Connection refused’.  Don't use the ‘load’
     command in GDB when using ‘target remote’ mode, since the program
     is already on the target.

20.3.3 Monitor Commands for ‘gdbserver’
---------------------------------------

During a GDB session using ‘gdbserver’, you can use the ‘monitor’
command to send special requests to ‘gdbserver’.  Here are the available
commands.

‘monitor help’
     List the available monitor commands.

‘monitor set debug off’
     Disable all internal logging from gdbserver.

‘monitor set debug on’
     Enable some general logging from within gdbserver.  Currently this
     is equivalent to ‘monitor set debug threads on’, but this might
     change in future releases of gdbserver.

‘monitor set debug threads off’
‘monitor set debug threads on’
     Disable or enable specific logging messages associated with thread
     handling in gdbserver.  Currently this category also includes
     additional output not specifically related to thread handling, this
     could change in future releases of gdbserver.

‘monitor set debug remote off’
‘monitor set debug remote on’
     Disable or enable specific logging messages associated with the
     remote protocol (*note Remote Protocol::).

‘monitor set debug event-loop off’
‘monitor set debug event-loop on’
     Disable or enable specific logging messages associated with
     gdbserver's event-loop.

‘monitor set debug-file filename’
‘monitor set debug-file’
     Send any debug output to the given file, or to stderr.

‘monitor set debug-format option1[,option2,...]’
     Specify additional text to add to debugging messages.  Possible
     options are:

     ‘none’
          Turn off all extra information in debugging output.
     ‘all’
          Turn on all extra information in debugging output.
     ‘timestamps’
          Include a timestamp in each line of debugging output.

     Options are processed in order.  Thus, for example, if ‘none’
     appears last then no additional information is added to debugging
     output.

‘monitor set libthread-db-search-path [PATH]’
     When this command is issued, PATH is a colon-separated list of
     directories to search for ‘libthread_db’ (*note set
     libthread-db-search-path: Threads.).  If you omit PATH,
     ‘libthread-db-search-path’ will be reset to its default value.

     The special entry ‘$pdir’ for ‘libthread-db-search-path’ is not
     supported in ‘gdbserver’.

‘monitor exit’
     Tell gdbserver to exit immediately.  This command should be
     followed by ‘disconnect’ to close the debugging session.
     ‘gdbserver’ will detach from any attached processes and kill any
     processes it created.  Use ‘monitor exit’ to terminate ‘gdbserver’
     at the end of a multi-process mode debug session.

20.3.4 Tracepoints support in ‘gdbserver’
-----------------------------------------

On some targets, ‘gdbserver’ supports tracepoints and fast tracepoints.

   For fast tracepoints to work, a special library called the
“in-process agent” (IPA), must be loaded in the inferior process.  This
library is built and distributed as an integral part of ‘gdbserver’.

   There are several ways to load the in-process agent in your program:

‘Specifying it as dependency at link time’

     You can link your program dynamically with the in-process agent
     library.  On most systems, this is accomplished by adding
     ‘-linproctrace’ to the link command.

‘Using the system's preloading mechanisms’

     You can force loading the in-process agent at startup time by using
     your system's support for preloading shared libraries.  Many Unixes
     support the concept of preloading user defined libraries.  In most
     cases, you do that by specifying ‘LD_PRELOAD=libinproctrace.so’ in
     the environment.  See also the description of ‘gdbserver’'s
     ‘--wrapper’ command line option.

‘Using GDB to force loading the agent at run time’

     On some systems, you can force the inferior to load a shared
     library, by calling a dynamic loader function in the inferior that
     takes care of dynamically looking up and loading a shared library.
     On most Unix systems, the function is ‘dlopen’.  You'll use the
     ‘call’ command for that.  For example:

          (gdb) call dlopen ("libinproctrace.so", ...)

     Note that on most Unix systems, for the ‘dlopen’ function to be
     available, the program needs to be linked with ‘-ldl’.

   On systems that have a userspace dynamic loader, like most Unix
systems, when you connect to ‘gdbserver’ using ‘target remote’, you'll
find that the program is stopped at the dynamic loader's entry point,
and no shared library has been loaded in the program's address space
yet, including the in-process agent.  In that case, before being able to
use any of the fast or static tracepoints features, you need to let the
loader run and load the shared libraries.  The simplest way to do that
is to run the program to the main procedure.  E.g., if debugging a C or
C++ program, start ‘gdbserver’ like so:

     $ gdbserver :9999 myprogram

   Start GDB and connect to ‘gdbserver’ like so, and run to main:

     $ gdb myprogram
     (gdb) target remote myhost:9999
     0x00007f215893ba60 in ?? () from /lib64/ld-linux-x86-64.so.2
     (gdb) b main
     (gdb) continue

   The in-process tracing agent library should now be loaded into the
process; you can confirm it with the ‘info sharedlibrary’ command, which
will list ‘libinproctrace.so’ as loaded in the process.  You are now
ready to install fast tracepoints, list static tracepoint markers, probe
static tracepoints markers, and start tracing.

   ---------- Footnotes ----------

   (1) If you choose a port number that conflicts with another service,
‘gdbserver’ prints an error message and exits.

