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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
18.0.50.20260501-37cb3cd8f89.

   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: Remote Protocol,  Next: Agent Expressions,  Prev: Maintenance Commands,  Up: Top

Appendix E GDB Remote Serial Protocol
*************************************

* Menu:

* Overview::
* Standard Replies::
* Packets::
* Stop Reply Packets::
* General Query Packets::
* Architecture-Specific Protocol Details::
* Tracepoint Packets::
* Host I/O Packets::
* Interrupts::
* Notification Packets::
* Remote Non-Stop::
* Packet Acknowledgment::
* Examples::
* File-I/O Remote Protocol Extension::
* Library List Format::
* Library List Format for SVR4 Targets::
* Memory Map Format::
* Thread List Format::
* Traceframe Info Format::
* Branch Trace Format::
* Branch Trace Configuration Format::


File: gdb.info,  Node: Overview,  Next: Standard Replies,  Up: Remote Protocol

E.1 Overview
============

There may be occasions when you need to know something about the
protocol--for example, if there is only one serial port to your target
machine, you might want your program to do something special if it
recognizes a packet meant for GDB.

   In the examples below, ‘->’ and ‘<-’ are used to indicate transmitted
and received data, respectively.

   All GDB commands and responses (other than acknowledgments and
notifications, see *note Notification Packets::) are sent as a PACKET.
A PACKET is introduced with the character ‘$’, the actual PACKET-DATA,
and the terminating character ‘#’ followed by a two-digit CHECKSUM:

     $PACKET-DATA#CHECKSUM

The two-digit CHECKSUM is computed as the modulo 256 sum of all
characters between the leading ‘$’ and the trailing ‘#’ (an eight bit
unsigned checksum).

   Implementors should note that prior to GDB 5.0 the protocol
specification also included an optional two-digit SEQUENCE-ID:

     $SEQUENCE-ID:PACKET-DATA#CHECKSUM

That SEQUENCE-ID was appended to the acknowledgment.  GDB has never
output SEQUENCE-IDs.  Stubs that handle packets added since GDB 5.0 must
not accept SEQUENCE-ID.

   When either the host or the target machine receives a packet, the
first response expected is an acknowledgment: either ‘+’ (to indicate
the package was received correctly) or ‘-’ (to request retransmission):

     -> $PACKET-DATA#CHECKSUM
     <- +

   The ‘+’/‘-’ acknowledgments can be disabled once a connection is
established.  *Note Packet Acknowledgment::, for details.

   The host (GDB) sends COMMANDs, and the target (the debugging stub
incorporated in your program) sends a RESPONSE.  In the case of step and
continue COMMANDs, the response is only sent when the operation has
completed, and the target has again stopped all threads in all attached
processes.  This is the default all-stop mode behavior, but the remote
protocol also supports GDB's non-stop execution mode; see *note Remote
Non-Stop::, for details.

   PACKET-DATA consists of a sequence of characters with the exception
of ‘#’ and ‘$’ (see ‘X’ packet for additional exceptions).

   Fields within the packet should be separated using ‘,’ ‘;’ or ‘:’.
Except where otherwise noted all numbers are represented in HEX with
leading zeros suppressed.

   Implementors should note that prior to GDB 5.0, the character ‘:’
could not appear as the third character in a packet (as it would
potentially conflict with the SEQUENCE-ID).

   Binary data in most packets is encoded as two hexadecimal digits per
byte of binary data.  This allowed the traditional remote protocol to
work over connections which were only seven-bit clean.  Some packets
designed more recently assume an eight-bit clean connection, and use a
more efficient encoding to send and receive binary data.

   The binary data representation uses ‘7d’ (ASCII ‘}’) as an escape
character.  Any escaped byte is transmitted as the escape character
followed by the original character XORed with ‘0x20’.  For example, the
byte ‘0x7d’ would be transmitted as the two bytes ‘0x7d 0x5d’.  The
bytes ‘0x23’ (ASCII ‘#’), ‘0x24’ (ASCII ‘$’), and ‘0x7d’ (ASCII ‘}’)
must always be escaped.  Responses sent by the stub must also escape
‘0x2a’ (ASCII ‘*’), so that it is not interpreted as the start of a
run-length encoded sequence (described next).

   Response DATA can be run-length encoded to save space.  Run-length
encoding replaces runs of identical characters with one instance of the
repeated character, followed by a ‘*’ and a repeat count.  The repeat
count is itself sent encoded, to avoid binary characters in DATA: a
value of N is sent as ‘N+29’.  For a repeat count greater or equal to 3,
this produces a printable ASCII character, e.g. a space (ASCII code 32)
for a repeat count of 3.  (This is because run-length encoding starts to
win for counts 3 or more.)  Thus, for example, ‘0* ’ is a run-length
encoding of "0000": the space character after ‘*’ means repeat the
leading ‘0’ ‘32 - 29 = 3’ more times.

   The printable characters ‘#’ and ‘$’ or with a numeric value greater
than 126 must not be used.  Runs of six repeats (‘#’) or seven repeats
(‘$’) can be expanded using a repeat count of only five (‘"’).  For
example, ‘00000000’ can be encoded as ‘0*"00’.

   *Note Standard Replies::, for standard error responses, and how to
respond indicating a command is not supported.

   In describing packets (commands and responses), each description has
a template showing the overall syntax, followed by an explanation of the
packet's meaning.  We include spaces in some of the templates for
clarity; these are not part of the packet's syntax.  No GDB packet uses
spaces to separate its components.  For example, a template like ‘foo
BAR BAZ’ describes a packet beginning with the three ASCII bytes ‘foo’,
followed by a BAR, followed directly by a BAZ.  GDB does not transmit a
space character between the ‘foo’ and the BAR, or between the BAR and
the BAZ.

   We place optional portions of a packet in [square brackets]; for
example, a template like ‘c [ADDR]’ describes a packet beginning with
the single ASCII character ‘c’, possibly followed by an ADDR.

   At a minimum, a stub is required to support the ‘?’ command to tell
GDB the reason for halting, ‘g’ and ‘G’ commands for register access,
and the ‘m’ and ‘M’ commands for memory access.  Stubs that only control
single-threaded targets can implement run control with the ‘c’
(continue) command, and if the target architecture supports
hardware-assisted single-stepping, the ‘s’ (step) command.  Stubs that
support multi-threading targets should support the ‘vCont’ command.  All
other commands are optional.


File: gdb.info,  Node: Standard Replies,  Next: Packets,  Prev: Overview,  Up: Remote Protocol

E.2 Standard Replies
====================

The remote protocol specifies a few standard replies.  All commands
support these, except as noted in the individual command descriptions.

empty response

     An empty response (raw character sequence ‘$#00’) means the COMMAND
     is not supported by the stub.  This way it is possible to extend
     the protocol.  A newer GDB can tell if a command is supported based
     on that response (but see also *note qSupported::).

‘E XX’
     An error has occurred; XX is a two-digit hexadecimal error number.
     In almost all cases, the protocol does not specify the meaning of
     the error numbers; GDB usually ignores the numbers, or displays
     them to the user without further interpretation.

‘E.ERRTEXT’
     An error has occurred; ERRTEXT is the textual error message,
     encoded in ASCII.


File: gdb.info,  Node: Packets,  Next: Stop Reply Packets,  Prev: Standard Replies,  Up: Remote Protocol

E.3 Packets
===========

The following table provides a complete list of all currently defined
COMMANDs and their corresponding response DATA.  *Note File-I/O Remote
Protocol Extension::, for details about the File I/O extension of the
remote protocol.

   Each packet's description has a template showing the packet's overall
syntax, followed by an explanation of the packet's meaning.  We include
spaces in some of the templates for clarity; these are not part of the
packet's syntax.  No GDB packet uses spaces to separate its components.
For example, a template like ‘foo BAR BAZ’ describes a packet beginning
with the three ASCII bytes ‘foo’, followed by a BAR, followed directly
by a BAZ.  GDB does not transmit a space character between the ‘foo’ and
the BAR, or between the BAR and the BAZ.

   Several packets and replies include a THREAD-ID field to identify a
thread.  Normally these are positive numbers with a target-specific
interpretation, formatted as big-endian hex strings.  A THREAD-ID can
also be a literal ‘-1’ to indicate all threads, or ‘0’ to pick any
thread.

   In addition, the remote protocol supports a multiprocess feature in
which the THREAD-ID syntax is extended to optionally include both
process and thread ID fields, as ‘pPID.TID’.  The PID (process) and TID
(thread) components each have the format described above: a positive
number with target-specific interpretation formatted as a big-endian hex
string, literal ‘-1’ to indicate all processes or threads
(respectively), or ‘0’ to indicate an arbitrary process or thread.
Specifying just a process, as ‘pPID’, is equivalent to ‘pPID.-1’.  It is
an error to specify all processes but a specific thread, such as
‘p-1.TID’.  Note that the ‘p’ prefix is _not_ used for those packets and
replies explicitly documented to include a process ID, rather than a
THREAD-ID.

   The multiprocess THREAD-ID syntax extensions are only used if both
GDB and the stub report support for the ‘multiprocess’ feature using
‘qSupported’.  *Note multiprocess extensions::, for more information.

   Note that all packet forms beginning with an upper- or lower-case
letter, other than those described here, are reserved for future use.

   Here are the packet descriptions.

‘!’
     Enable extended mode.  In extended mode, the remote server is made
     persistent.  The ‘R’ packet is used to restart the program being
     debugged.

     Reply:
     ‘OK’
          The remote target both supports and has enabled extended mode.

‘?’
     This is sent when connection is first established to query the
     reason the target halted.  The reply is the same as for step and
     continue.  This packet has a special interpretation when the target
     is in non-stop mode; see *note Remote Non-Stop::.

     Reply: *Note Stop Reply Packets::, for the reply specifications.

‘A ARGLEN,ARGNUM,ARG,...’
     Initialized ‘argv[]’ array passed into program.  ARGLEN specifies
     the number of bytes in the hex encoded byte stream ARG.  See
     ‘gdbserver’ for more details.

     Reply:
     ‘OK’
          The arguments were set.

‘b BAUD’
     (Don't use this packet; its behavior is not well-defined.)  Change
     the serial line speed to BAUD.

     JTC: _When does the transport layer state change?  When it's
     received, or after the ACK is transmitted.  In either case, there
     are problems if the command or the acknowledgment packet is
     dropped._

     Stan: _If people really wanted to add something like this, and get
     it working for the first time, they ought to modify ser-unix.c to
     send some kind of out-of-band message to a specially-setup stub and
     have the switch happen "in between" packets, so that from remote
     protocol's point of view, nothing actually happened._

‘B ADDR,MODE’
     Set (MODE is ‘S’) or clear (MODE is ‘C’) a breakpoint at ADDR.

     Don't use this packet.  Use the ‘Z’ and ‘z’ packets instead (*note
     insert breakpoint or watchpoint packet::).

‘bc’
     Backward continue.  Execute the target system in reverse.  No
     parameter.  *Note Reverse Execution::, for more information.

     Reply: *Note Stop Reply Packets::, for the reply specifications.

‘bs’
     Backward single step.  Execute one instruction in reverse.  No
     parameter.  *Note Reverse Execution::, for more information.

     Reply: *Note Stop Reply Packets::, for the reply specifications.

‘c [ADDR]’
     Continue at ADDR, which is the address to resume.  If ADDR is
     omitted, resume at current address.

     This packet is deprecated for multi-threading support.  *Note vCont
     packet::.

     Reply: *Note Stop Reply Packets::, for the reply specifications.

‘C SIG[;ADDR]’
     Continue with signal SIG (hex signal number).  If ‘;ADDR’ is
     omitted, resume at same address.

     This packet is deprecated for multi-threading support.  *Note vCont
     packet::.

     Reply: *Note Stop Reply Packets::, for the reply specifications.

‘d’
     Toggle debug flag.

     Don't use this packet; instead, define a general set packet (*note
     General Query Packets::).

‘D’
‘D;PID’
     The first form of the packet is used to detach GDB from the remote
     system.  It is sent to the remote target before GDB disconnects via
     the ‘detach’ command.

     The second form, including a process ID, is used when multiprocess
     protocol extensions are enabled (*note multiprocess extensions::),
     to detach only a specific process.  The PID is specified as a
     big-endian hex string.

     Reply:
     ‘OK’
          for success

‘F RC,EE,CF;XX’
     A reply from GDB to an ‘F’ packet sent by the target.  This is part
     of the File-I/O protocol extension.  *Note File-I/O Remote Protocol
     Extension::, for the specification.

‘g’
     Read general registers.

     Reply:
     ‘XX...’
          Each byte of register data is described by two hex digits.
          The bytes with the register are transmitted in target byte
          order.  The size of each register and their position within
          the ‘g’ packet are determined by the target description (*note
          Target Descriptions::); in the absence of a target
          description, this is done using code internal to GDB;
          typically this is some customary register layout for the
          architecture in question.

          When reading registers, the stub may also return a string of
          literal ‘x’'s in place of the register data digits, to
          indicate that the corresponding register's value is
          unavailable.  For example, when reading registers from a trace
          frame (*note Using the Collected Data: Analyze Collected
          Data.), this means that the register has not been collected in
          the trace frame.  When reading registers from a live program,
          this indicates that the stub has no means to access the
          register contents, even though the corresponding register is
          known to exist.  Note that if a register truly does not exist
          on the target, then it is better to not include it in the
          target description in the first place.

          For example, for an architecture with 4 registers of 4 bytes
          each, the following reply indicates to GDB that registers 0
          and 2 are unavailable, while registers 1 and 3 are available,
          and both have zero value:

               -> g
               <- xxxxxxxx00000000xxxxxxxx00000000

‘G XX...’
     Write general registers.  *Note read registers packet::, for a
     description of the XX... data.

     Reply:
     ‘OK’
          for success

‘H OP THREAD-ID’
     Set thread for subsequent operations (‘m’, ‘M’, ‘g’, ‘G’, et.al.).
     Depending on the operation to be performed, OP should be ‘c’ for
     step and continue operations (note that this is deprecated,
     supporting the ‘vCont’ command is a better option), and ‘g’ for
     other operations.  The thread designator THREAD-ID has the format
     and interpretation described in *note thread-id syntax::.

     Reply:
     ‘OK’
          for success

‘i [ADDR[,NNN]]’
     Step the remote target by a single clock cycle.  If ‘,NNN’ is
     present, cycle step NNN cycles.  If ADDR is present, cycle step
     starting at that address.

‘I’
     Signal, then cycle step.  *Note step with signal packet::.  *Note
     cycle step packet::.

‘k’
     Kill request.

     The exact effect of this packet is not specified.

     For a bare-metal target, it may power cycle or reset the target
     system.  For that reason, the ‘k’ packet has no reply.

     For a single-process target, it may kill that process if possible.

     A multiple-process target may choose to kill just one process, or
     all that are under GDB's control.  For more precise control, use
     the vKill packet (*note vKill packet::).

     If the target system immediately closes the connection in response
     to ‘k’, GDB does not consider the lack of packet acknowledgment to
     be an error, and assumes the kill was successful.

     If connected using ‘target extended-remote’, and the target does
     not close the connection in response to a kill request, GDB probes
     the target state as if a new connection was opened (*note ?
     packet::).

‘m ADDR,LENGTH’
     Read LENGTH addressable memory units starting at address ADDR
     (*note addressable memory unit::).  Note that ADDR does not have to
     be aligned to any particular boundary.

     The stub need not use any particular size or alignment when
     gathering data from memory for the response; even if ADDR is
     word-aligned and LENGTH is a multiple of the word size, the stub is
     free to use byte accesses, or not.  For this reason, this packet
     may not be suitable for accessing memory-mapped I/O devices.

     Reply:
     ‘XX...’
          Memory contents; each byte is transmitted as a two-digit
          hexadecimal number.  The reply may contain fewer addressable
          memory units than requested if the server was reading from a
          trace frame memory and was able to read only part of the
          region of memory.

     Unlike most packets, this packet does not support ‘E.ERRTEXT’-style
     textual error replies (*note textual error reply::) by default.
     Stubs should be careful to only send such a reply if GDB reported
     support for it with the ‘error-message’ feature (*note
     error-message::).

‘M ADDR,LENGTH:XX...’
     Write LENGTH addressable memory units starting at address ADDR
     (*note addressable memory unit::).  The data is given by XX...;
     each byte is transmitted as a two-digit hexadecimal number.

     Reply:
     ‘OK’
          All the data was written successfully.  (If only part of the
          data was written, this command returns an error.)

‘p N’
     Read the value of register N; N is in hex.  *Note read registers
     packet::, for a description of how the returned register value is
     encoded.

     Reply:
     ‘XX...’
          the register's value

‘P N...=R...’
     Write register N... with value R....  The register number N is in
     hexadecimal, and R... contains two hex digits for each byte in the
     register (target byte order).

     Reply:
     ‘OK’
          for success

‘q NAME PARAMS...’
‘Q NAME PARAMS...’
     General query (‘q’) and set (‘Q’).  These packets are described
     fully in *note General Query Packets::.

‘r’
     Reset the entire system.

     Don't use this packet; use the ‘R’ packet instead.

‘R XX’
     Restart the program being debugged.  The XX, while needed, is
     ignored.  This packet is only available in extended mode (*note
     extended mode::).

     The ‘R’ packet has no reply.

‘s [ADDR]’
     Single step, resuming at ADDR.  If ADDR is omitted, resume at same
     address.

     This packet is deprecated for multi-threading support.  *Note vCont
     packet::.

     Reply: *Note Stop Reply Packets::, for the reply specifications.

‘S SIG[;ADDR]’
     Step with signal.  This is analogous to the ‘C’ packet, but
     requests a single-step, rather than a normal resumption of
     execution.

     This packet is deprecated for multi-threading support.  *Note vCont
     packet::.

     Reply: *Note Stop Reply Packets::, for the reply specifications.

‘t ADDR:PP,MM’
     Search backwards starting at address ADDR for a match with pattern
     PP and mask MM, both of which are are 4 byte long.  There must be
     at least 3 digits in ADDR.

‘T THREAD-ID’
     Find out if the thread THREAD-ID is alive.  *Note thread-id
     syntax::.

     Reply:
     ‘OK’
          thread is still alive

‘v’
     Packets starting with ‘v’ are identified by a multi-letter name, up
     to the first ‘;’ or ‘?’ (or the end of the packet).

‘vAttach;PID’
     Attach to a new process with the specified process ID PID.  The
     process ID is a hexadecimal integer identifying the process.  In
     all-stop mode, all threads in the attached process are stopped; in
     non-stop mode, it may be attached without being stopped if that is
     supported by the target.

     This packet is only available in extended mode (*note extended
     mode::).

     Reply:
     ‘Any stop packet’
          for success in all-stop mode (*note Stop Reply Packets::)
     ‘OK’
          for success in non-stop mode (*note Remote Non-Stop::)

‘vCont[;ACTION[:THREAD-ID]]...’
     Resume the inferior, specifying different actions for each thread.

     For each inferior thread, the leftmost action with a matching
     THREAD-ID is applied.  Threads that don't match any action remain
     in their current state.  Thread IDs are specified using the syntax
     described in *note thread-id syntax::.  If multiprocess extensions
     (*note multiprocess extensions::) are supported, actions can be
     specified to match all threads in a process by using the ‘pPID.-1’
     form of the THREAD-ID.  An action with no THREAD-ID matches all
     threads.  Specifying no actions is an error.

     Currently supported actions are:

     ‘c’
          Continue.
     ‘C SIG’
          Continue with signal SIG.  The signal SIG should be two hex
          digits.
     ‘s’
          Step.
     ‘S SIG’
          Step with signal SIG.  The signal SIG should be two hex
          digits.
     ‘t’
          Stop.
     ‘r START,END’
          Step once, and then keep stepping as long as the thread stops
          at addresses between START (inclusive) and END (exclusive).
          The remote stub reports a stop reply when either the thread
          goes out of the range or is stopped due to an unrelated
          reason, such as hitting a breakpoint.  *Note range stepping::.

          If the range is empty (START == END), then the action becomes
          equivalent to the ‘s’ action.  In other words, single-step
          once, and report the stop (even if the stepped instruction
          jumps to START).

          (A stop reply may be sent at any point even if the PC is still
          within the stepping range; for example, it is valid to
          implement this packet in a degenerate way as a single
          instruction step operation.)

     The optional argument ADDR normally associated with the ‘c’, ‘C’,
     ‘s’, and ‘S’ packets is not supported in ‘vCont’.

     The ‘t’ action is only relevant in non-stop mode (*note Remote
     Non-Stop::) and may be ignored by the stub otherwise.  A stop reply
     should be generated for any affected thread not already stopped.
     When a thread is stopped by means of a ‘t’ action, the
     corresponding stop reply should indicate that the thread has
     stopped with signal ‘0’, regardless of whether the target uses some
     other signal as an implementation detail.

     The server must ignore ‘c’, ‘C’, ‘s’, ‘S’, and ‘r’ actions for
     threads that are already running.  Conversely, the server must
     ignore ‘t’ actions for threads that are already stopped.

     _Note:_ In non-stop mode, a thread is considered running until GDB
     acknowledges an asynchronous stop notification for it with the
     ‘vStopped’ packet (*note Remote Non-Stop::).

     The stub must support ‘vCont’ if it reports support for
     multiprocess extensions (*note multiprocess extensions::).

     Reply: *Note Stop Reply Packets::, for the reply specifications.

‘vCont?’
     Request a list of actions supported by the ‘vCont’ packet.

     Reply:
     ‘vCont[;ACTION...]’
          The ‘vCont’ packet is supported.  Each ACTION is a supported
          command in the ‘vCont’ packet.

‘vCtrlC’
     Interrupt remote target as if a control-C was pressed on the remote
     terminal.  This is the equivalent to reacting to the ‘^C’ (‘\003’,
     the control-C character) character in all-stop mode while the
     target is running, except this works in non-stop mode.  *Note
     interrupting remote targets::, for more info on the all-stop
     variant.

     Reply:
     ‘OK’
          for success

‘vFile:OPERATION:PARAMETER...’
     Perform a file operation on the target system.  For details, see
     *note Host I/O Packets::.

‘vFlashErase:ADDR,LENGTH’
     Direct the stub to erase LENGTH bytes of flash starting at ADDR.
     The region may enclose any number of flash blocks, but its start
     and end must fall on block boundaries, as indicated by the flash
     block size appearing in the memory map (*note Memory Map Format::).
     GDB groups flash memory programming operations together, and sends
     a ‘vFlashDone’ request after each group; the stub is allowed to
     delay erase operation until the ‘vFlashDone’ packet is received.

     Reply:
     ‘OK’
          for success

‘vFlashWrite:ADDR:XX...’
     Direct the stub to write data to flash address ADDR.  The data is
     passed in binary form using the same encoding as for the ‘X’ packet
     (*note Binary Data::).  The memory ranges specified by
     ‘vFlashWrite’ packets preceding a ‘vFlashDone’ packet must not
     overlap, and must appear in order of increasing addresses (although
     ‘vFlashErase’ packets for higher addresses may already have been
     received; the ordering is guaranteed only between ‘vFlashWrite’
     packets).  If a packet writes to an address that was neither erased
     by a preceding ‘vFlashErase’ packet nor by some other
     target-specific method, the results are unpredictable.

     Reply:
     ‘OK’
          for success
     ‘E.memtype’
          for vFlashWrite addressing non-flash memory

‘vFlashDone’
     Indicate to the stub that flash programming operation is finished.
     The stub is permitted to delay or batch the effects of a group of
     ‘vFlashErase’ and ‘vFlashWrite’ packets until a ‘vFlashDone’ packet
     is received.  The contents of the affected regions of flash memory
     are unpredictable until the ‘vFlashDone’ request is completed.

‘vKill;PID’
     Kill the process with the specified process ID PID, which is a
     hexadecimal integer identifying the process.  This packet is used
     in preference to ‘k’ when multiprocess protocol extensions are
     supported; see *note multiprocess extensions::.

     Reply:
     ‘OK’
          for success

‘vMustReplyEmpty’
     The correct reply to an unknown ‘v’ packet is to return the empty
     string, however, some older versions of ‘gdbserver’ would
     incorrectly return ‘OK’ for unknown ‘v’ packets.

     The ‘vMustReplyEmpty’ is used as a feature test to check how
     ‘gdbserver’ handles unknown packets, it is important that this
     packet be handled in the same way as other unknown ‘v’ packets.  If
     this packet is handled differently to other unknown ‘v’ packets
     then it is possible that GDB may run into problems in other areas,
     specifically around use of ‘vFile:setfs:’.

‘vRun;FILENAME[;ARGUMENT]...’
     Run the program FILENAME, passing it each ARGUMENT on its command
     line.  The file and arguments are hex-encoded strings.  If FILENAME
     is an empty string, the stub may use a default program (e.g. the
     last program run).  The program is created in the stopped state.

     If GDB sent the ‘single-inf-arg’ feature in the ‘qSupported’ packet
     (*note single-inf-arg::), and the stub replied with
     ‘single-inf-arg+’, then there will only be a single ARGUMENT
     string, which includes all inferior arguments, separated with
     whitespace.

     This packet is only available in extended mode (*note extended
     mode::).

     Reply:
     ‘Any stop packet’
          for success (*note Stop Reply Packets::)

‘vStopped’
     *Note Notification Packets::.

‘x ADDR,LENGTH’
     Read LENGTH addressable memory units starting at address ADDR
     (*note addressable memory unit::).  Note that ADDR does not have to
     be aligned to any particular boundary.

     The stub need not use any particular size or alignment when
     gathering data from memory for the response; even if ADDR is
     word-aligned and LENGTH is a multiple of the word size, the stub is
     free to use byte accesses, or not.  For this reason, this packet
     may not be suitable for accessing memory-mapped I/O devices.

     GDB will only use this packet if the stub reports the
     ‘binary-upload’ feature is supported in its ‘qSupported’ reply
     (*note qSupported::).

     Reply:
     ‘b XX...’
          Memory contents as binary data (*note Binary Data::).  The
          reply may contain fewer addressable memory units than
          requested if the server was reading from a trace frame memory
          and was able to read only part of the region of memory.
     ‘E NN’
          for an error

‘X ADDR,LENGTH:XX...’
     Write data to memory, where the data is transmitted in binary.
     Memory is specified by its address ADDR and number of addressable
     memory units LENGTH (*note addressable memory unit::); ‘XX...’ is
     binary data (*note Binary Data::).

     Reply:
     ‘OK’
          for success

‘z TYPE,ADDR,KIND’
‘Z TYPE,ADDR,KIND’
     Insert (‘Z’) or remove (‘z’) a TYPE breakpoint or watchpoint
     starting at address ADDRESS of kind KIND.

     Each breakpoint and watchpoint packet TYPE is documented
     separately.

     _Implementation notes: A remote target shall return an empty string
     for an unrecognized breakpoint or watchpoint packet TYPE.  A remote
     target shall support either both or neither of a given ‘ZTYPE...’
     and ‘zTYPE...’ packet pair.  To avoid potential problems with
     duplicate packets, the operations should be implemented in an
     idempotent way._

‘z0,ADDR,KIND’
‘Z0,ADDR,KIND[;COND_LIST...][;cmds:PERSIST,CMD_LIST...]’
     Insert (‘Z0’) or remove (‘z0’) a software breakpoint at address
     ADDR of type KIND.

     A software breakpoint is implemented by replacing the instruction
     at ADDR with a software breakpoint or trap instruction.  The KIND
     is target-specific and typically indicates the size of the
     breakpoint in bytes that should be inserted.  E.g., the ARM and
     MIPS can insert either a 2 or 4 byte breakpoint.  Some
     architectures have additional meanings for KIND (*note
     Architecture-Specific Protocol Details::); if no
     architecture-specific value is being used, it should be ‘0’.  KIND
     is hex-encoded.  COND_LIST is an optional list of conditional
     expressions in bytecode form that should be evaluated on the
     target's side.  These are the conditions that should be taken into
     consideration when deciding if the breakpoint trigger should be
     reported back to GDB.

     See also the ‘swbreak’ stop reason (*note swbreak stop reason::)
     for how to best report a software breakpoint event to GDB.

     The COND_LIST parameter is comprised of a series of expressions,
     concatenated without separators.  Each expression has the following
     form:

     ‘X LEN,EXPR’
          LEN is the length of the bytecode expression and EXPR is the
          actual conditional expression in bytecode form.

     The optional CMD_LIST parameter introduces commands that may be run
     on the target, rather than being reported back to GDB.  The
     parameter starts with a numeric flag PERSIST; if the flag is
     nonzero, then the breakpoint may remain active and the commands
     continue to be run even when GDB disconnects from the target.
     Following this flag is a series of expressions concatenated with no
     separators.  Each expression has the following form:

     ‘X LEN,EXPR’
          LEN is the length of the bytecode expression and EXPR is the
          actual commands expression in bytecode form.

     _Implementation note: It is possible for a target to copy or move
     code that contains software breakpoints (e.g., when implementing
     overlays).  The behavior of this packet, in the presence of such a
     target, is not defined._

     Reply:
     ‘OK’
          success

‘z1,ADDR,KIND’
‘Z1,ADDR,KIND[;COND_LIST...][;cmds:PERSIST,CMD_LIST...]’
     Insert (‘Z1’) or remove (‘z1’) a hardware breakpoint at address
     ADDR.

     A hardware breakpoint is implemented using a mechanism that is not
     dependent on being able to modify the target's memory.  The KIND,
     COND_LIST, and CMD_LIST arguments have the same meaning as in ‘Z0’
     packets.

     _Implementation note: A hardware breakpoint is not affected by code
     movement._

     Reply:
     ‘OK’
          success

‘z2,ADDR,KIND’
‘Z2,ADDR,KIND’
     Insert (‘Z2’) or remove (‘z2’) a write watchpoint at ADDR.  The
     number of bytes to watch is specified by KIND.

     Reply:
     ‘OK’
          success

‘z3,ADDR,KIND’
‘Z3,ADDR,KIND’
     Insert (‘Z3’) or remove (‘z3’) a read watchpoint at ADDR.  The
     number of bytes to watch is specified by KIND.

     Reply:
     ‘OK’
          success

‘z4,ADDR,KIND’
‘Z4,ADDR,KIND’
     Insert (‘Z4’) or remove (‘z4’) an access watchpoint at ADDR.  The
     number of bytes to watch is specified by KIND.

     Reply:
     ‘OK’
          success


File: gdb.info,  Node: Stop Reply Packets,  Next: General Query Packets,  Prev: Packets,  Up: Remote Protocol

E.4 Stop Reply Packets
======================

The ‘C’, ‘c’, ‘S’, ‘s’, ‘vCont’, ‘vAttach’, ‘vRun’, ‘vStopped’, and ‘?’
packets can receive any of the below as a reply.  Except for ‘?’ and
‘vStopped’, that reply is only returned when the target halts.  In the
below the exact meaning of “signal number” is defined by the header
‘include/gdb/signals.h’ in the GDB source code.

   In non-stop mode, the server will simply reply ‘OK’ to commands such
as ‘vCont’; any stop will be the subject of a future notification.
*Note Remote Non-Stop::.

   As in the description of request packets, we include spaces in the
reply templates for clarity; these are not part of the reply packet's
syntax.  No GDB stop reply packet uses spaces to separate its
components.

‘S AA’
     The program received signal number AA (a two-digit hexadecimal
     number).  This is equivalent to a ‘T’ response with no N:R pairs.

‘T AA N1:R1;N2:R2;...’
     The program received signal number AA (a two-digit hexadecimal
     number).  This is equivalent to an ‘S’ response, except that the
     ‘N:R’ pairs can carry values of important registers and other
     information directly in the stop reply packet, reducing round-trip
     latency.  Single-step and breakpoint traps are reported this way.
     Each ‘N:R’ pair is interpreted as follows:

        • If N is a hexadecimal number, it is a register number, and the
          corresponding R gives that register's value.  The data R is a
          series of bytes in target byte order, with each byte given by
          a two-digit hex number.

        • If N is ‘thread’, then R is the thread ID of the stopped
          thread, as specified in *note thread-id syntax::.

        • If N is ‘core’, then R is the hexadecimal number of the core
          on which the stop event was detected.

        • If N is a recognized “stop reason”, it describes a more
          specific event that stopped the target.  The currently defined
          stop reasons are listed below.  The AA should be ‘05’, the
          trap signal.  At most one stop reason should be present.

        • Otherwise, GDB should ignore this ‘N:R’ pair and go on to the
          next; this allows us to extend the protocol in the future.

     The currently defined stop reasons are:

     ‘watch’
     ‘rwatch’
     ‘awatch’
          The packet indicates a watchpoint hit, and R is the data
          address, in hex.

          Some targets, for example AArch64, are unable to accurately
          report the address which triggered a watchpoint trap.  As a
          consequence, multiple watched addresses could explain a single
          watchpoint trap.

          If GDB sent the ‘multi-wp-addr’ feature flag in its
          ‘qSupported’ packet (*note multi-wp-addr feature::), then
          multiple instances of these stop reasons can appear in a
          single ‘T’ stop reply packet.  GDB will select between the
          multiple reported watchpoint addresses when displaying the
          stop to the user.

          If the ‘multi-wp-addr’ was not sent by GDB, then GDB only
          expects one watchpoint related stop address in a single ‘T’
          packet.  The server must select the most likely watchpoint
          address.

     ‘syscall_entry’
     ‘syscall_return’
          The packet indicates a syscall entry or return, and R is the
          syscall number, in hex.

     ‘library’
          The packet indicates that the loaded libraries have changed.
          GDB should use ‘qXfer:libraries:read’ to fetch a new list of
          loaded libraries.  The R part is ignored.

     ‘replaylog’
          The packet indicates that the target cannot continue replaying
          logged execution events, because it has reached the end (or
          the beginning when executing backward) of the log.  The value
          of R will be either ‘begin’ or ‘end’.  *Note Reverse
          Execution::, for more information.

     ‘swbreak’
          The packet indicates a software breakpoint instruction was
          executed, irrespective of whether it was GDB that planted the
          breakpoint or the breakpoint is hardcoded in the program.  The
          R part must be left empty.

          On some architectures, such as x86, at the architecture level,
          when a breakpoint instruction executes the program counter
          points at the breakpoint address plus an offset.  On such
          targets, the stub is responsible for adjusting the PC to point
          back at the breakpoint address.

          This packet should not be sent by default; older GDB versions
          did not support it.  GDB requests it, by supplying an
          appropriate ‘qSupported’ feature (*note qSupported::).  The
          remote stub must also supply the appropriate ‘qSupported’
          feature indicating support.

          This packet is required for correct non-stop mode operation.

     ‘hwbreak’
          The packet indicates the target stopped for a hardware
          breakpoint.  The R part must be left empty.

          The same remarks about ‘qSupported’ and non-stop mode above
          apply.

     ‘fork’
          The packet indicates that ‘fork’ was called, and R is the
          thread ID of the new child process, as specified in *note
          thread-id syntax::.  This packet is only applicable to targets
          that support fork events.

          This packet should not be sent by default; older GDB versions
          did not support it.  GDB requests it, by supplying an
          appropriate ‘qSupported’ feature (*note qSupported::).  The
          remote stub must also supply the appropriate ‘qSupported’
          feature indicating support.

     ‘vfork’
          The packet indicates that ‘vfork’ was called, and R is the
          thread ID of the new child process, as specified in *note
          thread-id syntax::.  This packet is only applicable to targets
          that support vfork events.

          This packet should not be sent by default; older GDB versions
          did not support it.  GDB requests it, by supplying an
          appropriate ‘qSupported’ feature (*note qSupported::).  The
          remote stub must also supply the appropriate ‘qSupported’
          feature indicating support.

     ‘vforkdone’
          The packet indicates that a child process created by a vfork
          has either called ‘exec’ or terminated, so that the address
          spaces of the parent and child process are no longer shared.
          The R part is ignored.  This packet is only applicable to
          targets that support vforkdone events.

          This packet should not be sent by default; older GDB versions
          did not support it.  GDB requests it, by supplying an
          appropriate ‘qSupported’ feature (*note qSupported::).  The
          remote stub must also supply the appropriate ‘qSupported’
          feature indicating support.

     ‘exec’
          The packet indicates that ‘execve’ was called, and R is the
          absolute pathname of the file that was executed, in hex.  This
          packet is only applicable to targets that support exec events.

          This packet should not be sent by default; older GDB versions
          did not support it.  GDB requests it, by supplying an
          appropriate ‘qSupported’ feature (*note qSupported::).  The
          remote stub must also supply the appropriate ‘qSupported’
          feature indicating support.

     ‘clone’
          The packet indicates that ‘clone’ was called, and R is the
          thread ID of the new child thread, as specified in *note
          thread-id syntax::.  This packet is only applicable to targets
          that support clone events.

          This packet should not be sent by default; GDB requests it
          with the *note QThreadOptions:: packet.

     ‘create’
          The packet indicates that the thread was just created.  The
          new thread is stopped until GDB sets it running with a
          resumption packet (*note vCont packet::).  This packet should
          not be sent by default; GDB requests it with the *note
          QThreadEvents:: packet.  See also the ‘w’ (*note thread exit
          event::) remote reply below.  The R part is ignored.

‘W AA’
‘W AA ; process:PID’
     The process exited, and AA is the exit status.  This is only
     applicable to certain targets.

     The second form of the response, including the process ID of the
     exited process, can be used only when GDB has reported support for
     multiprocess protocol extensions; see *note multiprocess
     extensions::.  Both AA and PID are formatted as big-endian hex
     strings.

‘X AA’
‘X AA ; process:PID’
     The process terminated with signal AA.

     The second form of the response, including the process ID of the
     terminated process, can be used only when GDB has reported support
     for multiprocess protocol extensions; see *note multiprocess
     extensions::.  Both AA and PID are formatted as big-endian hex
     strings.

‘w AA ; TID’

     The thread exited, and AA is the exit status.  This response should
     not be sent by default; GDB requests it with either the *note
     QThreadEvents:: or *note QThreadOptions:: packets.  See also *note
     thread create event:: above.  AA is formatted as a big-endian hex
     string.

‘N’
     There are no resumed threads left in the target.  In other words,
     even though the process is alive, the last resumed thread has
     exited.  For example, say the target process has two threads:
     thread 1 and thread 2.  The client leaves thread 1 stopped, and
     resumes thread 2, which subsequently exits.  At this point, even
     though the process is still alive, and thus no ‘W’ stop reply is
     sent, no thread is actually executing either.  The ‘N’ stop reply
     thus informs the client that it can stop waiting for stop replies.
     This packet should not be sent by default; older GDB versions did
     not support it.  GDB requests it, by supplying an appropriate
     ‘qSupported’ feature (*note qSupported::).  The remote stub must
     also supply the appropriate ‘qSupported’ feature indicating
     support.

‘O XX...’
     ‘XX...’ is hex encoding of ASCII data, to be written as the
     program's console output.  This can happen at any time while the
     program is running and the debugger should continue to wait for
     ‘W’, ‘T’, etc.  This reply is not permitted in non-stop mode.

‘F CALL-ID,PARAMETER...’
     CALL-ID is the identifier which says which host system call should
     be called.  This is just the name of the function.  Translation
     into the correct system call is only applicable as it's defined in
     GDB.  *Note File-I/O Remote Protocol Extension::, for a list of
     implemented system calls.

     ‘PARAMETER...’ is a list of parameters as defined for this very
     system call.

     The target replies with this packet when it expects GDB to call a
     host system call on behalf of the target.  GDB replies with an
     appropriate ‘F’ packet and keeps up waiting for the next reply
     packet from the target.  The latest ‘C’, ‘c’, ‘S’ or ‘s’ action is
     expected to be continued.  *Note File-I/O Remote Protocol
     Extension::, for more details.


File: gdb.info,  Node: General Query Packets,  Next: Architecture-Specific Protocol Details,  Prev: Stop Reply Packets,  Up: Remote Protocol

E.5 General Query Packets
=========================

Packets starting with ‘q’ are “general query packets”; packets starting
with ‘Q’ are “general set packets”.  General query and set packets are a
semi-unified form for retrieving and sending information to and from the
stub.

   The initial letter of a query or set packet is followed by a name
indicating what sort of thing the packet applies to.  For example, GDB
may use a ‘qSymbol’ packet to exchange symbol definitions with the stub.
These packet names follow some conventions:

   • The name must not contain commas, colons or semicolons.
   • Most GDB query and set packets have a leading upper case letter.
   • The names of custom vendor packets should use a company prefix, in
     lower case, followed by a period.  For example, packets designed at
     the Acme Corporation might begin with ‘qacme.foo’ (for querying
     foos) or ‘Qacme.bar’ (for setting bars).

   The name of a query or set packet should be separated from any
parameters by a ‘:’; the parameters themselves should be separated by
‘,’ or ‘;’.  Stubs must be careful to match the full packet name, and
check for a separator or the end of the packet, in case two packet names
share a common prefix.  New packets should not begin with ‘qC’, ‘qP’, or
‘qL’(1).

   Like the descriptions of the other packets, each description here has
a template showing the packet's overall syntax, followed by an
explanation of the packet's meaning.  We include spaces in some of the
templates for clarity; these are not part of the packet's syntax.  No
GDB packet uses spaces to separate its components.

   Here are the currently defined query and set packets:

‘QAgent:1’
‘QAgent:0’
     Turn on or off the agent as a helper to perform some debugging
     operations delegated from GDB (*note Control Agent::).

‘QAllow:OP:VAL...’
     Specify which operations GDB expects to request of the target, as a
     semicolon-separated list of operation name and value pairs.
     Possible values for OP include ‘WriteReg’, ‘WriteMem’,
     ‘InsertBreak’, ‘InsertTrace’, ‘InsertFastTrace’, and ‘Stop’.  VAL
     is either 0, indicating that GDB will not request the operation, or
     1, indicating that it may.  (The target can then use this to set up
     its own internals optimally, for instance if the debugger never
     expects to insert breakpoints, it may not need to install its own
     trap handler.)

‘qC’
     Return the current thread ID.

     Reply:
     ‘QC THREAD-ID’
          Where THREAD-ID is a thread ID as documented in *note
          thread-id syntax::.
     ‘(anything else)’
          Any other reply implies the old thread ID.

‘qCRC:ADDR,LENGTH’
     Compute the CRC checksum of a block of memory using CRC-32 defined
     in IEEE 802.3.  The CRC is computed byte at a time, taking the most
     significant bit of each byte first.  The initial pattern code
     ‘0xffffffff’ is used to ensure leading zeros affect the CRC.

     _Note:_ This is the same CRC used in validating separate debug
     files (*note Debugging Information in Separate Files: Separate
     Debug Files.).  However the algorithm is slightly different.  When
     validating separate debug files, the CRC is computed taking the
     _least_ significant bit of each byte first, and the final result is
     inverted to detect trailing zeros.

     Reply:
     ‘C CRC32’
          The specified memory region's checksum is CRC32.

‘QDisableRandomization:VALUE’
     Some target operating systems will randomize the virtual address
     space of the inferior process as a security feature, but provide a
     feature to disable such randomization, e.g. to allow for a more
     deterministic debugging experience.  On such systems, this packet
     with a VALUE of 1 directs the target to disable address space
     randomization for processes subsequently started via ‘vRun’
     packets, while a packet with a VALUE of 0 tells the target to
     enable address space randomization.

     This packet is only available in extended mode (*note extended
     mode::).

     Reply:
     ‘OK’
          The request succeeded.

     This packet is not probed by default; the remote stub must request
     it, by supplying an appropriate ‘qSupported’ response (*note
     qSupported::).  This should only be done on targets that actually
     support disabling address space randomization.

‘QStartupWithShell:VALUE’
     On UNIX-like targets, it is possible to start the inferior using a
     shell program.  This is the default behavior on both GDB and
     ‘gdbserver’ (*note set startup-with-shell::).  This packet is used
     to inform ‘gdbserver’ whether it should start the inferior using a
     shell or not.

     If VALUE is ‘0’, ‘gdbserver’ will not use a shell to start the
     inferior.  If VALUE is ‘1’, ‘gdbserver’ will use a shell to start
     the inferior.  All other values are considered an error.

     This packet is only available in extended mode (*note extended
     mode::).

     Reply:
     ‘OK’
          The request succeeded.

     This packet is not probed by default; the remote stub must request
     it, by supplying an appropriate ‘qSupported’ response (*note
     qSupported::).  This should only be done on targets that actually
     support starting the inferior using a shell.

     Use of this packet is controlled by the ‘set startup-with-shell’
     command; *note set startup-with-shell::.

‘QEnvironmentHexEncoded:HEX-VALUE’
     On UNIX-like targets, it is possible to set environment variables
     that will be passed to the inferior during the startup process.
     This packet is used to inform ‘gdbserver’ of an environment
     variable that has been defined by the user on GDB (*note set
     environment::).

     The packet is composed by HEX-VALUE, an hex encoded representation
     of the NAME=VALUE format representing an environment variable.  The
     name of the environment variable is represented by NAME, and the
     value to be assigned to the environment variable is represented by
     VALUE.  If the variable has no value (i.e., the value is ‘null’),
     then VALUE will not be present.

     This packet is only available in extended mode (*note extended
     mode::).

     Reply:
     ‘OK’
          The request succeeded.

     This packet is not probed by default; the remote stub must request
     it, by supplying an appropriate ‘qSupported’ response (*note
     qSupported::).  This should only be done on targets that actually
     support passing environment variables to the starting inferior.

     This packet is related to the ‘set environment’ command; *note set
     environment::.

‘QEnvironmentUnset:HEX-VALUE’
     On UNIX-like targets, it is possible to unset environment variables
     before starting the inferior in the remote target.  This packet is
     used to inform ‘gdbserver’ of an environment variable that has been
     unset by the user on GDB (*note unset environment::).

     The packet is composed by HEX-VALUE, an hex encoded representation
     of the name of the environment variable to be unset.

     This packet is only available in extended mode (*note extended
     mode::).

     Reply:
     ‘OK’
          The request succeeded.

     This packet is not probed by default; the remote stub must request
     it, by supplying an appropriate ‘qSupported’ response (*note
     qSupported::).  This should only be done on targets that actually
     support passing environment variables to the starting inferior.

     This packet is related to the ‘unset environment’ command; *note
     unset environment::.

‘QEnvironmentReset’
     On UNIX-like targets, this packet is used to reset the state of
     environment variables in the remote target before starting the
     inferior.  In this context, reset means unsetting all environment
     variables that were previously set by the user (i.e., were not
     initially present in the environment).  It is sent to ‘gdbserver’
     before the ‘QEnvironmentHexEncoded’ (*note
     QEnvironmentHexEncoded::) and the ‘QEnvironmentUnset’ (*note
     QEnvironmentUnset::) packets.

     This packet is only available in extended mode (*note extended
     mode::).

     Reply:
     ‘OK’
          The request succeeded.

     This packet is not probed by default; the remote stub must request
     it, by supplying an appropriate ‘qSupported’ response (*note
     qSupported::).  This should only be done on targets that actually
     support passing environment variables to the starting inferior.

‘QSetWorkingDir:[DIRECTORY]’
     This packet is used to inform the remote server of the intended
     current working directory for programs that are going to be
     executed.

     The packet is composed by DIRECTORY, an hex encoded representation
     of the directory that the remote inferior will use as its current
     working directory.  If DIRECTORY is an empty string, the remote
     server should reset the inferior's current working directory to its
     original, empty value.

     This packet is only available in extended mode (*note extended
     mode::).

     Reply:
     ‘OK’
          The request succeeded.

‘qfThreadInfo’
‘qsThreadInfo’
     Obtain a list of all active thread IDs from the target (OS). Since
     there may be too many active threads to fit into one reply packet,
     this query works iteratively: it may require more than one
     query/reply sequence to obtain the entire list of threads.  The
     first query of the sequence will be the ‘qfThreadInfo’ query;
     subsequent queries in the sequence will be the ‘qsThreadInfo’
     query.

     NOTE: This packet replaces the ‘qL’ query (see below).

     Reply:
     ‘m THREAD-ID’
          A single thread ID
     ‘m THREAD-ID,THREAD-ID...’
          a comma-separated list of thread IDs
     ‘l’
          (lower case letter ‘L’) denotes end of list.

     In response to each query, the target will reply with a list of one
     or more thread IDs, separated by commas.  GDB will respond to each
     reply with a request for more thread ids (using the ‘qs’ form of
     the query), until the target responds with ‘l’ (lower-case ell, for
     “last”).  Refer to *note thread-id syntax::, for the format of the
     THREAD-ID fields.

     _Note: GDB will send the ‘qfThreadInfo’ query during the initial
     connection with the remote target, and the very first thread ID
     mentioned in the reply will be stopped by GDB in a subsequent
     message.  Therefore, the stub should ensure that the first thread
     ID in the ‘qfThreadInfo’ reply is suitable for being stopped by
     GDB._

‘qGetTLSAddr:THREAD-ID,OFFSET,LM’
     Fetch the address associated with thread local storage specified by
     THREAD-ID, OFFSET, and LM.

     THREAD-ID is the thread ID associated with the thread for which to
     fetch the TLS address.  *Note thread-id syntax::.

     OFFSET is the (big endian, hex encoded) offset associated with the
     thread local variable.  (This offset is obtained from the debug
     information associated with the variable.)

     LM is the (big endian, hex encoded) OS/ABI-specific encoding of the
     load module associated with the thread local storage.  For example,
     a GNU/Linux system will pass the link map address of the shared
     object associated with the thread local storage under
     consideration.  Other operating environments may choose to
     represent the load module differently, so the precise meaning of
     this parameter will vary.

     Reply:
     ‘XX...’
          Hex encoded (big endian) bytes representing the address of the
          thread local storage requested.

‘qGetTIBAddr:THREAD-ID’
     Fetch address of the Windows OS specific Thread Information Block.

     THREAD-ID is the thread ID associated with the thread.

     Reply:
     ‘XX...’
          Hex encoded (big endian) bytes representing the linear address
          of the thread information block.

‘qL STARTFLAG THREADCOUNT NEXTTHREAD’
     Obtain thread information from RTOS. Where: STARTFLAG (one hex
     digit) is one to indicate the first query and zero to indicate a
     subsequent query; THREADCOUNT (two hex digits) is the maximum
     number of threads the response packet can contain; and NEXTTHREAD
     (eight hex digits), for subsequent queries (STARTFLAG is zero), is
     returned in the response as ARGTHREAD.

     Don't use this packet; use the ‘qfThreadInfo’ query instead (see
     above).

     Reply:
     ‘qM COUNT DONE ARGTHREAD THREAD...’
          Where: COUNT (two hex digits) is the number of threads being
          returned; DONE (one hex digit) is zero to indicate more
          threads and one indicates no further threads; ARGTHREADID
          (eight hex digits) is NEXTTHREAD from the request packet;
          THREAD... is a sequence of thread IDs, THREADID (eight hex
          digits), from the target.  See
          ‘remote.c:parse_threadlist_response()’.

‘qMemTags:START ADDRESS,LENGTH:TYPE’
     Fetch memory tags of type TYPE from the address range
     [START ADDRESS, START ADDRESS + LENGTH).  The target is responsible
     for calculating how many tags will be returned, as this is
     architecture-specific.

     START ADDRESS is the starting address of the memory range.

     LENGTH is the length, in bytes, of the memory range.

     TYPE is the type of tag the request wants to fetch.  The type is a
     signed integer.

     GDB will only send this packet if the stub has advertised support
     for memory tagging via ‘qSupported’.

     Reply:
     ‘MXX...’
          Hex encoded sequence of uninterpreted bytes, XX...,
          representing the tags found in the requested memory range.

‘qIsAddressTagged:ADDRESS’
     Check if address ADDRESS is in a memory tagged region; if it is,
     it's said to be “tagged”.  The target is responsible for checking
     it, as this is architecture-specific.

     ADDRESS is the address to be checked.

     Reply:
          Replies to this packet should all be in two hex digit format,
          as follows:

     ‘‘01’’
          Address ADDRESS is tagged.

     ‘‘00’’
          Address ADDRESS is not tagged.

‘QMemTags:START ADDRESS,LENGTH:TYPE:TAG BYTES’
     Store memory tags of type TYPE to the address range
     [START ADDRESS, START ADDRESS + LENGTH).  The target is responsible
     for interpreting the type, the tag bytes and modifying the memory
     tag granules accordingly, given this is architecture-specific.

     The interpretation of how many tags (NT) should be written to how
     many memory tag granules (NG) is also architecture-specific.  The
     behavior is implementation-specific, but the following is
     suggested.

     If the number of memory tags, NT, is greater than or equal to the
     number of memory tag granules, NG, only NG tags will be stored.

     If NT is less than NG, the behavior is that of a fill operation,
     and the tag bytes will be used as a pattern that will get repeated
     until NG tags are stored.

     START ADDRESS is the starting address of the memory range.  The
     address does not have any restriction on alignment or size.

     LENGTH is the length, in bytes, of the memory range.

     TYPE is the type of tag the request wants to fetch.  The type is a
     signed integer.

     TAG BYTES is a sequence of hex encoded uninterpreted bytes which
     will be interpreted by the target.  Each pair of hex digits is
     interpreted as a single byte.

     GDB will only send this packet if the stub has advertised support
     for memory tagging via ‘qSupported’.

     Reply:
     ‘OK’
          The request was successful and the memory tag granules were
          modified accordingly.

‘qOffsets’
     Get section offsets that the target used when relocating the
     downloaded image.

     Reply:
     ‘Text=XXX;Data=YYY[;Bss=ZZZ]’
          Relocate the ‘Text’ section by XXX from its original address.
          Relocate the ‘Data’ section by YYY from its original address.
          If the object file format provides segment information (e.g.
          ELF ‘PT_LOAD’ program headers), GDB will relocate entire
          segments by the supplied offsets.

          _Note: while a ‘Bss’ offset may be included in the response,
          GDB ignores this and instead applies the ‘Data’ offset to the
          ‘Bss’ section._

     ‘TextSeg=XXX[;DataSeg=YYY]’
          Relocate the first segment of the object file, which
          conventionally contains program code, to a starting address of
          XXX.  If ‘DataSeg’ is specified, relocate the second segment,
          which conventionally contains modifiable data, to a starting
          address of YYY.  GDB will report an error if the object file
          does not contain segment information, or does not contain at
          least as many segments as mentioned in the reply.  Extra
          segments are kept at fixed offsets relative to the last
          relocated segment.

‘qP MODE THREAD-ID’
     Returns information on THREAD-ID.  Where: MODE is a hex encoded 32
     bit mode; THREAD-ID is a thread ID (*note thread-id syntax::).

     Don't use this packet; use the ‘qThreadExtraInfo’ query instead
     (see below).

     Reply: see ‘remote.c:remote_unpack_thread_info_response()’.

‘QNonStop:1’
‘QNonStop:0’
     Enter non-stop (‘QNonStop:1’) or all-stop (‘QNonStop:0’) mode.
     *Note Remote Non-Stop::, for more information.

     Reply:
     ‘OK’
          The request succeeded.

     This packet is not probed by default; the remote stub must request
     it, by supplying an appropriate ‘qSupported’ response (*note
     qSupported::).  Use of this packet is controlled by the ‘set
     non-stop’ command; *note Non-Stop Mode::.

‘QCatchSyscalls:1 [;SYSNO]...’
‘QCatchSyscalls:0’
     Enable (‘QCatchSyscalls:1’) or disable (‘QCatchSyscalls:0’)
     catching syscalls from the inferior process.

     For ‘QCatchSyscalls:1’, each listed syscall SYSNO (encoded in hex)
     should be reported to GDB.  If no syscall SYSNO is listed, every
     system call should be reported.

     Note that if a syscall not in the list is reported, GDB will still
     filter the event according to its own list from all corresponding
     ‘catch syscall’ commands.  However, it is more efficient to only
     report the requested syscalls.

     Multiple ‘QCatchSyscalls:1’ packets do not combine; any earlier
     ‘QCatchSyscalls:1’ list is completely replaced by the new list.

     If the inferior process execs, the state of ‘QCatchSyscalls’ is
     kept for the new process too.  On targets where exec may affect
     syscall numbers, for example with exec between 32 and 64-bit
     processes, the client should send a new packet with the new syscall
     list.

     Reply:
     ‘OK’
          The request succeeded.

     Use of this packet is controlled by the ‘set remote catch-syscalls’
     command (*note set remote catch-syscalls: Remote Configuration.).
     This packet is not probed by default; the remote stub must request
     it, by supplying an appropriate ‘qSupported’ response (*note
     qSupported::).

‘QPassSignals: SIGNAL [;SIGNAL]...’
     Each listed SIGNAL should be passed directly to the inferior
     process.  Signals are numbered identically to continue packets and
     stop replies (*note Stop Reply Packets::).  Each SIGNAL list item
     should be strictly greater than the previous item.  These signals
     do not need to stop the inferior, or be reported to GDB.  All other
     signals should be reported to GDB.  Multiple ‘QPassSignals’ packets
     do not combine; any earlier ‘QPassSignals’ list is completely
     replaced by the new list.  This packet improves performance when
     using ‘handle SIGNAL nostop noprint pass’.

     Reply:
     ‘OK’
          The request succeeded.

     Use of this packet is controlled by the ‘set remote pass-signals’
     command (*note set remote pass-signals: Remote Configuration.).
     This packet is not probed by default; the remote stub must request
     it, by supplying an appropriate ‘qSupported’ response (*note
     qSupported::).

‘QProgramSignals: SIGNAL [;SIGNAL]...’
     Each listed SIGNAL may be delivered to the inferior process.
     Others should be silently discarded.

     In some cases, the remote stub may need to decide whether to
     deliver a signal to the program or not without GDB involvement.
     One example of that is while detaching -- the program's threads may
     have stopped for signals that haven't yet had a chance of being
     reported to GDB, and so the remote stub can use the signal list
     specified by this packet to know whether to deliver or ignore those
     pending signals.

     This does not influence whether to deliver a signal as requested by
     a resumption packet (*note vCont packet::).

     Signals are numbered identically to continue packets and stop
     replies (*note Stop Reply Packets::).  Each SIGNAL list item should
     be strictly greater than the previous item.  Multiple
     ‘QProgramSignals’ packets do not combine; any earlier
     ‘QProgramSignals’ list is completely replaced by the new list.

     Reply:
     ‘OK’
          The request succeeded.

     Use of this packet is controlled by the ‘set remote
     program-signals’ command (*note set remote program-signals: Remote
     Configuration.).  This packet is not probed by default; the remote
     stub must request it, by supplying an appropriate ‘qSupported’
     response (*note qSupported::).

‘QThreadEvents:1’
‘QThreadEvents:0’

     Enable (‘QThreadEvents:1’) or disable (‘QThreadEvents:0’) reporting
     of thread create and exit events.  *Note thread create event::, for
     the reply specifications.  For example, this is used in non-stop
     mode when GDB stops a set of threads and synchronously waits for
     the their corresponding stop replies.  Without exit events, if one
     of the threads exits, GDB would hang forever not knowing that it
     should no longer expect a stop for that same thread.  GDB does not
     enable this feature unless the stub reports that it supports it by
     including ‘QThreadEvents+’ in its ‘qSupported’ reply.

     This packet always enables/disables event reporting for all threads
     of all processes under control of the remote stub.  For per-thread
     control of optional event reporting, see the *note QThreadOptions::
     packet.

     Reply:
     ‘OK’
          The request succeeded.

     Use of this packet is controlled by the ‘set remote thread-events’
     command (*note set remote thread-events: Remote Configuration.).

‘QThreadOptions[;OPTIONS[:THREAD-ID]]...’

     For each inferior thread, the last OPTIONS in the list with a
     matching THREAD-ID are applied.  Any options previously set on a
     thread are discarded and replaced by the new options specified.
     Threads that do not match any THREAD-ID retain their previously-set
     options.  Thread IDs are specified using the syntax described in
     *note thread-id syntax::.  If multiprocess extensions (*note
     multiprocess extensions::) are supported, options can be specified
     to apply to all threads of a process by using the ‘pPID.-1’ form of
     THREAD-ID.  Options with no THREAD-ID apply to all threads.
     Specifying no options value is an error.  Zero is a valid value.

     OPTIONS is an hexadecimal integer specifying the enabled thread
     options, and is the bitwise ‘OR’ of the following values.  All
     values are given in hexadecimal representation.

     ‘GDB_THREAD_OPTION_CLONE (0x1)’
          Report thread clone events (*note thread clone event::).  This
          is only meaningful for targets that support clone events
          (e.g., GNU/Linux systems).

     ‘GDB_THREAD_OPTION_EXIT (0x2)’
          Report thread exit events (*note thread exit event::).

     For example, GDB enables the ‘GDB_THREAD_OPTION_EXIT’ and
     ‘GDB_THREAD_OPTION_CLONE’ options when single-stepping a thread
     past a breakpoint, for the following reasons:

        • If the single-stepped thread exits (e.g., it executes a thread
          exit system call), enabling ‘GDB_THREAD_OPTION_EXIT’ prevents
          GDB from waiting forever, not knowing that it should no longer
          expect a stop for that same thread, and blocking other threads
          from progressing.

        • If the single-stepped thread spawns a new clone child (i.e.,
          it executes a clone system call), enabling
          ‘GDB_THREAD_OPTION_CLONE’ halts the cloned thread before it
          executes any instructions, and thus prevents the following
          problematic situations:

             − If the breakpoint is stepped-over in-line, the spawned
               thread would incorrectly run free while the breakpoint
               being stepped over is not inserted, and thus the cloned
               thread may potentially run past the breakpoint without
               stopping for it;

             − If displaced (out-of-line) stepping is used, the cloned
               thread starts running at the out-of-line PC, leading to
               undefined behavior, usually crashing or corrupting data.

     New threads start with thread options cleared.

     GDB does not enable this feature unless the stub reports that it
     supports it by including ‘QThreadOptions=SUPPORTED_OPTIONS’ in its
     ‘qSupported’ reply.

     Reply:
     ‘OK’
          The request succeeded.

     Use of this packet is controlled by the ‘set remote thread-options’
     command (*note set remote thread-options: Remote Configuration.).

‘qRcmd,COMMAND’
     COMMAND (hex encoded) is passed to the local interpreter for
     execution.  Invalid commands should be reported using the output
     string.  Before the final result packet, the target may also
     respond with a number of intermediate ‘OOUTPUT’ console output
     packets.  _Implementors should note that providing access to a
     stubs's interpreter may have security implications_.

     Reply:
     ‘OK’
          A command response with no output.
     ‘OUTPUT’
          A command response with the hex encoded output string OUTPUT.

     Unlike most packets, this packet does not support ‘E.ERRTEXT’-style
     textual error replies (*note textual error reply::) by default.
     Stubs should be careful to only send such a reply if GDB reported
     support for it with the ‘error-message’ feature (*note
     error-message::).

     (Note that the ‘qRcmd’ packet's name is separated from the command
     by a ‘,’, not a ‘:’, contrary to the naming conventions above.
     Please don't use this packet as a model for new packets.)

‘qSearch:memory:ADDRESS;LENGTH;SEARCH-PATTERN’
     Search LENGTH bytes at ADDRESS for SEARCH-PATTERN.  Both ADDRESS
     and LENGTH are encoded in hex; SEARCH-PATTERN is a sequence of
     binary-encoded bytes (*note Binary Data::).

     Reply:
     ‘0’
          The pattern was not found.
     ‘1,address’
          The pattern was found at ADDRESS.

‘QStartNoAckMode’
     Request that the remote stub disable the normal ‘+’/‘-’ protocol
     acknowledgments (*note Packet Acknowledgment::).

     Reply:
     ‘OK’
          The stub has switched to no-acknowledgment mode.  GDB
          acknowledges this response, but neither the stub nor GDB shall
          send or expect further ‘+’/‘-’ acknowledgments in the current
          connection.

‘qSupported [:GDBFEATURE [;GDBFEATURE]... ]’
     Tell the remote stub about features supported by GDB, and query the
     stub for features it supports.  This packet allows GDB and the
     remote stub to take advantage of each others' features.
     ‘qSupported’ also consolidates multiple feature probes at startup,
     to improve GDB performance--a single larger packet performs better
     than multiple smaller probe packets on high-latency links.  Some
     features may enable behavior which must not be on by default, e.g.
     because it would confuse older clients or stubs.  Other features
     may describe packets which could be automatically probed for, but
     are not.  These features must be reported before GDB will use them.
     This "default unsupported" behavior is not appropriate for all
     packets, but it helps to keep the initial connection time under
     control with new versions of GDB which support increasing numbers
     of packets.

     Reply:
     ‘STUBFEATURE [;STUBFEATURE]...’
          The stub supports or does not support each returned
          STUBFEATURE, depending on the form of each STUBFEATURE (see
          below for the possible forms).

     The allowed forms for each feature (either a GDBFEATURE in the
     ‘qSupported’ packet, or a STUBFEATURE in the response) are:

     ‘NAME=VALUE’
          The remote protocol feature NAME is supported, and associated
          with the specified VALUE.  The format of VALUE depends on the
          feature, but it must not include a semicolon.
     ‘NAME+’
          The remote protocol feature NAME is supported, and does not
          need an associated value.
     ‘NAME-’
          The remote protocol feature NAME is not supported.
     ‘NAME?’
          The remote protocol feature NAME may be supported, and GDB
          should auto-detect support in some other way when it is
          needed.  This form will not be used for GDBFEATURE
          notifications, but may be used for STUBFEATURE responses.

     Whenever the stub receives a ‘qSupported’ request, the supplied set
     of GDB features should override any previous request.  This allows
     GDB to put the stub in a known state, even if the stub had
     previously been communicating with a different version of GDB.

     The following values of GDBFEATURE (for the packet sent by GDB) are
     defined:

     ‘multiprocess’
          This feature indicates whether GDB supports multiprocess
          extensions to the remote protocol.  GDB does not use such
          extensions unless the stub also reports that it supports them
          by including ‘multiprocess+’ in its ‘qSupported’ reply.  *Note
          multiprocess extensions::, for details.

     ‘xmlRegisters’
          This feature indicates that GDB supports the XML target
          description.  If the stub sees ‘xmlRegisters=’ with target
          specific strings separated by a comma, it will report register
          description.

     ‘qRelocInsn’
          This feature indicates whether GDB supports the ‘qRelocInsn’
          packet (*note Relocate instruction reply packet: Tracepoint
          Packets.).

     ‘swbreak’
          This feature indicates whether GDB supports the swbreak stop
          reason in stop replies.  *Note swbreak stop reason::, for
          details.

     ‘hwbreak’
          This feature indicates whether GDB supports the hwbreak stop
          reason in stop replies.  *Note swbreak stop reason::, for
          details.

     ‘fork-events’
          This feature indicates whether GDB supports fork event
          extensions to the remote protocol.  GDB does not use such
          extensions unless the stub also reports that it supports them
          by including ‘fork-events+’ in its ‘qSupported’ reply.

     ‘vfork-events’
          This feature indicates whether GDB supports vfork event
          extensions to the remote protocol.  GDB does not use such
          extensions unless the stub also reports that it supports them
          by including ‘vfork-events+’ in its ‘qSupported’ reply.

     ‘exec-events’
          This feature indicates whether GDB supports exec event
          extensions to the remote protocol.  GDB does not use such
          extensions unless the stub also reports that it supports them
          by including ‘exec-events+’ in its ‘qSupported’ reply.

     ‘vContSupported’
          This feature indicates whether GDB wants to know the supported
          actions in the reply to ‘vCont?’ packet.

     ‘error-message’
          This feature indicates whether GDB supports accepting a reply
          in ‘E.ERRTEXT’ format (*Note textual error reply::) from the
          ‘qRcmd’ and ‘m’ packets.  These packets, historically, didn't
          support ‘E.ERRTEXT’, and older versions of GDB didn't expect
          to see a reply in this format.

          New packets should be written to support ‘E.ERRTEXT’
          regardless of this feature being true or not.

     ‘single-inf-arg’
          This feature indicates that GDB would like to send the
          inferior arguments as a single string within the ‘vRun’
          packet.  GDB will not send the arguments as a single string
          unless the stub also reports that it supports this behaviour
          by including ‘single-inf-arg+’ in its ‘qSupported’ reply.

     ‘multi-wp-addr’
          This features indicates that GDB supports receiving multiple
          watchpoint addresses in the ‘T’ stop reply packet (*note Stop
          Reply Packets::).

          Use of this feature is controlled by the ‘set remote
          multiple-watchpoint-addresses-packet’ command (*note set
          remote multiple-watchpoint-addresses-packet: Remote
          Configuration.).

     Stubs should ignore any unknown values for GDBFEATURE.  Any GDB
     which sends a ‘qSupported’ packet supports receiving packets of
     unlimited length (earlier versions of GDB may reject overly long
     responses).  Additional values for GDBFEATURE may be defined in the
     future to let the stub take advantage of new features in GDB, e.g.
     incompatible improvements in the remote protocol--the
     ‘multiprocess’ feature is an example of such a feature.  The stub's
     reply should be independent of the GDBFEATURE entries sent by GDB;
     first GDB describes all the features it supports, and then the stub
     replies with all the features it supports.

     Similarly, GDB will silently ignore unrecognized stub feature
     responses, as long as each response uses one of the standard forms.

     Some features are flags.  A stub which supports a flag feature
     should respond with a ‘+’ form response.  Other features require
     values, and the stub should respond with an ‘=’ form response.

     Each feature has a default value, which GDB will use if
     ‘qSupported’ is not available or if the feature is not mentioned in
     the ‘qSupported’ response.  The default values are fixed; a stub is
     free to omit any feature responses that match the defaults.

     Not all features can be probed, but for those which can, the
     probing mechanism is useful: in some cases, a stub's internal
     architecture may not allow the protocol layer to know some
     information about the underlying target in advance.  This is
     especially common in stubs which may be configured for multiple
     targets.

     These are the currently defined stub features and their properties:

     Feature Name              Value          Default   Probe
                               Required                 Allowed
                                                        
     ‘PacketSize’              Yes            ‘-’       No
                                                        
     ‘qXfer:auxv:read’         No             ‘-’       Yes
                                                        
     ‘qXfer:btrace:read’       No             ‘-’       Yes
                                                        
     ‘qXfer:btrace-conf:read’  No             ‘-’       Yes
                                                        
     ‘qXfer:exec-file:read’    No             ‘-’       Yes
                                                        
     ‘qXfer:features:read’     No             ‘-’       Yes
                                                        
     ‘qXfer:libraries:read’    No             ‘-’       Yes
                                                        
     ‘qXfer:libraries-svr4:read’No            ‘-’       Yes
                                                        
     ‘augmented-libraries-svr4-read’No        ‘-’       No
                                                        
     ‘qXfer:memory-map:read’   No             ‘-’       Yes
                                                        
     ‘qXfer:sdata:read’        No             ‘-’       Yes
                                                        
     ‘qXfer:siginfo:read’      No             ‘-’       Yes
                                                        
     ‘qXfer:siginfo:write’     No             ‘-’       Yes
                                                        
     ‘qXfer:threads:read’      No             ‘-’       Yes
                                                        
     ‘qXfer:traceframe-info:read’No           ‘-’       Yes
                                                        
     ‘qXfer:uib:read’          No             ‘-’       Yes
                                                        
     ‘qXfer:fdpic:read’        No             ‘-’       Yes
                                                        
     ‘Qbtrace:off’             Yes            ‘-’       Yes
                                                        
     ‘Qbtrace:bts’             Yes            ‘-’       Yes
                                                        
     ‘Qbtrace:pt’              Yes            ‘-’       Yes
                                                        
     ‘Qbtrace-conf:bts:size’   Yes            ‘-’       Yes
                                                        
     ‘Qbtrace-conf:pt:size’    Yes            ‘-’       Yes
                                                        
     ‘Qbtrace-conf:pt:ptwrite’ Yes            ‘-’       Yes
                                                        
     ‘Qbtrace-conf:pt:event-tracing’Yes       ‘-’       Yes
                                                        
     ‘QNonStop’                No             ‘-’       Yes
                                                        
     ‘QCatchSyscalls’          No             ‘-’       Yes
                                                        
     ‘QPassSignals’            No             ‘-’       Yes
                                                        
     ‘QStartNoAckMode’         No             ‘-’       Yes
                                                        
     ‘multiprocess’            No             ‘-’       No
                                                        
     ‘ConditionalBreakpoints’  No             ‘-’       No
                                                        
     ‘ConditionalTracepoints’  No             ‘-’       No
                                                        
     ‘ReverseContinue’         No             ‘-’       No
                                                        
     ‘ReverseStep’             No             ‘-’       No
                                                        
     ‘TracepointSource’        No             ‘-’       No
                                                        
     ‘QAgent’                  No             ‘-’       No
                                                        
     ‘QAllow’                  No             ‘-’       No
                                                        
     ‘QDisableRandomization’   No             ‘-’       No
                                                        
     ‘EnableDisableTracepoints’No             ‘-’       No
                                                        
     ‘QTBuffer:size’           No             ‘-’       No
                                                        
     ‘tracenz’                 No             ‘-’       No
                                                        
     ‘BreakpointCommands’      No             ‘-’       No
                                                        
     ‘swbreak’                 No             ‘-’       No
                                                        
     ‘hwbreak’                 No             ‘-’       No
                                                        
     ‘fork-events’             No             ‘-’       No
                                                        
     ‘vfork-events’            No             ‘-’       No
                                                        
     ‘exec-events’             No             ‘-’       No
                                                        
     ‘QThreadEvents’           No             ‘-’       No
                                                        
     ‘QThreadOptions’          Yes            ‘-’       No
                                                        
     ‘no-resumed’              No             ‘-’       No
                                                        
     ‘memory-tagging’          No             ‘-’       No
                                                        
     ‘error-message’           No             ‘+’       No
                                                        
     ‘binary-upload’           No             ‘-’       No
                                                        
     ‘single-inf-arg’          No             ‘-’       No
                                                        
     ‘multi-wp-addr’           No             ‘+’       No
                                                        

     These are the currently defined stub features, in more detail:

     ‘PacketSize=BYTES’
          The remote stub can accept packets up to at least BYTES in
          length.  GDB will send packets up to this size for bulk
          transfers, and will never send larger packets.  This is a
          limit on the data characters in the packet, not including the
          frame and checksum.  There is no trailing NUL byte in a remote
          protocol packet; if the stub stores packets in a
          NUL-terminated format, it should allow an extra byte in its
          buffer for the NUL. If this stub feature is not supported, GDB
          guesses based on the size of the ‘g’ packet response.

     ‘qXfer:auxv:read’
          The remote stub understands the ‘qXfer:auxv:read’ packet
          (*note qXfer auxiliary vector read::).

     ‘qXfer:btrace:read’
          The remote stub understands the ‘qXfer:btrace:read’ packet
          (*note qXfer btrace read::).

     ‘qXfer:btrace-conf:read’
          The remote stub understands the ‘qXfer:btrace-conf:read’
          packet (*note qXfer btrace-conf read::).

     ‘qXfer:exec-file:read’
          The remote stub understands the ‘qXfer:exec-file:read’ packet
          (*note qXfer executable filename read::).

     ‘qXfer:features:read’
          The remote stub understands the ‘qXfer:features:read’ packet
          (*note qXfer target description read::).

     ‘qXfer:libraries:read’
          The remote stub understands the ‘qXfer:libraries:read’ packet
          (*note qXfer library list read::).

     ‘qXfer:libraries-svr4:read’
          The remote stub understands the ‘qXfer:libraries-svr4:read’
          packet (*note qXfer svr4 library list read::).

     ‘augmented-libraries-svr4-read’
          The remote stub understands the augmented form of the
          ‘qXfer:libraries-svr4:read’ packet (*note qXfer svr4 library
          list read::).

     ‘qXfer:memory-map:read’
          The remote stub understands the ‘qXfer:memory-map:read’ packet
          (*note qXfer memory map read::).

     ‘qXfer:sdata:read’
          The remote stub understands the ‘qXfer:sdata:read’ packet
          (*note qXfer sdata read::).

     ‘qXfer:siginfo:read’
          The remote stub understands the ‘qXfer:siginfo:read’ packet
          (*note qXfer siginfo read::).

     ‘qXfer:siginfo:write’
          The remote stub understands the ‘qXfer:siginfo:write’ packet
          (*note qXfer siginfo write::).

     ‘qXfer:threads:read’
          The remote stub understands the ‘qXfer:threads:read’ packet
          (*note qXfer threads read::).

     ‘qXfer:traceframe-info:read’
          The remote stub understands the ‘qXfer:traceframe-info:read’
          packet (*note qXfer traceframe info read::).

     ‘qXfer:uib:read’
          The remote stub understands the ‘qXfer:uib:read’ packet (*note
          qXfer unwind info block::).

     ‘qXfer:fdpic:read’
          The remote stub understands the ‘qXfer:fdpic:read’ packet
          (*note qXfer fdpic loadmap read::).

     ‘QNonStop’
          The remote stub understands the ‘QNonStop’ packet (*note
          QNonStop::).

     ‘QCatchSyscalls’
          The remote stub understands the ‘QCatchSyscalls’ packet (*note
          QCatchSyscalls::).

     ‘QPassSignals’
          The remote stub understands the ‘QPassSignals’ packet (*note
          QPassSignals::).

     ‘QStartNoAckMode’
          The remote stub understands the ‘QStartNoAckMode’ packet and
          prefers to operate in no-acknowledgment mode.  *Note Packet
          Acknowledgment::.

     ‘multiprocess’
          The remote stub understands the multiprocess extensions to the
          remote protocol syntax.  The multiprocess extensions affect
          the syntax of thread IDs in both packets and replies (*note
          thread-id syntax::), and add process IDs to the ‘D’ packet and
          ‘W’ and ‘X’ replies.  Note that reporting this feature
          indicates support for the syntactic extensions only, not that
          the stub necessarily supports debugging of more than one
          process at a time.  The stub must not use multiprocess
          extensions in packet replies unless GDB has also indicated it
          supports them in its ‘qSupported’ request.

     ‘qXfer:osdata:read’
          The remote stub understands the ‘qXfer:osdata:read’ packet
          ((*note qXfer osdata read::).

     ‘ConditionalBreakpoints’
          The target accepts and implements evaluation of conditional
          expressions defined for breakpoints.  The target will only
          report breakpoint triggers when such conditions are true
          (*note Break Conditions: Conditions.).

     ‘ConditionalTracepoints’
          The remote stub accepts and implements conditional expressions
          defined for tracepoints (*note Tracepoint Conditions::).

     ‘ReverseContinue’
          The remote stub accepts and implements the reverse continue
          packet (*note bc::).

     ‘ReverseStep’
          The remote stub accepts and implements the reverse step packet
          (*note bs::).

     ‘TracepointSource’
          The remote stub understands the ‘QTDPsrc’ packet that supplies
          the source form of tracepoint definitions.

     ‘QAgent’
          The remote stub understands the ‘QAgent’ packet.

     ‘QAllow’
          The remote stub understands the ‘QAllow’ packet.

     ‘QDisableRandomization’
          The remote stub understands the ‘QDisableRandomization’
          packet.

     ‘StaticTracepoint’
          The remote stub supports static tracepoints.

     ‘InstallInTrace’
          The remote stub supports installing tracepoint in tracing.

     ‘EnableDisableTracepoints’
          The remote stub supports the ‘QTEnable’ (*note QTEnable::) and
          ‘QTDisable’ (*note QTDisable::) packets that allow tracepoints
          to be enabled and disabled while a trace experiment is
          running.

     ‘QTBuffer:size’
          The remote stub supports the ‘QTBuffer:size’ (*note
          QTBuffer-size::) packet that allows to change the size of the
          trace buffer.

     ‘tracenz’
          The remote stub supports the ‘tracenz’ bytecode for collecting
          strings.  See *note Bytecode Descriptions:: for details about
          the bytecode.

     ‘BreakpointCommands’
          The remote stub supports running a breakpoint's command list
          itself, rather than reporting the hit to GDB.

     ‘Qbtrace:off’
          The remote stub understands the ‘Qbtrace:off’ packet.

     ‘Qbtrace:bts’
          The remote stub understands the ‘Qbtrace:bts’ packet.

     ‘Qbtrace:pt’
          The remote stub understands the ‘Qbtrace:pt’ packet.

     ‘Qbtrace-conf:bts:size’
          The remote stub understands the ‘Qbtrace-conf:bts:size’
          packet.

     ‘Qbtrace-conf:pt:size’
          The remote stub understands the ‘Qbtrace-conf:pt:size’ packet.

     ‘Qbtrace-conf:pt:ptwrite’
          The remote stub understands the ‘Qbtrace-conf:pt:ptwrite’
          packet.

     ‘Qbtrace-conf:pt:event-tracing’
          The remote stub understands the
          ‘Qbtrace-conf:pt:event-tracing’ packet.

     ‘swbreak’
          The remote stub reports the ‘swbreak’ stop reason for memory
          breakpoints.

     ‘hwbreak’
          The remote stub reports the ‘hwbreak’ stop reason for hardware
          breakpoints.

     ‘fork-events’
          The remote stub reports the ‘fork’ stop reason for fork
          events.

     ‘vfork-events’
          The remote stub reports the ‘vfork’ stop reason for vfork
          events and vforkdone events.

     ‘exec-events’
          The remote stub reports the ‘exec’ stop reason for exec
          events.

     ‘vContSupported’
          The remote stub reports the supported actions in the reply to
          ‘vCont?’ packet.

     ‘QThreadEvents’
          The remote stub understands the ‘QThreadEvents’ packet.

     ‘QThreadOptions=SUPPORTED_OPTIONS’
          The remote stub understands the ‘QThreadOptions’ packet.
          SUPPORTED_OPTIONS indicates the set of thread options the
          remote stub supports.  SUPPORTED_OPTIONS has the same format
          as the OPTIONS parameter of the ‘QThreadOptions’ packet,
          described at *note QThreadOptions::.

     ‘no-resumed’
          The remote stub reports the ‘N’ stop reply.

     ‘memory-tagging’
          The remote stub supports and implements the required memory
          tagging functionality and understands the ‘qMemTags’ (*note
          qMemTags::) and ‘QMemTags’ (*note QMemTags::) packets.

          For AArch64 GNU/Linux systems, this feature can require access
          to the ‘/proc/PID/smaps’ file so memory mapping page flags can
          be inspected, if ‘qIsAddressTagged’ (*note qIsAddressTagged::)
          packet is not supported by the stub.  Access to the
          ‘/proc/PID/smaps’ file is done via ‘vFile’ requests.

     ‘error-message’
          The remote stub supports replying with an error in a
          ‘E.ERRTEXT’ (*Note textual error reply::) format from the ‘m’
          and ‘qRcmd’ packets.  It is not usually necessary to send this
          feature back to GDB in the ‘qSupported’ reply, GDB will always
          support ‘E.ERRTEXT’ format replies if it sent the
          ‘error-message’ feature.

     ‘binary-upload’
          The remote stub supports the ‘x’ packet (*note x packet::).

     ‘single-inf-arg’
          The remote stub would like to receive the inferior arguments
          as a single string within the ‘vRun’ packet.  The stub should
          only send this feature if GDB sent ‘single-inf-arg+’ in the
          ‘qSupported’ packet.

     ‘multi-wp-addr’
          The remote stub supports sending multiple watchpoint addresses
          within ‘T’ stop reply packet.  Stubs that don't support this
          feature don't need to tell GDB.  Not supporting this feature
          just means sending back one watchpoint address instead of
          multiple, and GDB has always supported receiving a single
          watchpoint address.

‘qSymbol::’
     Notify the target that GDB is prepared to serve symbol lookup
     requests.  Accept requests from the target for the values of
     symbols.

     Reply:
     ‘OK’
          The target does not need to look up any (more) symbols.
     ‘qSymbol:SYM_NAME’
          The target requests the value of symbol SYM_NAME (hex
          encoded).  GDB may provide the value by using the
          ‘qSymbol:SYM_VALUE:SYM_NAME’ message, described below.

‘qSymbol:SYM_VALUE:SYM_NAME’
     Set the value of SYM_NAME to SYM_VALUE.

     SYM_NAME (hex encoded) is the name of a symbol whose value the
     target has previously requested.

     SYM_VALUE (hex) is the value for symbol SYM_NAME.  If GDB cannot
     supply a value for SYM_NAME, then this field will be empty.

     Reply:
     ‘OK’
          The target does not need to look up any (more) symbols.
     ‘qSymbol:SYM_NAME’
          The target requests the value of a new symbol SYM_NAME (hex
          encoded).  GDB will continue to supply the values of symbols
          (if available), until the target ceases to request them.

‘qTBuffer’
‘QTBuffer’
‘QTDisconnected’
‘QTDP’
‘QTDPsrc’
‘QTDV’
‘qTfP’
‘qTfV’
‘QTFrame’
‘qTMinFTPILen’

     *Note Tracepoint Packets::.

‘qThreadExtraInfo,THREAD-ID’
     Obtain from the target OS a printable string description of thread
     attributes for the thread THREAD-ID; see *note thread-id syntax::,
     for the forms of THREAD-ID.  This string may contain anything that
     the target OS thinks is interesting for GDB to tell the user about
     the thread.  The string is displayed in GDB's ‘info threads’
     display.  Some examples of possible thread extra info strings are
     ‘Runnable’, or ‘Blocked on Mutex’.

     Reply:
     ‘XX...’
          Where ‘XX...’ is a hex encoding of ASCII data, comprising the
          printable string containing the extra information about the
          thread's attributes.

     (Note that the ‘qThreadExtraInfo’ packet's name is separated from
     the command by a ‘,’, not a ‘:’, contrary to the naming conventions
     above.  Please don't use this packet as a model for new packets.)

‘QTNotes’
‘qTP’
‘QTSave’
‘qTsP’
‘qTsV’
‘QTStart’
‘QTStop’
‘QTEnable’
‘QTDisable’
‘QTinit’
‘QTro’
‘qTStatus’
‘qTV’
‘qTfSTM’
‘qTsSTM’
‘qTSTMat’
     *Note Tracepoint Packets::.

‘qXfer:OBJECT:read:ANNEX:OFFSET,LENGTH’
     Read uninterpreted bytes from the target's special data area
     identified by the keyword OBJECT.  Request LENGTH bytes starting at
     OFFSET bytes into the data.  The content and encoding of ANNEX is
     specific to OBJECT; it can supply additional details about what
     data to access.

     Reply:
     ‘m DATA’
          Data DATA (*note Binary Data::) has been read from the target.
          There may be more data at a higher address (although it is
          permitted to return ‘m’ even for the last valid block of data,
          as long as at least one byte of data was read).  It is
          possible for DATA to have fewer bytes than the LENGTH in the
          request.

     ‘l DATA’
          Data DATA (*note Binary Data::) has been read from the target.
          There is no more data to be read.  It is possible for DATA to
          have fewer bytes than the LENGTH in the request.

     ‘l’
          The OFFSET in the request is at the end of the data.  There is
          no more data to be read.

     Here are the specific requests of this form defined so far.  All
     the ‘qXfer:OBJECT:read:...’ requests use the same reply formats,
     listed above.

     ‘qXfer:auxv:read::OFFSET,LENGTH’
          Access the target's “auxiliary vector”.  *Note auxiliary
          vector: OS Information.  Note ANNEX must be empty.

          This packet is not probed by default; the remote stub must
          request it, by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

     ‘qXfer:btrace:read:ANNEX:OFFSET,LENGTH’

          Return a description of the current branch trace.  *Note
          Branch Trace Format::.  The annex part of the generic ‘qXfer’
          packet may have one of the following values:

          ‘all’
               Returns all available branch trace.

          ‘new’
               Returns all available branch trace if the branch trace
               changed since the last read request.

          ‘delta’
               Returns the new branch trace since the last read request.
               Adds a new block to the end of the trace that begins at
               zero and ends at the source location of the first branch
               in the trace buffer.  This extra block is used to stitch
               traces together.

               If the trace buffer overflowed, returns an error
               indicating the overflow.

          This packet is not probed by default; the remote stub must
          request it by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

     ‘qXfer:btrace-conf:read::OFFSET,LENGTH’

          Return a description of the current branch trace
          configuration.  *Note Branch Trace Configuration Format::.

          This packet is not probed by default; the remote stub must
          request it by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

     ‘qXfer:exec-file:read:ANNEX:OFFSET,LENGTH’
          Return the full absolute name of the file that was executed to
          create a process running on the remote system.  The annex
          specifies the numeric process ID of the process to query,
          encoded as a hexadecimal number.  If the annex part is empty
          the remote stub should return the filename corresponding to
          the currently executing process.

          This packet is not probed by default; the remote stub must
          request it, by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

     ‘qXfer:features:read:ANNEX:OFFSET,LENGTH’
          Access the “target description”.  *Note Target Descriptions::.
          The annex specifies which XML document to access.  The main
          description is always loaded from the ‘target.xml’ annex.

          This packet is not probed by default; the remote stub must
          request it, by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

     ‘qXfer:libraries:read:ANNEX:OFFSET,LENGTH’
          Access the target's list of loaded libraries.  *Note Library
          List Format::.  The annex part of the generic ‘qXfer’ packet
          must be empty (*note qXfer read::).

          Targets which maintain a list of libraries in the program's
          memory do not need to implement this packet; it is designed
          for platforms where the operating system manages the list of
          loaded libraries.

          This packet is not probed by default; the remote stub must
          request it, by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

     ‘qXfer:libraries-svr4:read:ANNEX:OFFSET,LENGTH’
          Access the target's list of loaded libraries when the target
          is an SVR4 platform.  *Note Library List Format for SVR4
          Targets::.  The annex part of the generic ‘qXfer’ packet must
          be empty unless the remote stub indicated it supports the
          augmented form of this packet by supplying an appropriate
          ‘qSupported’ response (*note qXfer read::, *note
          qSupported::).

          This packet is optional for better performance on SVR4
          targets.  GDB uses memory read packets to read the SVR4
          library list otherwise.

          This packet is not probed by default; the remote stub must
          request it, by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

          If the remote stub indicates it supports the augmented form of
          this packet then the annex part of the generic ‘qXfer’ packet
          may contain a semicolon-separated list of ‘NAME=VALUE’
          arguments.  The currently supported arguments are:

          ‘start=ADDRESS’
               A hexadecimal number specifying the address of the
               ‘struct link_map’ to start reading the library list from.
               If unset or zero then the first ‘struct link_map’ in the
               library list will be chosen as the starting point.

          ‘prev=ADDRESS’
               A hexadecimal number specifying the address of the
               ‘struct link_map’ immediately preceding the ‘struct
               link_map’ specified by the ‘start’ argument.  If unset or
               zero then the remote stub will expect that no ‘struct
               link_map’ exists prior to the starting point.

          ‘lmid=LMID’
               A hexadecimal number specifying a namespace identifier.
               This is currently only used together with ‘start’ to
               provide the namespace identifier back to GDB in the
               response.  GDB will only provide values that were
               previously reported to it.  If unset, the response will
               include ‘lmid="0x0"’.

          Arguments that are not understood by the remote stub will be
          silently ignored.

     ‘qXfer:memory-map:read::OFFSET,LENGTH’
          Access the target's “memory-map”.  *Note Memory Map Format::.
          The annex part of the generic ‘qXfer’ packet must be empty
          (*note qXfer read::).

          This packet is not probed by default; the remote stub must
          request it, by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

     ‘qXfer:sdata:read::OFFSET,LENGTH’

          Read contents of the extra collected static tracepoint marker
          information.  The annex part of the generic ‘qXfer’ packet
          must be empty (*note qXfer read::).  *Note Tracepoint Action
          Lists: Tracepoint Actions.

          This packet is not probed by default; the remote stub must
          request it, by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

     ‘qXfer:siginfo:read::OFFSET,LENGTH’
          Read contents of the extra signal information on the target
          system.  The annex part of the generic ‘qXfer’ packet must be
          empty (*note qXfer read::).

          This packet is not probed by default; the remote stub must
          request it, by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

     ‘qXfer:threads:read::OFFSET,LENGTH’
          Access the list of threads on target.  *Note Thread List
          Format::.  The annex part of the generic ‘qXfer’ packet must
          be empty (*note qXfer read::).

          This packet is not probed by default; the remote stub must
          request it, by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

     ‘qXfer:traceframe-info:read::OFFSET,LENGTH’

          Return a description of the current traceframe's contents.
          *Note Traceframe Info Format::.  The annex part of the generic
          ‘qXfer’ packet must be empty (*note qXfer read::).

          This packet is not probed by default; the remote stub must
          request it, by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

     ‘qXfer:uib:read:PC:OFFSET,LENGTH’

          Return the unwind information block for PC.  This packet is
          used on OpenVMS/ia64 to ask the kernel unwind information.

          This packet is not probed by default.

     ‘qXfer:fdpic:read:ANNEX:OFFSET,LENGTH’
          Read contents of ‘loadmap’s on the target system.  The annex,
          either ‘exec’ or ‘interp’, specifies which ‘loadmap’,
          executable ‘loadmap’ or interpreter ‘loadmap’ to read.

          This packet is not probed by default; the remote stub must
          request it, by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

     ‘qXfer:osdata:read::OFFSET,LENGTH’
          Access the target's “operating system information”.  *Note
          Operating System Information::.

‘qXfer:OBJECT:write:ANNEX:OFFSET:DATA...’
     Write uninterpreted bytes into the target's special data area
     identified by the keyword OBJECT, starting at OFFSET bytes into the
     data.  The binary-encoded data (*note Binary Data::) to be written
     is given by DATA....  The content and encoding of ANNEX is specific
     to OBJECT; it can supply additional details about what data to
     access.

     Reply:
     ‘NN’
          NN (hex encoded) is the number of bytes written.  This may be
          fewer bytes than supplied in the request.

     Here are the specific requests of this form defined so far.  All
     the ‘qXfer:OBJECT:write:...’ requests use the same reply formats,
     listed above.

     ‘qXfer:siginfo:write::OFFSET:DATA...’
          Write DATA to the extra signal information on the target
          system.  The annex part of the generic ‘qXfer’ packet must be
          empty (*note qXfer write::).

          This packet is not probed by default; the remote stub must
          request it, by supplying an appropriate ‘qSupported’ response
          (*note qSupported::).

‘qXfer:OBJECT:OPERATION:...’
     Requests of this form may be added in the future.  When a stub does
     not recognize the OBJECT keyword, or its support for OBJECT does
     not recognize the OPERATION keyword, the stub must respond with an
     empty packet.

‘qAttached:PID’
     Return an indication of whether the remote server attached to an
     existing process or created a new process.  When the multiprocess
     protocol extensions are supported (*note multiprocess
     extensions::), PID is an integer in hexadecimal format identifying
     the target process.  Otherwise, GDB will omit the PID field and the
     query packet will be simplified as ‘qAttached’.

     This query is used, for example, to know whether the remote process
     should be detached or killed when a GDB session is ended with the
     ‘quit’ command.

     Reply:
     ‘1’
          The remote server attached to an existing process.
     ‘0’
          The remote server created a new process.

‘qExecAndArgs’
     Return the program filename and arguments string with which the
     remote server was started, if the remote server was started with
     such things.  If the remote server was started without the filename
     of a program to execute, or without any arguments, then the reply
     indicates this.

     Reply:
     ‘U’
          The program filename and arguments are unset.  If GDB wants to
          start a new inferior, for example with ‘vRun’, then it will
          need to provide the filename of a program to use.

     ‘S;PROG;ARGS;’
     ‘S;;ARGS;’
          The program filename provided to the remote server when it
          started was PROG, which is a hex encoded string.  The complete
          argument string passed to the inferior when it started as
          ARGS, this is also a hex encoded string.

          If no arguments were passed when the inferior started then
          ARGS be an empty string.

          It is valid for PROG to be the empty string, this indicates
          that the server has no program set, GDB will need to supply a
          program name in order to start a new inferior.  It is valid to
          reply with an empty PROG and non-empty ARGS, GDB will set the
          inferior arguments, but the user will need to supply a remote
          exec-file before an inferior can be started.

          The argument string ARGS is passed directly to GDB as if to
          the ‘set args’ command.  And escaping required should be
          present in ARGS, no changes are made by GDB.

‘Qbtrace:bts’
     Enable branch tracing for the current thread using Branch Trace
     Store.

     Reply:
     ‘OK’
          Branch tracing has been enabled.

‘Qbtrace:pt’
     Enable branch tracing for the current thread using Intel Processor
     Trace.

     Reply:
     ‘OK’
          Branch tracing has been enabled.

‘Qbtrace:off’
     Disable branch tracing for the current thread.

     Reply:
     ‘OK’
          Branch tracing has been disabled.

‘Qbtrace-conf:bts:size=VALUE’
     Set the requested ring buffer size for new threads that use the
     btrace recording method in bts format.

     Reply:
     ‘OK’
          The ring buffer size has been set.

‘Qbtrace-conf:pt:size=VALUE’
     Set the requested ring buffer size for new threads that use the
     btrace recording method in pt format.

     Reply:
     ‘OK’
          The ring buffer size has been set.

‘Qbtrace-conf:pt:ptwrite=(YES|NO)’
     Indicate support for ‘PTWRITE’ packets.  This allows for backwards
     compatibility.

     Reply:
     ‘OK’
          The ptwrite config parameter has been set.
     ‘E.errtext’
          A badly formed request or an error was encountered.

‘Qbtrace-conf:pt:event-tracing=(YES|NO)’
     Indicate support for event-tracing packets.  This allows for
     backwards compatibility.

     Reply:
     ‘OK’
          The event-tracing config parameter has been set.
     ‘E.errtext’
          A badly formed request or an error was encountered.

   ---------- Footnotes ----------

   (1) The ‘qP’ and ‘qL’ packets predate these conventions, and have
arguments without any terminator for the packet name; we suspect they
are in widespread use in places that are difficult to upgrade.  The ‘qC’
packet has no arguments, but some existing stubs (e.g. RedBoot) are
known to not check for the end of the packet.


File: gdb.info,  Node: Architecture-Specific Protocol Details,  Next: Tracepoint Packets,  Prev: General Query Packets,  Up: Remote Protocol

E.6 Architecture-Specific Protocol Details
==========================================

This section describes how the remote protocol is applied to specific
target architectures.  Also see *note Standard Target Features::, for
details of XML target descriptions for each architecture.

* Menu:

* ARM-Specific Protocol Details::
* MIPS-Specific Protocol Details::


File: gdb.info,  Node: ARM-Specific Protocol Details,  Next: MIPS-Specific Protocol Details,  Up: Architecture-Specific Protocol Details

E.6.1 ARM-specific Protocol Details
-----------------------------------

* Menu:

* ARM Breakpoint Kinds::
* ARM Memory Tag Types::


File: gdb.info,  Node: ARM Breakpoint Kinds,  Next: ARM Memory Tag Types,  Up: ARM-Specific Protocol Details

E.6.1.1 ARM Breakpoint Kinds
............................

These breakpoint kinds are defined for the ‘Z0’ and ‘Z1’ packets.

2
     16-bit Thumb mode breakpoint.

3
     32-bit Thumb mode (Thumb-2) breakpoint.

4
     32-bit ARM mode breakpoint.


File: gdb.info,  Node: ARM Memory Tag Types,  Prev: ARM Breakpoint Kinds,  Up: ARM-Specific Protocol Details

E.6.1.2 ARM Memory Tag Types
............................

These memory tag types are defined for the ‘qMemTag’ and ‘QMemTag’
packets.

0
     MTE logical tag

1
     MTE allocation tag


File: gdb.info,  Node: MIPS-Specific Protocol Details,  Prev: ARM-Specific Protocol Details,  Up: Architecture-Specific Protocol Details

E.6.2 MIPS-specific Protocol Details
------------------------------------

* Menu:

* MIPS Register packet Format::
* MIPS Breakpoint Kinds::


File: gdb.info,  Node: MIPS Register packet Format,  Next: MIPS Breakpoint Kinds,  Up: MIPS-Specific Protocol Details

E.6.2.1 MIPS Register Packet Format
...................................

The following ‘g’/‘G’ packets have previously been defined.  In the
below, some thirty-two bit registers are transferred as sixty-four bits.
Those registers should be zero/sign extended (which?)  to fill the space
allocated.  Register bytes are transferred in target byte order.  The
two nibbles within a register byte are transferred most-significant -
least-significant.

MIPS32
     All registers are transferred as thirty-two bit quantities in the
     order: 32 general-purpose; sr; lo; hi; bad; cause; pc; 32
     floating-point registers; fsr; fir; fp.

MIPS64
     All registers are transferred as sixty-four bit quantities
     (including thirty-two bit registers such as ‘sr’).  The ordering is
     the same as ‘MIPS32’.


File: gdb.info,  Node: MIPS Breakpoint Kinds,  Prev: MIPS Register packet Format,  Up: MIPS-Specific Protocol Details

E.6.2.2 MIPS Breakpoint Kinds
.............................

These breakpoint kinds are defined for the ‘Z0’ and ‘Z1’ packets.

2
     16-bit MIPS16 mode breakpoint.

3
     16-bit microMIPS mode breakpoint.

4
     32-bit standard MIPS mode breakpoint.

5
     32-bit microMIPS mode breakpoint.


File: gdb.info,  Node: Tracepoint Packets,  Next: Host I/O Packets,  Prev: Architecture-Specific Protocol Details,  Up: Remote Protocol

E.7 Tracepoint Packets
======================

Here we describe the packets GDB uses to implement tracepoints (*note
Tracepoints::).

‘QTDP:N:ADDR:ENA:STEP:PASS[:FFLEN][:S][:XLEN,BYTES][-]’
     Create a new tracepoint, number N, at ADDR.  If ENA is ‘E’, then
     the tracepoint is enabled; if it is ‘D’, then the tracepoint is
     disabled.  The STEP gives the tracepoint's step count, and PASS
     gives its pass count.  If an ‘F’ is present, then the tracepoint is
     to be a fast tracepoint, and the FLEN is the number of bytes that
     the target should copy elsewhere to make room for the tracepoint.
     If an ‘S’ is present, the tracepoint is to be a static tracepoint.
     If an ‘X’ is present, it introduces a tracepoint condition, which
     consists of a hexadecimal length, followed by a comma and
     hex-encoded bytes, in a manner similar to action encodings as
     described below.  If the trailing ‘-’ is present, further ‘QTDP’
     packets will follow to specify this tracepoint's actions.

     Replies:
     ‘OK’
          The packet was understood and carried out.
     ‘qRelocInsn’
          *Note Relocate instruction reply packet: Tracepoint Packets.

‘QTDP:-N:ADDR:[S]ACTION...[-]’
     Define actions to be taken when a tracepoint is hit.  The N and
     ADDR must be the same as in the initial ‘QTDP’ packet for this
     tracepoint.  This packet may only be sent immediately after another
     ‘QTDP’ packet that ended with a ‘-’.  If the trailing ‘-’ is
     present, further ‘QTDP’ packets will follow, specifying more
     actions for this tracepoint.

     In the series of action packets for a given tracepoint, at most one
     can have an ‘S’ before its first ACTION.  If such a packet is sent,
     it and the following packets define "while-stepping" actions.  Any
     prior packets define ordinary actions -- that is, those taken when
     the tracepoint is first hit.  If no action packet has an ‘S’, then
     all the packets in the series specify ordinary tracepoint actions.

     The ‘ACTION...’ portion of the packet is a series of actions,
     concatenated without separators.  Each action has one of the
     following forms:

     ‘R MASK’
          Collect the registers whose bits are set in MASK, a
          hexadecimal number whose I'th bit is set if register number I
          should be collected.  (The least significant bit is numbered
          zero.)  Note that MASK may be any number of digits long; it
          may not fit in a 32-bit word.

     ‘M BASEREG,OFFSET,LEN’
          Collect LEN bytes of memory starting at the address in
          register number BASEREG, plus OFFSET.  If BASEREG is ‘-1’,
          then the range has a fixed address: OFFSET is the address of
          the lowest byte to collect.  The BASEREG, OFFSET, and LEN
          parameters are all unsigned hexadecimal values (the ‘-1’ value
          for BASEREG is a special case).

     ‘X LEN,EXPR’
          Evaluate EXPR, whose length is LEN, and collect memory as it
          directs.  The agent expression EXPR is as described in *note
          Agent Expressions::.  Each byte of the expression is encoded
          as a two-digit hex number in the packet; LEN is the number of
          bytes in the expression (and thus one-half the number of hex
          digits in the packet).

     ‘L’
          Collect static trace data.

     Any number of actions may be packed together in a single ‘QTDP’
     packet, as long as the packet does not exceed the maximum packet
     length (400 bytes, for many stubs).  There may be only one ‘R’
     action per tracepoint, and it must precede any ‘M’ or ‘X’ actions.
     Any registers referred to by ‘M’ and ‘X’ actions must be collected
     by a preceding ‘R’ action.  (The "while-stepping" actions are
     treated as if they were attached to a separate tracepoint, as far
     as these restrictions are concerned.)

     Replies:
     ‘OK’
          The packet was understood and carried out.
     ‘qRelocInsn’
          *Note Relocate instruction reply packet: Tracepoint Packets.

‘QTDPsrc:N:ADDR:TYPE:START:SLEN:BYTES’
     Specify a source string of tracepoint N at address ADDR.  This is
     useful to get accurate reproduction of the tracepoints originally
     downloaded at the beginning of the trace run.  The TYPE is the name
     of the tracepoint part, such as ‘cond’ for the tracepoint's
     conditional expression (see below for a list of types), while BYTES
     is the string, encoded in hexadecimal.

     START is the offset of the BYTES within the overall source string,
     while SLEN is the total length of the source string.  This is
     intended for handling source strings that are longer than will fit
     in a single packet.

     The available string types are ‘at’ for the location, ‘cond’ for
     the conditional, and ‘cmd’ for an action command.  GDB sends a
     separate packet for each command in the action list, in the same
     order in which the commands are stored in the list.

     The target does not need to do anything with source strings except
     report them back as part of the replies to the ‘qTfP’/‘qTsP’ query
     packets.

     Although this packet is optional, and GDB will only send it if the
     target replies with ‘TracepointSource’ *Note General Query
     Packets::, it makes both disconnected tracing and trace files much
     easier to use.  Otherwise the user must be careful that the
     tracepoints in effect while looking at trace frames are identical
     to the ones in effect during the trace run; even a small
     discrepancy could cause ‘tdump’ not to work, or a particular trace
     frame not be found.

‘QTDV:N:VALUE:BUILTIN:NAME’
     Create a new trace state variable, number N, with an initial value
     of VALUE, which is a 64-bit signed integer.  Both N and VALUE are
     encoded as hexadecimal values.  GDB has the option of not using
     this packet for initial values of zero; the target should simply
     create the trace state variables as they are mentioned in
     expressions.  The value BUILTIN should be 1 (one) if the trace
     state variable is builtin and 0 (zero) if it is not builtin.  GDB
     only sets BUILTIN to 1 if a previous ‘qTfV’ or ‘qTsV’ packet had it
     set.  The contents of NAME is the hex-encoded name (without the
     leading ‘$’) of the trace state variable.

‘QTFrame:N’
     Select the N'th tracepoint frame from the buffer, and use the
     register and memory contents recorded there to answer subsequent
     request packets from GDB.

     A successful reply from the stub indicates that the stub has found
     the requested frame.  The response is a series of parts,
     concatenated without separators, describing the frame we selected.
     Each part has one of the following forms:

     ‘F F’
          The selected frame is number N in the trace frame buffer; F is
          a hexadecimal number.  If F is ‘-1’, then there was no frame
          matching the criteria in the request packet.

     ‘T T’
          The selected trace frame records a hit of tracepoint number T;
          T is a hexadecimal number.

‘QTFrame:pc:ADDR’
     Like ‘QTFrame:N’, but select the first tracepoint frame after the
     currently selected frame whose PC is ADDR; ADDR is a hexadecimal
     number.

‘QTFrame:tdp:T’
     Like ‘QTFrame:N’, but select the first tracepoint frame after the
     currently selected frame that is a hit of tracepoint T; T is a
     hexadecimal number.

‘QTFrame:range:START:END’
     Like ‘QTFrame:N’, but select the first tracepoint frame after the
     currently selected frame whose PC is between START (inclusive) and
     END (inclusive); START and END are hexadecimal numbers.

‘QTFrame:outside:START:END’
     Like ‘QTFrame:range:START:END’, but select the first frame
     _outside_ the given range of addresses (exclusive).

‘qTMinFTPILen’
     This packet requests the minimum length of instruction at which a
     fast tracepoint (*note Set Tracepoints::) may be placed.  For
     instance, on the 32-bit x86 architecture, it is possible to use a
     4-byte jump, but it depends on the target system being able to
     create trampolines in the first 64K of memory, which might or might
     not be possible for that system.  So the reply to this packet will
     be 4 if it is able to arrange for that.

     Replies:

     ‘0’
          The minimum instruction length is currently unknown.
     ‘LENGTH’
          The minimum instruction length is LENGTH, where LENGTH is a
          hexadecimal number greater or equal to 1.  A reply of 1 means
          that a fast tracepoint may be placed on any instruction
          regardless of size.
     ‘E’
          An error has occurred.

‘QTStart’
     Begin the tracepoint experiment.  Begin collecting data from
     tracepoint hits in the trace frame buffer.  This packet supports
     the ‘qRelocInsn’ reply (*note Relocate instruction reply packet:
     Tracepoint Packets.).

‘QTStop’
     End the tracepoint experiment.  Stop collecting trace frames.

‘QTEnable:N:ADDR’
     Enable tracepoint N at address ADDR in a started tracepoint
     experiment.  If the tracepoint was previously disabled, then
     collection of data from it will resume.

‘QTDisable:N:ADDR’
     Disable tracepoint N at address ADDR in a started tracepoint
     experiment.  No more data will be collected from the tracepoint
     unless ‘QTEnable:N:ADDR’ is subsequently issued.

‘QTinit’
     Clear the table of tracepoints, and empty the trace frame buffer.

‘QTro:START1,END1:START2,END2:...’
     Establish the given ranges of memory as "transparent".  The stub
     will answer requests for these ranges from memory's current
     contents, if they were not collected as part of the tracepoint hit.

     GDB uses this to mark read-only regions of memory, like those
     containing program code.  Since these areas never change, they
     should still have the same contents they did when the tracepoint
     was hit, so there's no reason for the stub to refuse to provide
     their contents.

‘QTDisconnected:VALUE’
     Set the choice to what to do with the tracing run when GDB
     disconnects from the target.  A VALUE of 1 directs the target to
     continue the tracing run, while 0 tells the target to stop tracing
     if GDB is no longer in the picture.

‘qTStatus’
     Ask the stub if there is a trace experiment running right now.

     The reply has the form:

     ‘TRUNNING[;FIELD]...’
          RUNNING is a single digit ‘1’ if the trace is presently
          running, or ‘0’ if not.  It is followed by semicolon-separated
          optional fields that an agent may use to report additional
          status.

     If the trace is not running, the agent may report any of several
     explanations as one of the optional fields:

     ‘tnotrun:0’
          No trace has been run yet.

     ‘tstop[:TEXT]:0’
          The trace was stopped by a user-originated stop command.  The
          optional TEXT field is a user-supplied string supplied as part
          of the stop command (for instance, an explanation of why the
          trace was stopped manually).  It is hex-encoded.

     ‘tfull:0’
          The trace stopped because the trace buffer filled up.

     ‘tdisconnected:0’
          The trace stopped because GDB disconnected from the target.

     ‘tpasscount:TPNUM’
          The trace stopped because tracepoint TPNUM exceeded its pass
          count.

     ‘terror:TEXT:TPNUM’
          The trace stopped because tracepoint TPNUM had an error.  The
          string TEXT is available to describe the nature of the error
          (for instance, a divide by zero in the condition expression);
          it is hex encoded.

     ‘tunknown:0’
          The trace stopped for some other reason.

     Additional optional fields supply statistical and other
     information.  Although not required, they are extremely useful for
     users monitoring the progress of a trace run.  If a trace has
     stopped, and these numbers are reported, they must reflect the
     state of the just-stopped trace.

     ‘tframes:N’
          The number of trace frames in the buffer.

     ‘tcreated:N’
          The total number of trace frames created during the run.  This
          may be larger than the trace frame count, if the buffer is
          circular.

     ‘tsize:N’
          The total size of the trace buffer, in bytes.

     ‘tfree:N’
          The number of bytes still unused in the buffer.

     ‘circular:N’
          The value of the circular trace buffer flag.  ‘1’ means that
          the trace buffer is circular and old trace frames will be
          discarded if necessary to make room, ‘0’ means that the trace
          buffer is linear and may fill up.

     ‘disconn:N’
          The value of the disconnected tracing flag.  ‘1’ means that
          tracing will continue after GDB disconnects, ‘0’ means that
          the trace run will stop.

‘qTP:TP:ADDR’
     Ask the stub for the current state of tracepoint number TP at
     address ADDR.

     Replies:
     ‘VHITS:USAGE’
          The tracepoint has been hit HITS times so far during the trace
          run, and accounts for USAGE in the trace buffer.  Note that
          ‘while-stepping’ steps are not counted as separate hits, but
          the steps' space consumption is added into the usage number.

‘qTV:VAR’
     Ask the stub for the value of the trace state variable number VAR.

     Replies:
     ‘VVALUE’
          The value of the variable is VALUE.  This will be the current
          value of the variable if the user is examining a running
          target, or a saved value if the variable was collected in the
          trace frame that the user is looking at.  Note that multiple
          requests may result in different reply values, such as when
          requesting values while the program is running.

     ‘U’
          The value of the variable is unknown.  This would occur, for
          example, if the user is examining a trace frame in which the
          requested variable was not collected.

‘qTfP’
‘qTsP’
     These packets request data about tracepoints that are being used by
     the target.  GDB sends ‘qTfP’ to get the first piece of data, and
     multiple ‘qTsP’ to get additional pieces.  Replies to these packets
     generally take the form of the ‘QTDP’ packets that define
     tracepoints.  (FIXME add detailed syntax)

‘qTfV’
‘qTsV’
     These packets request data about trace state variables that are on
     the target.  GDB sends ‘qTfV’ to get the first vari of data, and
     multiple ‘qTsV’ to get additional variables.  Replies to these
     packets follow the syntax of the ‘QTDV’ packets that define trace
     state variables.

‘qTfSTM’
‘qTsSTM’
     These packets request data about static tracepoint markers that
     exist in the target program.  GDB sends ‘qTfSTM’ to get the first
     piece of data, and multiple ‘qTsSTM’ to get additional pieces.
     Replies to these packets take the following form:

     Reply:
     ‘m ADDRESS:ID:EXTRA’
          A single marker
     ‘m ADDRESS:ID:EXTRA,ADDRESS:ID:EXTRA...’
          a comma-separated list of markers
     ‘l’
          (lower case letter ‘L’) denotes end of list.

     The ADDRESS is encoded in hex; ID and EXTRA are strings encoded in
     hex.

     In response to each query, the target will reply with a list of one
     or more markers, separated by commas.  GDB will respond to each
     reply with a request for more markers (using the ‘qs’ form of the
     query), until the target responds with ‘l’ (lower-case ell, for
     “last”).

‘qTSTMat:ADDRESS’
     This packet requests data about static tracepoint markers in the
     target program at ADDRESS.  Replies to this packet follow the
     syntax of the ‘qTfSTM’ and ‘qTsSTM’ packets that list static
     tracepoint markers.

‘QTSave:FILENAME’
     This packet directs the target to save trace data to the file name
     FILENAME in the target's filesystem.  The FILENAME is encoded as a
     hex string; the interpretation of the file name (relative vs
     absolute, wild cards, etc) is up to the target.

‘qTBuffer:OFFSET,LEN’
     Return up to LEN bytes of the current contents of trace buffer,
     starting at OFFSET.  The trace buffer is treated as if it were a
     contiguous collection of traceframes, as per the trace file format.
     The reply consists as many hex-encoded bytes as the target can
     deliver in a packet; it is not an error to return fewer than were
     asked for.  A reply consisting of just ‘l’ indicates that no bytes
     are available.

‘QTBuffer:circular:VALUE’
     This packet directs the target to use a circular trace buffer if
     VALUE is 1, or a linear buffer if the value is 0.

‘QTBuffer:size:SIZE’
     This packet directs the target to make the trace buffer be of size
     SIZE if possible.  A value of ‘-1’ tells the target to use whatever
     size it prefers.

‘QTNotes:[TYPE:TEXT][;TYPE:TEXT]...’
     This packet adds optional textual notes to the trace run.
     Allowable types include ‘user’, ‘notes’, and ‘tstop’, the TEXT
     fields are arbitrary strings, hex-encoded.

E.7.1 Relocate instruction reply packet
---------------------------------------

When installing fast tracepoints in memory, the target may need to
relocate the instruction currently at the tracepoint address to a
different address in memory.  For most instructions, a simple copy is
enough, but, for example, call instructions that implicitly push the
return address on the stack, and relative branches or other PC-relative
instructions require offset adjustment, so that the effect of executing
the instruction at a different address is the same as if it had executed
in the original location.

   In response to several of the tracepoint packets, the target may also
respond with a number of intermediate ‘qRelocInsn’ request packets
before the final result packet, to have GDB handle this relocation
operation.  If a packet supports this mechanism, its documentation will
explicitly say so.  See for example the above descriptions for the
‘QTStart’ and ‘QTDP’ packets.  The format of the request is:

‘qRelocInsn:FROM;TO’

     This requests GDB to copy instruction at address FROM to address
     TO, possibly adjusted so that executing the instruction at TO has
     the same effect as executing it at FROM.  GDB writes the adjusted
     instruction to target memory starting at TO.

   Replies:
‘qRelocInsn:ADJUSTED_SIZE’
     Informs the stub the relocation is complete.  The ADJUSTED_SIZE is
     the length in bytes of resulting relocated instruction sequence.


File: gdb.info,  Node: Host I/O Packets,  Next: Interrupts,  Prev: Tracepoint Packets,  Up: Remote Protocol

E.8 Host I/O Packets
====================

The “Host I/O” packets allow GDB to perform I/O operations on the far
side of a remote link.  For example, Host I/O is used to upload and
download files to a remote target with its own filesystem.  Host I/O
uses the same constant values and data structure layout as the
target-initiated File-I/O protocol.  However, the Host I/O packets are
structured differently.  The target-initiated protocol relies on target
memory to store parameters and buffers.  Host I/O requests are initiated
by GDB, and the target's memory is not involved.  *Note File-I/O Remote
Protocol Extension::, for more details on the target-initiated protocol.

   The Host I/O request packets all encode a single operation along with
its arguments.  They have this format:

‘vFile:OPERATION: PARAMETER...’
     OPERATION is the name of the particular request; the target should
     compare the entire packet name up to the second colon when checking
     for a supported operation.  The format of PARAMETER depends on the
     operation.  Numbers are always passed in hexadecimal.  Negative
     numbers have an explicit minus sign (i.e. two's complement is not
     used).  Strings (e.g. filenames) are encoded as a series of
     hexadecimal bytes.  The last argument to a system call may be a
     buffer of escaped binary data (*note Binary Data::).

   The valid responses to Host I/O packets are:

‘F RESULT [, ERRNO] [; ATTACHMENT]’
     RESULT is the integer value returned by this operation, usually
     non-negative for success and -1 for errors.  If an error has
     occurred, ERRNO will be included in the result specifying a value
     defined by the File-I/O protocol (*note Errno Values::).  For
     operations which return data, ATTACHMENT supplies the data as a
     binary buffer.  Binary buffers in response packets are escaped in
     the normal way (*note Binary Data::).  See the individual packet
     documentation for the interpretation of RESULT and ATTACHMENT.

‘’
     An empty response indicates that this operation is not recognized.

   These are the supported Host I/O operations:

‘vFile:open: FILENAME, FLAGS, MODE’
     Open a file at FILENAME and return a file descriptor for it, or
     return -1 if an error occurs.  The FILENAME is a string, FLAGS is
     an integer indicating a mask of open flags (*note Open Flags::),
     and MODE is an integer indicating a mask of mode bits to use if the
     file is created (*note mode_t Values::).  *Note open::, for details
     of the open flags and mode values.

‘vFile:close: FD’
     Close the open file corresponding to FD and return 0, or -1 if an
     error occurs.

‘vFile:pread: FD, COUNT, OFFSET’
     Read data from the open file corresponding to FD.  Up to COUNT
     bytes will be read from the file, starting at OFFSET relative to
     the start of the file.  The target may read fewer bytes; common
     reasons include packet size limits and an end-of-file condition.
     The number of bytes read is returned.  Zero should only be returned
     for a successful read at the end of the file, or if COUNT was zero.

     The data read should be returned as a binary attachment on success.
     If zero bytes were read, the response should include an empty
     binary attachment (i.e. a trailing semicolon).  The return value is
     the number of target bytes read; the binary attachment may be
     longer if some characters were escaped.

‘vFile:pwrite: FD, OFFSET, DATA’
     Write DATA (a binary buffer) to the open file corresponding to FD.
     Start the write at OFFSET from the start of the file.  Unlike many
     ‘write’ system calls, there is no separate COUNT argument; the
     length of DATA in the packet is used.  ‘vFile:pwrite’ returns the
     number of bytes written, which may be shorter than the length of
     DATA, or -1 if an error occurred.

‘vFile:fstat: FD’
     Get information about the open file corresponding to FD.  On
     success the information is returned as a binary attachment and the
     return value is the size of this attachment in bytes.  If an error
     occurs the return value is -1.  The format of the returned binary
     attachment is as described in *note struct stat::.

‘vFile:stat: FILENAME’
     Get information about the file FILENAME on the target as if from a
     ‘stat’ call.  On success the information is returned as a binary
     attachment and the return value is the size of this attachment in
     bytes.  If an error occurs the return value is -1.  The format of
     the returned binary attachment is as described in *note struct
     stat::.

     If FILENAME is a symbolic link, then the information returned is
     about the file the link refers to, this is inline with the ‘stat’
     library call.

‘vFile:lstat: FILENAME’
     Get information about the file FILENAME on the target as if from an
     ‘lstat’ call.  On success the information is returned as a binary
     attachment and the return value is the size of this attachment in
     bytes.  If an error occurs the return value is -1.  The format of
     the returned binary attachment is as described in *note struct
     stat::.

     This packet is identical to ‘vFile:stat’, except if FILENAME is a
     symbolic link, then this packet returns information about the link
     itself, not the file that the link refers to, this is inline with
     the ‘lstat’ library call.

‘vFile:unlink: FILENAME’
     Delete the file at FILENAME on the target.  Return 0, or -1 if an
     error occurs.  The FILENAME is a string.

‘vFile:readlink: FILENAME’
     Read value of symbolic link FILENAME on the target.  Return the
     number of bytes read, or -1 if an error occurs.

     The data read should be returned as a binary attachment on success.
     If zero bytes were read, the response should include an empty
     binary attachment (i.e. a trailing semicolon).  The return value is
     the number of target bytes read; the binary attachment may be
     longer if some characters were escaped.

‘vFile:setfs: PID’
     Select the filesystem on which ‘vFile’ operations with FILENAME
     arguments will operate.  This is required for GDB to be able to
     access files on remote targets where the remote stub does not share
     a common filesystem with the inferior(s).

     If PID is nonzero, select the filesystem as seen by process PID.
     If PID is zero, select the filesystem as seen by the remote stub.
     Return 0 on success, or -1 if an error occurs.  If ‘vFile:setfs:’
     indicates success, the selected filesystem remains selected until
     the next successful ‘vFile:setfs:’ operation.


File: gdb.info,  Node: Interrupts,  Next: Notification Packets,  Prev: Host I/O Packets,  Up: Remote Protocol

E.9 Interrupts
==============

In all-stop mode, when a program on the remote target is running, GDB
may attempt to interrupt it by sending a ‘Ctrl-C’, ‘BREAK’ or a ‘BREAK’
followed by ‘g’, control of which is specified via GDB's
‘interrupt-sequence’.

   The precise meaning of ‘BREAK’ is defined by the transport mechanism
and may, in fact, be undefined.  GDB does not currently define a ‘BREAK’
mechanism for any of the network interfaces except for TCP, in which
case GDB sends the ‘telnet’ BREAK sequence.

   ‘Ctrl-C’, on the other hand, is defined and implemented for all
transport mechanisms.  It is represented by sending the single byte
‘0x03’ without any of the usual packet overhead described in the
Overview section (*note Overview::).  When a ‘0x03’ byte is transmitted
as part of a packet, it is considered to be packet data and does _not_
represent an interrupt.  E.g., an ‘X’ packet (*note X packet::), used
for binary downloads, may include an unescaped ‘0x03’ as part of its
packet.

   ‘BREAK’ followed by ‘g’ is also known as Magic SysRq g.  When Linux
kernel receives this sequence from serial port, it stops execution and
connects to gdb.

   In non-stop mode, because packet resumptions are asynchronous (*note
vCont packet::), GDB is always free to send a remote command to the
remote stub, even when the target is running.  For that reason, GDB
instead sends a regular packet (*note vCtrlC packet::) with the usual
packet framing instead of the single byte ‘0x03’.

   Stubs are not required to recognize these interrupt mechanisms and
the precise meaning associated with receipt of the interrupt is
implementation defined.  If the target supports debugging of multiple
threads and/or processes, it should attempt to interrupt all
currently-executing threads and processes.  If the stub is successful at
interrupting the running program, it should send one of the stop reply
packets (*note Stop Reply Packets::) to GDB as a result of successfully
stopping the program in all-stop mode, and a stop reply for each stopped
thread in non-stop mode.  Interrupts received while the program is
stopped are queued and the program will be interrupted when it is
resumed next time.


File: gdb.info,  Node: Notification Packets,  Next: Remote Non-Stop,  Prev: Interrupts,  Up: Remote Protocol

E.10 Notification Packets
=========================

The GDB remote serial protocol includes “notifications”, packets that
require no acknowledgment.  Both the GDB and the stub may send
notifications (although the only notifications defined at present are
sent by the stub).  Notifications carry information without incurring
the round-trip latency of an acknowledgment, and so are useful for
low-impact communications where occasional packet loss is not a problem.

   A notification packet has the form ‘% DATA # CHECKSUM’, where DATA is
the content of the notification, and CHECKSUM is a checksum of DATA,
computed and formatted as for ordinary GDB packets.  A notification's
DATA never contains ‘$’, ‘%’ or ‘#’ characters.  Upon receiving a
notification, the recipient sends no ‘+’ or ‘-’ to acknowledge the
notification's receipt or to report its corruption.

   Every notification's DATA begins with a name, which contains no colon
characters, followed by a colon character.

   Recipients should silently ignore corrupted notifications and
notifications they do not understand.  Recipients should restart timeout
periods on receipt of a well-formed notification, whether or not they
understand it.

   Senders should only send the notifications described here when this
protocol description specifies that they are permitted.  In the future,
we may extend the protocol to permit existing notifications in new
contexts; this rule helps older senders avoid confusing newer
recipients.

   (Older versions of GDB ignore bytes received until they see the ‘$’
byte that begins an ordinary packet, so new stubs may transmit
notifications without fear of confusing older clients.  There are no
notifications defined for GDB to send at the moment, but we assume that
most older stubs would ignore them, as well.)

   Each notification is comprised of three parts:
‘NAME:EVENT’
     The notification packet is sent by the side that initiates the
     exchange (currently, only the stub does that), with EVENT carrying
     the specific information about the notification, and NAME
     specifying the name of the notification.
‘ACK’
     The acknowledge sent by the other side, usually GDB, to acknowledge
     the exchange and request the event.

   The purpose of an asynchronous notification mechanism is to report to
GDB that something interesting happened in the remote stub.

   The remote stub may send notification NAME:EVENT at any time, but GDB
acknowledges the notification when appropriate.  The notification event
is pending before GDB acknowledges.  Only one notification at a time may
be pending; if additional events occur before GDB has acknowledged the
previous notification, they must be queued by the stub for later
synchronous transmission in response to ACK packets from GDB.  Because
the notification mechanism is unreliable, the stub is permitted to
resend a notification if it believes GDB may not have received it.

   Specifically, notifications may appear when GDB is not otherwise
reading input from the stub, or when GDB is expecting to read a normal
synchronous response or a ‘+’/‘-’ acknowledgment to a packet it has
sent.  Notification packets are distinct from any other communication
from the stub so there is no ambiguity.

   After receiving a notification, GDB shall acknowledge it by sending a
ACK packet as a regular, synchronous request to the stub.  Such
acknowledgment is not required to happen immediately, as GDB is
permitted to send other, unrelated packets to the stub first, which the
stub should process normally.

   Upon receiving a ACK packet, if the stub has other queued events to
report to GDB, it shall respond by sending a normal EVENT.  GDB shall
then send another ACK packet to solicit further responses; again, it is
permitted to send other, unrelated packets as well which the stub should
process normally.

   If the stub receives a ACK packet and there are no additional EVENT
to report, the stub shall return an ‘OK’ response.  At this point, GDB
has finished processing a notification and the stub has completed
sending any queued events.  GDB won't accept any new notifications until
the final ‘OK’ is received .  If further notification events occur, the
stub shall send a new notification, GDB shall accept the notification,
and the process shall be repeated.

   The process of asynchronous notification can be illustrated by the
following example:
     <- %Stop:T0505:98e7ffbf;04:4ce6ffbf;08:b1b6e54c;thread:p7526.7526;core:0;
     ...
     -> vStopped
     <- T0505:68f37db7;04:40f37db7;08:63850408;thread:p7526.7528;core:0;
     -> vStopped
     <- T0505:68e3fdb6;04:40e3fdb6;08:63850408;thread:p7526.7529;core:0;
     -> vStopped
     <- OK

   The following notifications are defined:

NotificationAck     Event                       Description
                                                
Stop      vStopped  REPLY.  The REPLY has the   Report an asynchronous
                    form of a stop reply, as    stop event in non-stop
                    described in                mode.
                    *note Stop Reply Packets::. 
                    Refer to
                    *note Remote Non-Stop::,
                    for information on how
                    these notifications are
                    acknowledged by GDB.


File: gdb.info,  Node: Remote Non-Stop,  Next: Packet Acknowledgment,  Prev: Notification Packets,  Up: Remote Protocol

E.11 Remote Protocol Support for Non-Stop Mode
==============================================

GDB's remote protocol supports non-stop debugging of multi-threaded
programs, as described in *note Non-Stop Mode::.  If the stub supports
non-stop mode, it should report that to GDB by including ‘QNonStop+’ in
its ‘qSupported’ response (*note qSupported::).

   GDB typically sends a ‘QNonStop’ packet only when establishing a new
connection with the stub.  Entering non-stop mode does not alter the
state of any currently-running threads, but targets must stop all
threads in any already-attached processes when entering all-stop mode.
GDB uses the ‘?’ packet as necessary to probe the target state after a
mode change.

   In non-stop mode, when an attached process encounters an event that
would otherwise be reported with a stop reply, it uses the asynchronous
notification mechanism (*note Notification Packets::) to inform GDB.  In
contrast to all-stop mode, where all threads in all processes are
stopped when a stop reply is sent, in non-stop mode only the thread
reporting the stop event is stopped.  That is, when reporting a ‘S’ or
‘T’ response to indicate completion of a step operation, hitting a
breakpoint, or a fault, only the affected thread is stopped; any other
still-running threads continue to run.  When reporting a ‘W’ or ‘X’
response, all running threads belonging to other attached processes
continue to run.

   In non-stop mode, the target shall respond to the ‘?’ packet as
follows.  First, any incomplete stop reply notification/‘vStopped’
sequence in progress is abandoned.  The target must begin a new sequence
reporting stop events for all stopped threads, whether or not it has
previously reported those events to GDB.  The first stop reply is sent
as a synchronous reply to the ‘?’ packet, and subsequent stop replies
are sent as responses to ‘vStopped’ packets using the mechanism
described above.  The target must not send asynchronous stop reply
notifications until the sequence is complete.  If all threads are
running when the target receives the ‘?’ packet, or if the target is not
attached to any process, it shall respond ‘OK’.

   If the stub supports non-stop mode, it should also support the
‘swbreak’ stop reason if software breakpoints are supported, and the
‘hwbreak’ stop reason if hardware breakpoints are supported (*note
swbreak stop reason::).  This is because given the asynchronous nature
of non-stop mode, between the time a thread hits a breakpoint and the
time the event is finally processed by GDB, the breakpoint may have
already been removed from the target.  Due to this, GDB needs to be able
to tell whether a trap stop was caused by a delayed breakpoint event,
which should be ignored, as opposed to a random trap signal, which
should be reported to the user.  Note the ‘swbreak’ feature implies that
the target is responsible for adjusting the PC when a software
breakpoint triggers, if necessary, such as on the x86 architecture.


File: gdb.info,  Node: Packet Acknowledgment,  Next: Examples,  Prev: Remote Non-Stop,  Up: Remote Protocol

E.12 Packet Acknowledgment
==========================

By default, when either the host or the target machine receives a
packet, the first response expected is an acknowledgment: either ‘+’ (to
indicate the package was received correctly) or ‘-’ (to request
retransmission).  This mechanism allows the GDB remote protocol to
operate over unreliable transport mechanisms, such as a serial line.

   In cases where the transport mechanism is itself reliable (such as a
pipe or TCP connection), the ‘+’/‘-’ acknowledgments are redundant.  It
may be desirable to disable them in that case to reduce communication
overhead, or for other reasons.  This can be accomplished by means of
the ‘QStartNoAckMode’ packet; *note QStartNoAckMode::.

   When in no-acknowledgment mode, neither the stub nor GDB shall send
or expect ‘+’/‘-’ protocol acknowledgments.  The packet and response
format still includes the normal checksum, as described in *note
Overview::, but the checksum may be ignored by the receiver.

   If the stub supports ‘QStartNoAckMode’ and prefers to operate in
no-acknowledgment mode, it should report that to GDB by including
‘QStartNoAckMode+’ in its response to ‘qSupported’; *note qSupported::.
If GDB also supports ‘QStartNoAckMode’ and it has not been disabled via
the ‘set remote noack-packet off’ command (*note Remote
Configuration::), GDB may then send a ‘QStartNoAckMode’ packet to the
stub.  Only then may the stub actually turn off packet acknowledgments.
GDB sends a final ‘+’ acknowledgment of the stub's ‘OK’ response, which
can be safely ignored by the stub.

   Note that ‘set remote noack-packet’ command only affects negotiation
between GDB and the stub when subsequent connections are made; it does
not affect the protocol acknowledgment state for any current connection.
Since ‘+’/‘-’ acknowledgments are enabled by default when a new
connection is established, there is also no protocol request to
re-enable the acknowledgments for the current connection, once disabled.


File: gdb.info,  Node: Examples,  Next: File-I/O Remote Protocol Extension,  Prev: Packet Acknowledgment,  Up: Remote Protocol

E.13 Examples
=============

Example sequence of a target being re-started.  Notice how the restart
does not get any direct output:

     -> R00
     <- +
     _target restarts_
     -> ?
     <- +
     <- T001:1234123412341234
     -> +

   Example sequence of a target being stepped by a single instruction:

     -> G1445...
     <- +
     -> s
     <- +
     _time passes_
     <- T001:1234123412341234
     -> +
     -> g
     <- +
     <- 1455...
     -> +


File: gdb.info,  Node: File-I/O Remote Protocol Extension,  Next: Library List Format,  Prev: Examples,  Up: Remote Protocol

E.14 File-I/O Remote Protocol Extension
=======================================

* Menu:

* File-I/O Overview::
* Protocol Basics::
* The F Request Packet::
* The F Reply Packet::
* The Ctrl-C Message::
* Console I/O::
* List of Supported Calls::
* Protocol-specific Representation of Datatypes::
* Constants::
* File-I/O Examples::


File: gdb.info,  Node: File-I/O Overview,  Next: Protocol Basics,  Up: File-I/O Remote Protocol Extension

E.14.1 File-I/O Overview
------------------------

The “File I/O remote protocol extension” (short: File-I/O) allows the
target to use the host's file system and console I/O to perform various
system calls.  System calls on the target system are translated into a
remote protocol packet to the host system, which then performs the
needed actions and returns a response packet to the target system.  This
simulates file system operations even on targets that lack file systems.

   The protocol is defined to be independent of both the host and target
systems.  It uses its own internal representation of datatypes and
values.  Both GDB and the target's GDB stub are responsible for
translating the system-dependent value representations into the internal
protocol representations when data is transmitted.

   The communication is synchronous.  A system call is possible only
when GDB is waiting for a response from the ‘C’, ‘c’, ‘S’ or ‘s’
packets.  While GDB handles the request for a system call, the target is
stopped to allow deterministic access to the target's memory.  Therefore
File-I/O is not interruptible by target signals.  On the other hand, it
is possible to interrupt File-I/O by a user interrupt (‘Ctrl-C’) within
GDB.

   The target's request to perform a host system call does not finish
the latest ‘C’, ‘c’, ‘S’ or ‘s’ action.  That means, after finishing the
system call, the target returns to continuing the previous activity
(continue, step).  No additional continue or step request from GDB is
required.

     (gdb) continue
       <- target requests 'system call X'
       target is stopped, GDB executes system call
       -> GDB returns result
       ... target continues, GDB returns to wait for the target
       <- target hits breakpoint and sends a Txx packet

   The protocol only supports I/O on the console and to regular files on
the host file system.  Character or block special devices, pipes, named
pipes, sockets or any other communication method on the host system are
not supported by this protocol.

   File I/O is not supported in non-stop mode.


File: gdb.info,  Node: Protocol Basics,  Next: The F Request Packet,  Prev: File-I/O Overview,  Up: File-I/O Remote Protocol Extension

E.14.2 Protocol Basics
----------------------

The File-I/O protocol uses the ‘F’ packet as the request as well as
reply packet.  Since a File-I/O system call can only occur when GDB is
waiting for a response from the continuing or stepping target, the
File-I/O request is a reply that GDB has to expect as a result of a
previous ‘C’, ‘c’, ‘S’ or ‘s’ packet.  This ‘F’ packet contains all
information needed to allow GDB to call the appropriate host system
call:

   • A unique identifier for the requested system call.

   • All parameters to the system call.  Pointers are given as addresses
     in the target memory address space.  Pointers to strings are given
     as pointer/length pair.  Numerical values are given as they are.
     Numerical control flags are given in a protocol-specific
     representation.

   At this point, GDB has to perform the following actions.

   • If the parameters include pointer values to data needed as input to
     a system call, GDB requests this data from the target with a
     standard ‘m’ packet request.  This additional communication has to
     be expected by the target implementation and is handled as any
     other ‘m’ packet.

   • GDB translates all value from protocol representation to host
     representation as needed.  Datatypes are coerced into the host
     types.

   • GDB calls the system call.

   • It then coerces datatypes back to protocol representation.

   • If the system call is expected to return data in buffer space
     specified by pointer parameters to the call, the data is
     transmitted to the target using a ‘M’ or ‘X’ packet.  This packet
     has to be expected by the target implementation and is handled as
     any other ‘M’ or ‘X’ packet.

   Eventually GDB replies with another ‘F’ packet which contains all
necessary information for the target to continue.  This at least
contains

   • Return value.

   • ‘errno’, if has been changed by the system call.

   • "Ctrl-C" flag.

   After having done the needed type and value coercion, the target
continues the latest continue or step action.


File: gdb.info,  Node: The F Request Packet,  Next: The F Reply Packet,  Prev: Protocol Basics,  Up: File-I/O Remote Protocol Extension

E.14.3 The ‘F’ Request Packet
-----------------------------

The ‘F’ request packet has the following format:

‘FCALL-ID,PARAMETER...’

     CALL-ID is the identifier to indicate the host system call to be
     called.  This is just the name of the function.

     PARAMETER... are the parameters to the system call.  Parameters are
     hexadecimal integer values, either the actual values in case of
     scalar datatypes, pointers to target buffer space in case of
     compound datatypes and unspecified memory areas, or pointer/length
     pairs in case of string parameters.  These are appended to the
     CALL-ID as a comma-delimited list.  All values are transmitted in
     ASCII string representation, pointer/length pairs separated by a
     slash.


File: gdb.info,  Node: The F Reply Packet,  Next: The Ctrl-C Message,  Prev: The F Request Packet,  Up: File-I/O Remote Protocol Extension

E.14.4 The ‘F’ Reply Packet
---------------------------

The ‘F’ reply packet has the following format:

‘FRETCODE,ERRNO,CTRL-C FLAG;CALL-SPECIFIC ATTACHMENT’

     RETCODE is the return code of the system call as hexadecimal value.

     ERRNO is the ‘errno’ set by the call, in protocol-specific
     representation.  This parameter can be omitted if the call was
     successful.

     CTRL-C FLAG is only sent if the user requested a break.  In this
     case, ERRNO must be sent as well, even if the call was successful.
     The CTRL-C FLAG itself consists of the character ‘C’:

          F0,0,C

     or, if the call was interrupted before the host call has been
     performed:

          F-1,4,C

     assuming 4 is the protocol-specific representation of ‘EINTR’.


File: gdb.info,  Node: The Ctrl-C Message,  Next: Console I/O,  Prev: The F Reply Packet,  Up: File-I/O Remote Protocol Extension

E.14.5 The ‘Ctrl-C’ Message
---------------------------

If the ‘Ctrl-C’ flag is set in the GDB reply packet (*note The F Reply
Packet::), the target should behave as if it had gotten a break message.
The meaning for the target is "system call interrupted by ‘SIGINT’".
Consequently, the target should actually stop (as with a break message)
and return to GDB with a ‘T02’ packet.

   It's important for the target to know in which state the system call
was interrupted.  There are two possible cases:

   • The system call hasn't been performed on the host yet.

   • The system call on the host has been finished.

   These two states can be distinguished by the target by the value of
the returned ‘errno’.  If it's the protocol representation of ‘EINTR’,
the system call hasn't been performed.  This is equivalent to the
‘EINTR’ handling on POSIX systems.  In any other case, the target may
presume that the system call has been finished -- successfully or not --
and should behave as if the break message arrived right after the system
call.

   GDB must behave reliably.  If the system call has not been called
yet, GDB may send the ‘F’ reply immediately, setting ‘EINTR’ as ‘errno’
in the packet.  If the system call on the host has been finished before
the user requests a break, the full action must be finished by GDB.
This requires sending ‘M’ or ‘X’ packets as necessary.  The ‘F’ packet
may only be sent when either nothing has happened or the full action has
been completed.


File: gdb.info,  Node: Console I/O,  Next: List of Supported Calls,  Prev: The Ctrl-C Message,  Up: File-I/O Remote Protocol Extension

E.14.6 Console I/O
------------------

By default and if not explicitly closed by the target system, the file
descriptors 0, 1 and 2 are connected to the GDB console.  Output on the
GDB console is handled as any other file output operation (‘write(1,
...)’ or ‘write(2, ...)’).  Console input is handled by GDB so that
after the target read request from file descriptor 0 all following
typing is buffered until either one of the following conditions is met:

   • The user types ‘Ctrl-c’.  The behavior is as explained above, and
     the ‘read’ system call is treated as finished.

   • The user presses <RET>.  This is treated as end of input with a
     trailing newline.

   • The user types ‘Ctrl-d’.  This is treated as end of input.  No
     trailing character (neither newline nor ‘Ctrl-D’) is appended to
     the input.

   If the user has typed more characters than fit in the buffer given to
the ‘read’ call, the trailing characters are buffered in GDB until
either another ‘read(0, ...)’ is requested by the target, or debugging
is stopped at the user's request.


File: gdb.info,  Node: List of Supported Calls,  Next: Protocol-specific Representation of Datatypes,  Prev: Console I/O,  Up: File-I/O Remote Protocol Extension

E.14.7 List of Supported Calls
------------------------------

* Menu:

* open::
* close::
* read::
* write::
* lseek::
* rename::
* unlink::
* stat/fstat::
* gettimeofday::
* isatty::
* system::


File: gdb.info,  Node: open,  Next: close,  Up: List of Supported Calls

open
....

Synopsis:
          int open(const char *pathname, int flags);
          int open(const char *pathname, int flags, mode_t mode);

Request:
     ‘Fopen,PATHPTR/LEN,FLAGS,MODE’

     FLAGS is the bitwise ‘OR’ of the following values:

     ‘O_CREAT’
          If the file does not exist it will be created.  The host rules
          apply as far as file ownership and time stamps are concerned.

     ‘O_EXCL’
          When used with ‘O_CREAT’, if the file already exists it is an
          error and open() fails.

     ‘O_TRUNC’
          If the file already exists and the open mode allows writing
          (‘O_RDWR’ or ‘O_WRONLY’ is given) it will be truncated to zero
          length.

     ‘O_APPEND’
          The file is opened in append mode.

     ‘O_RDONLY’
          The file is opened for reading only.

     ‘O_WRONLY’
          The file is opened for writing only.

     ‘O_RDWR’
          The file is opened for reading and writing.

     Other bits are silently ignored.

     MODE is the bitwise ‘OR’ of the following values:

     ‘S_IRUSR’
          User has read permission.

     ‘S_IWUSR’
          User has write permission.

     ‘S_IRGRP’
          Group has read permission.

     ‘S_IWGRP’
          Group has write permission.

     ‘S_IROTH’
          Others have read permission.

     ‘S_IWOTH’
          Others have write permission.

     Other bits are silently ignored.

Return value:
     ‘open’ returns the new file descriptor or -1 if an error occurred.

Errors:

     ‘EEXIST’
          PATHNAME already exists and ‘O_CREAT’ and ‘O_EXCL’ were used.

     ‘EISDIR’
          PATHNAME refers to a directory.

     ‘EACCES’
          The requested access is not allowed.

     ‘ENAMETOOLONG’
          PATHNAME was too long.

     ‘ENOENT’
          A directory component in PATHNAME does not exist.

     ‘ENODEV’
          PATHNAME refers to a device, pipe, named pipe or socket.

     ‘EROFS’
          PATHNAME refers to a file on a read-only filesystem and write
          access was requested.

     ‘EFAULT’
          PATHNAME is an invalid pointer value.

     ‘ENOSPC’
          No space on device to create the file.

     ‘EMFILE’
          The process already has the maximum number of files open.

     ‘ENFILE’
          The limit on the total number of files open on the system has
          been reached.

     ‘EINTR’
          The call was interrupted by the user.


File: gdb.info,  Node: close,  Next: read,  Prev: open,  Up: List of Supported Calls

close
.....

Synopsis:
          int close(int fd);

Request:
     ‘Fclose,FD’

Return value:
     ‘close’ returns zero on success, or -1 if an error occurred.

Errors:

     ‘EBADF’
          FD isn't a valid open file descriptor.

     ‘EINTR’
          The call was interrupted by the user.


File: gdb.info,  Node: read,  Next: write,  Prev: close,  Up: List of Supported Calls

read
....

Synopsis:
          int read(int fd, void *buf, unsigned int count);

Request:
     ‘Fread,FD,BUFPTR,COUNT’

Return value:
     On success, the number of bytes read is returned.  Zero indicates
     end of file.  If count is zero, read returns zero as well.  On
     error, -1 is returned.

Errors:

     ‘EBADF’
          FD is not a valid file descriptor or is not open for reading.

     ‘EFAULT’
          BUFPTR is an invalid pointer value.

     ‘EINTR’
          The call was interrupted by the user.


File: gdb.info,  Node: write,  Next: lseek,  Prev: read,  Up: List of Supported Calls

write
.....

Synopsis:
          int write(int fd, const void *buf, unsigned int count);

Request:
     ‘Fwrite,FD,BUFPTR,COUNT’

Return value:
     On success, the number of bytes written are returned.  Zero
     indicates nothing was written.  On error, -1 is returned.

Errors:

     ‘EBADF’
          FD is not a valid file descriptor or is not open for writing.

     ‘EFAULT’
          BUFPTR is an invalid pointer value.

     ‘EFBIG’
          An attempt was made to write a file that exceeds the
          host-specific maximum file size allowed.

     ‘ENOSPC’
          No space on device to write the data.

     ‘EINTR’
          The call was interrupted by the user.


File: gdb.info,  Node: lseek,  Next: rename,  Prev: write,  Up: List of Supported Calls

lseek
.....

Synopsis:
          long lseek (int fd, long offset, int flag);

Request:
     ‘Flseek,FD,OFFSET,FLAG’

     FLAG is one of:

     ‘SEEK_SET’
          The offset is set to OFFSET bytes.

     ‘SEEK_CUR’
          The offset is set to its current location plus OFFSET bytes.

     ‘SEEK_END’
          The offset is set to the size of the file plus OFFSET bytes.

Return value:
     On success, the resulting unsigned offset in bytes from the
     beginning of the file is returned.  Otherwise, a value of -1 is
     returned.

Errors:

     ‘EBADF’
          FD is not a valid open file descriptor.

     ‘ESPIPE’
          FD is associated with the GDB console.

     ‘EINVAL’
          FLAG is not a proper value.

     ‘EINTR’
          The call was interrupted by the user.


File: gdb.info,  Node: rename,  Next: unlink,  Prev: lseek,  Up: List of Supported Calls

rename
......

Synopsis:
          int rename(const char *oldpath, const char *newpath);

Request:
     ‘Frename,OLDPATHPTR/LEN,NEWPATHPTR/LEN’

Return value:
     On success, zero is returned.  On error, -1 is returned.

Errors:

     ‘EISDIR’
          NEWPATH is an existing directory, but OLDPATH is not a
          directory.

     ‘EEXIST’
          NEWPATH is a non-empty directory.

     ‘EBUSY’
          OLDPATH or NEWPATH is a directory that is in use by some
          process.

     ‘EINVAL’
          An attempt was made to make a directory a subdirectory of
          itself.

     ‘ENOTDIR’
          A component used as a directory in OLDPATH or new path is not
          a directory.  Or OLDPATH is a directory and NEWPATH exists but
          is not a directory.

     ‘EFAULT’
          OLDPATHPTR or NEWPATHPTR are invalid pointer values.

     ‘EACCES’
          No access to the file or the path of the file.

     ‘ENAMETOOLONG’

          OLDPATH or NEWPATH was too long.

     ‘ENOENT’
          A directory component in OLDPATH or NEWPATH does not exist.

     ‘EROFS’
          The file is on a read-only filesystem.

     ‘ENOSPC’
          The device containing the file has no room for the new
          directory entry.

     ‘EINTR’
          The call was interrupted by the user.


File: gdb.info,  Node: unlink,  Next: stat/fstat,  Prev: rename,  Up: List of Supported Calls

unlink
......

Synopsis:
          int unlink(const char *pathname);

Request:
     ‘Funlink,PATHNAMEPTR/LEN’

Return value:
     On success, zero is returned.  On error, -1 is returned.

Errors:

     ‘EACCES’
          No access to the file or the path of the file.

     ‘EPERM’
          The system does not allow unlinking of directories.

     ‘EBUSY’
          The file PATHNAME cannot be unlinked because it's being used
          by another process.

     ‘EFAULT’
          PATHNAMEPTR is an invalid pointer value.

     ‘ENAMETOOLONG’
          PATHNAME was too long.

     ‘ENOENT’
          A directory component in PATHNAME does not exist.

     ‘ENOTDIR’
          A component of the path is not a directory.

     ‘EROFS’
          The file is on a read-only filesystem.

     ‘EINTR’
          The call was interrupted by the user.


File: gdb.info,  Node: stat/fstat,  Next: gettimeofday,  Prev: unlink,  Up: List of Supported Calls

stat/fstat
..........

Synopsis:
          int stat(const char *pathname, struct stat *buf);
          int fstat(int fd, struct stat *buf);

Request:
     ‘Fstat,PATHNAMEPTR/LEN,BUFPTR’
     ‘Ffstat,FD,BUFPTR’

Return value:
     On success, zero is returned.  On error, -1 is returned.

Errors:

     ‘EBADF’
          FD is not a valid open file.

     ‘ENOENT’
          A directory component in PATHNAME does not exist or the path
          is an empty string.

     ‘ENOTDIR’
          A component of the path is not a directory.

     ‘EFAULT’
          PATHNAMEPTR is an invalid pointer value.

     ‘EACCES’
          No access to the file or the path of the file.

     ‘ENAMETOOLONG’
          PATHNAME was too long.

     ‘EINTR’
          The call was interrupted by the user.


File: gdb.info,  Node: gettimeofday,  Next: isatty,  Prev: stat/fstat,  Up: List of Supported Calls

gettimeofday
............

Synopsis:
          int gettimeofday(struct timeval *tv, void *tz);

Request:
     ‘Fgettimeofday,TVPTR,TZPTR’

Return value:
     On success, 0 is returned, -1 otherwise.

Errors:

     ‘EINVAL’
          TZ is a non-NULL pointer.

     ‘EFAULT’
          TVPTR and/or TZPTR is an invalid pointer value.


File: gdb.info,  Node: isatty,  Next: system,  Prev: gettimeofday,  Up: List of Supported Calls

isatty
......

Synopsis:
          int isatty(int fd);

Request:
     ‘Fisatty,FD’

Return value:
     Returns 1 if FD refers to the GDB console, 0 otherwise.

Errors:

     ‘EINTR’
          The call was interrupted by the user.

   Note that the ‘isatty’ call is treated as a special case: it returns
1 to the target if the file descriptor is attached to the GDB console, 0
otherwise.  Implementing through system calls would require implementing
‘ioctl’ and would be more complex than needed.


File: gdb.info,  Node: system,  Prev: isatty,  Up: List of Supported Calls

system
......

Synopsis:
          int system(const char *command);

Request:
     ‘Fsystem,COMMANDPTR/LEN’

Return value:
     If LEN is zero, the return value indicates whether a shell is
     available.  A zero return value indicates a shell is not available.
     For non-zero LEN, the value returned is -1 on error and the return
     status of the command otherwise.  Only the exit status of the
     command is returned, which is extracted from the host's ‘system’
     return value by calling ‘WEXITSTATUS(retval)’.  In case ‘/bin/sh’
     could not be executed, 127 is returned.

Errors:

     ‘EINTR’
          The call was interrupted by the user.

   GDB takes over the full task of calling the necessary host calls to
perform the ‘system’ call.  The return value of ‘system’ on the host is
simplified before it's returned to the target.  Any termination signal
information from the child process is discarded, and the return value
consists entirely of the exit status of the called command.

   Due to security concerns, the ‘system’ call is by default refused by
GDB.  The user has to allow this call explicitly with the ‘set remote
system-call-allowed 1’ command.

‘set remote system-call-allowed’
     Control whether to allow the ‘system’ calls in the File I/O
     protocol for the remote target.  The default is zero (disabled).

‘show remote system-call-allowed’
     Show whether the ‘system’ calls are allowed in the File I/O
     protocol.


File: gdb.info,  Node: Protocol-specific Representation of Datatypes,  Next: Constants,  Prev: List of Supported Calls,  Up: File-I/O Remote Protocol Extension

E.14.8 Protocol-specific Representation of Datatypes
----------------------------------------------------

* Menu:

* Integral Datatypes::
* Pointer Values::
* Memory Transfer::
* struct stat::
* struct timeval::


File: gdb.info,  Node: Integral Datatypes,  Next: Pointer Values,  Up: Protocol-specific Representation of Datatypes

Integral Datatypes
..................

The integral datatypes used in the system calls are ‘int’, ‘unsigned
int’, ‘long’, ‘unsigned long’, ‘mode_t’, and ‘time_t’.

   ‘int’, ‘unsigned int’, ‘mode_t’ and ‘time_t’ are implemented as 32
bit values in this protocol.

   ‘long’ and ‘unsigned long’ are implemented as 64 bit types.

   *Note Limits::, for corresponding MIN and MAX values (similar to
those in ‘limits.h’) to allow range checking on host and target.

   ‘time_t’ datatypes are defined as seconds since the Epoch.

   All integral datatypes transferred as part of a memory read or write
of a structured datatype e.g. a ‘struct stat’ have to be given in big
endian byte order.


File: gdb.info,  Node: Pointer Values,  Next: Memory Transfer,  Prev: Integral Datatypes,  Up: Protocol-specific Representation of Datatypes

Pointer Values
..............

Pointers to target data are transmitted as they are.  An exception is
made for pointers to buffers for which the length isn't transmitted as
part of the function call, namely strings.  Strings are transmitted as a
pointer/length pair, both as hex values, e.g.

     1aaf/12

which is a pointer to data of length 18 bytes at position 0x1aaf.  The
length is defined as the full string length in bytes, including the
trailing null byte.  For example, the string ‘"hello world"’ at address
0x123456 is transmitted as

     123456/d


File: gdb.info,  Node: Memory Transfer,  Next: struct stat,  Prev: Pointer Values,  Up: Protocol-specific Representation of Datatypes

Memory Transfer
...............

Structured data which is transferred using a memory read or write (for
example, a ‘struct stat’) is expected to be in a protocol-specific
format with all scalar multibyte datatypes being big endian.
Translation to this representation needs to be done both by the target
before the ‘F’ packet is sent, and by GDB before it transfers memory to
the target.  Transferred pointers to structured data should point to the
already-coerced data at any time.


File: gdb.info,  Node: struct stat,  Next: struct timeval,  Prev: Memory Transfer,  Up: Protocol-specific Representation of Datatypes

struct stat
...........

The buffer of type ‘struct stat’ used by the target and GDB is defined
as follows:

     struct stat {
         unsigned int  st_dev;      /* device */
         unsigned int  st_ino;      /* inode */
         mode_t        st_mode;     /* protection */
         unsigned int  st_nlink;    /* number of hard links */
         unsigned int  st_uid;      /* user ID of owner */
         unsigned int  st_gid;      /* group ID of owner */
         unsigned int  st_rdev;     /* device type (if inode device) */
         unsigned long st_size;     /* total size, in bytes */
         unsigned long st_blksize;  /* blocksize for filesystem I/O */
         unsigned long st_blocks;   /* number of blocks allocated */
         time_t        st_atime;    /* time of last access */
         time_t        st_mtime;    /* time of last modification */
         time_t        st_ctime;    /* time of last change */
     };

   The integral datatypes conform to the definitions given in the
appropriate section (see *note Integral Datatypes::, for details) so
this structure is of size 64 bytes.

   The values of several fields have a restricted meaning and/or range
of values.

‘st_dev’
     A value of 0 represents a file, 1 the console.

‘st_ino’
     No valid meaning for the target.  Transmitted unchanged.

‘st_mode’
     Valid mode bits are described in *note Constants::.  Any other bits
     have currently no meaning for the target.

‘st_uid’
‘st_gid’
‘st_rdev’
     No valid meaning for the target.  Transmitted unchanged.

‘st_atime’
‘st_mtime’
‘st_ctime’
     These values have a host and file system dependent accuracy.
     Especially on Windows hosts, the file system may not support exact
     timing values.

   The target gets a ‘struct stat’ of the above representation and is
responsible for coercing it to the target representation before
continuing.

   Note that due to size differences between the host, target, and
protocol representations of ‘struct stat’ members, these members could
eventually get truncated on the target.


File: gdb.info,  Node: struct timeval,  Prev: struct stat,  Up: Protocol-specific Representation of Datatypes

struct timeval
..............

The buffer of type ‘struct timeval’ used by the File-I/O protocol is
defined as follows:

     struct timeval {
         time_t tv_sec;  /* second */
         long   tv_usec; /* microsecond */
     };

   The integral datatypes conform to the definitions given in the
appropriate section (see *note Integral Datatypes::, for details) so
this structure is of size 8 bytes.


File: gdb.info,  Node: Constants,  Next: File-I/O Examples,  Prev: Protocol-specific Representation of Datatypes,  Up: File-I/O Remote Protocol Extension

E.14.9 Constants
----------------

The following values are used for the constants inside of the protocol.
GDB and target are responsible for translating these values before and
after the call as needed.

* Menu:

* Open Flags::
* mode_t Values::
* Errno Values::
* Lseek Flags::
* Limits::


File: gdb.info,  Node: Open Flags,  Next: mode_t Values,  Up: Constants

Open Flags
..........

All values are given in hexadecimal representation.

       O_RDONLY        0x0
       O_WRONLY        0x1
       O_RDWR          0x2
       O_APPEND        0x8
       O_CREAT       0x200
       O_TRUNC       0x400
       O_EXCL        0x800


File: gdb.info,  Node: mode_t Values,  Next: Errno Values,  Prev: Open Flags,  Up: Constants

mode_t Values
.............

All values are given in octal representation.

       S_IFREG       0100000
       S_IFDIR        040000
       S_IRUSR          0400
       S_IWUSR          0200
       S_IXUSR          0100
       S_IRGRP           040
       S_IWGRP           020
       S_IXGRP           010
       S_IROTH            04
       S_IWOTH            02
       S_IXOTH            01


File: gdb.info,  Node: Errno Values,  Next: Lseek Flags,  Prev: mode_t Values,  Up: Constants

Errno Values
............

All values are given in decimal representation.

       EPERM           1
       ENOENT          2
       EINTR           4
       EIO             5
       EBADF           9
       EACCES         13
       EFAULT         14
       EBUSY          16
       EEXIST         17
       ENODEV         19
       ENOTDIR        20
       EISDIR         21
       EINVAL         22
       ENFILE         23
       EMFILE         24
       EFBIG          27
       ENOSPC         28
       ESPIPE         29
       EROFS          30
       ENOSYS         88
       ENAMETOOLONG   91
       EUNKNOWN       9999

   ‘EUNKNOWN’ is used as a fallback error value if a host system returns
any error value not in the list of supported error numbers.


File: gdb.info,  Node: Lseek Flags,  Next: Limits,  Prev: Errno Values,  Up: Constants

Lseek Flags
...........

       SEEK_SET      0
       SEEK_CUR      1
       SEEK_END      2


File: gdb.info,  Node: Limits,  Prev: Lseek Flags,  Up: Constants

Limits
......

All values are given in decimal representation.

       INT_MIN       -2147483648
       INT_MAX        2147483647
       UINT_MAX       4294967295
       LONG_MIN      -9223372036854775808
       LONG_MAX       9223372036854775807
       ULONG_MAX      18446744073709551615


File: gdb.info,  Node: File-I/O Examples,  Prev: Constants,  Up: File-I/O Remote Protocol Extension

E.14.10 File-I/O Examples
-------------------------

Example sequence of a write call, file descriptor 3, buffer is at target
address 0x1234, 6 bytes should be written:

     <- Fwrite,3,1234,6
     _request memory read from target_
     -> m1234,6
     <- XXXXXX
     _return "6 bytes written"_
     -> F6

   Example sequence of a read call, file descriptor 3, buffer is at
target address 0x1234, 6 bytes should be read:

     <- Fread,3,1234,6
     _request memory write to target_
     -> X1234,6:XXXXXX
     _return "6 bytes read"_
     -> F6

   Example sequence of a read call, call fails on the host due to
invalid file descriptor (‘EBADF’):

     <- Fread,3,1234,6
     -> F-1,9

   Example sequence of a read call, user presses ‘Ctrl-c’ before syscall
on host is called:

     <- Fread,3,1234,6
     -> F-1,4,C
     <- T02

   Example sequence of a read call, user presses ‘Ctrl-c’ after syscall
on host is called:

     <- Fread,3,1234,6
     -> X1234,6:XXXXXX
     <- T02


File: gdb.info,  Node: Library List Format,  Next: Library List Format for SVR4 Targets,  Prev: File-I/O Remote Protocol Extension,  Up: Remote Protocol

E.15 Library List Format
========================

On some platforms, a dynamic loader (e.g. ‘ld.so’) runs in the same
process as your application to manage libraries.  In this case, GDB can
use the loader's symbol table and normal memory operations to maintain a
list of shared libraries.  On other platforms, the operating system
manages loaded libraries.  GDB can not retrieve the list of currently
loaded libraries through memory operations, so it uses the
‘qXfer:libraries:read’ packet (*note qXfer library list read::) instead.
The remote stub queries the target's operating system and reports which
libraries are loaded.

   The ‘qXfer:libraries:read’ packet returns an XML document which lists
loaded libraries and their offsets.  Each library has an associated name
and one or more segment or section base addresses, which report where
the library was loaded in memory.

   For the common case of libraries that are fully linked binaries, the
library should have a list of segments.  If the target supports dynamic
linking of a relocatable object file, its library XML element should
instead include a list of allocated sections.  The segment or section
bases are start addresses, not relocation offsets; they do not depend on
the library's link-time base addresses.

   GDB must be linked with the Expat library to support XML library
lists.  *Note Expat::.

   A simple memory map, with one loaded library relocated by a single
offset, looks like this:

     <library-list>
       <library name="/lib/libc.so.6">
         <segment address="0x10000000"/>
       </library>
     </library-list>

   Another simple memory map, with one loaded library with three
allocated sections (.text, .data, .bss), looks like this:

     <library-list>
       <library name="sharedlib.o">
         <section address="0x10000000"/>
         <section address="0x20000000"/>
         <section address="0x30000000"/>
       </library>
     </library-list>

   The format of a library list is described by this DTD:

     <!-- library-list: Root element with versioning -->
     <!ELEMENT library-list  (library)*>
     <!ATTLIST library-list  version CDATA   #FIXED  "1.0">
     <!ELEMENT library       (segment*, section*)>
     <!ATTLIST library       name    CDATA   #REQUIRED>
     <!ELEMENT segment       EMPTY>
     <!ATTLIST segment       address CDATA   #REQUIRED>
     <!ELEMENT section       EMPTY>
     <!ATTLIST section       address CDATA   #REQUIRED>

   In addition, segments and section descriptors cannot be mixed within
a single library element, and you must supply at least one segment or
section for each library.


File: gdb.info,  Node: Library List Format for SVR4 Targets,  Next: Memory Map Format,  Prev: Library List Format,  Up: Remote Protocol

E.16 Library List Format for SVR4 Targets
=========================================

On SVR4 platforms GDB can use the symbol table of a dynamic loader (e.g.
‘ld.so’) and normal memory operations to maintain a list of shared
libraries.  Still a special library list provided by this packet is more
efficient for the GDB remote protocol.

   The ‘qXfer:libraries-svr4:read’ packet returns an XML document which
lists loaded libraries and their SVR4 linker parameters.  For each
library on SVR4 target, the following parameters are reported:

   − ‘name’, the absolute file name from the ‘l_name’ field of ‘struct
     link_map’.
   − ‘lm’ with address of ‘struct link_map’ used for TLS (Thread Local
     Storage) access.
   − ‘l_addr’, the displacement as read from the field ‘l_addr’ of
     ‘struct link_map’.  For prelinked libraries this is not an absolute
     memory address.  It is a displacement of absolute memory address
     against address the file was prelinked to during the library load.
   − ‘l_ld’, which is memory address of the ‘PT_DYNAMIC’ segment
   − ‘lmid’, which is an identifier for a linker namespace, such as the
     memory address of the ‘r_debug’ object that contains this
     namespace's load map or the namespace identifier returned by
     ‘dlinfo (3)’.

   Additionally the single ‘main-lm’ attribute specifies address of
‘struct link_map’ used for the main executable.  This parameter is used
for TLS access and its presence is optional.

   GDB must be linked with the Expat library to support XML SVR4 library
lists.  *Note Expat::.

   A simple memory map, with two loaded libraries (which do not use
prelink), looks like this:

     <library-list-svr4 version="1.0" main-lm="0xe4f8f8">
       <library name="/lib/ld-linux.so.2" lm="0xe4f51c" l_addr="0xe2d000"
                l_ld="0xe4eefc" lmid="0xfffe0"/>
       <library name="/lib/libc.so.6" lm="0xe4fbe8" l_addr="0x154000"
                l_ld="0x152350" lmid="0xfffe0"/>
     </library-list-svr>

   The format of an SVR4 library list is described by this DTD:

     <!-- library-list-svr4: Root element with versioning -->
     <!ELEMENT library-list-svr4  (library)*>
     <!ATTLIST library-list-svr4  version CDATA   #FIXED  "1.0">
     <!ATTLIST library-list-svr4  main-lm CDATA   #IMPLIED>
     <!ELEMENT library            EMPTY>
     <!ATTLIST library            name    CDATA   #REQUIRED>
     <!ATTLIST library            lm      CDATA   #REQUIRED>
     <!ATTLIST library            l_addr  CDATA   #REQUIRED>
     <!ATTLIST library            l_ld    CDATA   #REQUIRED>
     <!ATTLIST library            lmid    CDATA   #IMPLIED>


File: gdb.info,  Node: Memory Map Format,  Next: Thread List Format,  Prev: Library List Format for SVR4 Targets,  Up: Remote Protocol

E.17 Memory Map Format
======================

To be able to write into flash memory, GDB needs to obtain a memory map
from the target.  This section describes the format of the memory map.

   The memory map is obtained using the ‘qXfer:memory-map:read’ (*note
qXfer memory map read::) packet and is an XML document that lists memory
regions.

   GDB must be linked with the Expat library to support XML memory maps.
*Note Expat::.

   The top-level structure of the document is shown below:

     <?xml version="1.0"?>
     <!DOCTYPE memory-map
               PUBLIC "+//IDN gnu.org//DTD GDB Memory Map V1.0//EN"
                      "http://sourceware.org/gdb/gdb-memory-map.dtd">
     <memory-map>
         region...
     </memory-map>

   Each region can be either:

   • A region of RAM starting at ADDR and extending for LENGTH bytes
     from there:

          <memory type="ram" start="ADDR" length="LENGTH"/>

   • A region of read-only memory:

          <memory type="rom" start="ADDR" length="LENGTH"/>

   • A region of flash memory, with erasure blocks BLOCKSIZE bytes in
     length:

          <memory type="flash" start="ADDR" length="LENGTH">
            <property name="blocksize">BLOCKSIZE</property>
          </memory>

   Regions must not overlap.  GDB assumes that areas of memory not
covered by the memory map are RAM, and uses the ordinary ‘M’ and ‘X’
packets to write to addresses in such ranges.

   The formal DTD for memory map format is given below:

     <!-- ................................................... -->
     <!-- Memory Map XML DTD ................................ -->
     <!-- File: memory-map.dtd .............................. -->
     <!-- .................................... .............. -->
     <!-- memory-map.dtd -->
     <!-- memory-map: Root element with versioning -->
     <!ELEMENT memory-map (memory)*>
     <!ATTLIST memory-map    version CDATA   #FIXED  "1.0.0">
     <!ELEMENT memory (property)*>
     <!-- memory: Specifies a memory region,
                  and its type, or device. -->
     <!ATTLIST memory        type    (ram|rom|flash) #REQUIRED
                             start   CDATA   #REQUIRED
                             length  CDATA   #REQUIRED>
     <!-- property: Generic attribute tag -->
     <!ELEMENT property (#PCDATA | property)*>
     <!ATTLIST property      name    (blocksize) #REQUIRED>


File: gdb.info,  Node: Thread List Format,  Next: Traceframe Info Format,  Prev: Memory Map Format,  Up: Remote Protocol

E.18 Thread List Format
=======================

To efficiently update the list of threads and their attributes, GDB
issues the ‘qXfer:threads:read’ packet (*note qXfer threads read::) and
obtains the XML document with the following structure:

     <?xml version="1.0"?>
     <threads>
         <thread id="id" core="0" name="name" id_str="Thread 12.34" handle="1a2b3c">
         ... description ...
         </thread>
     </threads>

   Each ‘thread’ element must have the ‘id’ attribute that identifies
the thread (*note thread-id syntax::).  The ‘core’ attribute, if
present, specifies which processor core the thread was last executing
on.  The ‘name’ attribute, if present, specifies the human-readable name
of the thread.  The content of the of ‘thread’ element is interpreted as
human-readable auxiliary information.  The ‘id_str’ attribute, if
present, specifies what GDB should print as the target ID of the thread
(e.g. in the ‘info threads’ command or when switching to the thread).
The ‘handle’ attribute, if present, is a hex encoded representation of
the thread handle.


File: gdb.info,  Node: Traceframe Info Format,  Next: Branch Trace Format,  Prev: Thread List Format,  Up: Remote Protocol

E.19 Traceframe Info Format
===========================

To be able to know which objects in the inferior can be examined when
inspecting a tracepoint hit, GDB needs to obtain the list of memory
ranges, registers and trace state variables that have been collected in
a traceframe.

   This list is obtained using the ‘qXfer:traceframe-info:read’ (*note
qXfer traceframe info read::) packet and is an XML document.

   GDB must be linked with the Expat library to support XML traceframe
info discovery.  *Note Expat::.

   The top-level structure of the document is shown below:

     <?xml version="1.0"?>
     <!DOCTYPE traceframe-info
               PUBLIC "+//IDN gnu.org//DTD GDB Memory Map V1.0//EN"
                      "http://sourceware.org/gdb/gdb-traceframe-info.dtd">
     <traceframe-info>
        block...
     </traceframe-info>

   Each traceframe block can be either:

   • A region of collected memory starting at ADDR and extending for
     LENGTH bytes from there:

          <memory start="ADDR" length="LENGTH"/>

   • A block indicating trace state variable numbered NUMBER has been
     collected:

          <tvar id="NUMBER"/>

   The formal DTD for the traceframe info format is given below:

     <!ELEMENT traceframe-info  (memory | tvar)* >
     <!ATTLIST traceframe-info  version CDATA   #FIXED  "1.0">

     <!ELEMENT memory        EMPTY>
     <!ATTLIST memory        start   CDATA   #REQUIRED
                             length  CDATA   #REQUIRED>
     <!ELEMENT tvar>
     <!ATTLIST tvar          id      CDATA   #REQUIRED>


File: gdb.info,  Node: Branch Trace Format,  Next: Branch Trace Configuration Format,  Prev: Traceframe Info Format,  Up: Remote Protocol

E.20 Branch Trace Format
========================

In order to display the branch trace of an inferior thread, GDB needs to
obtain the list of branches.  This list is represented as list of
sequential code blocks that are connected via branches.  The code in
each block has been executed sequentially.

   This list is obtained using the ‘qXfer:btrace:read’ (*note qXfer
btrace read::) packet and is an XML document.

   GDB must be linked with the Expat library to support XML traceframe
info discovery.  *Note Expat::.

   The top-level structure of the document is shown below:

     <?xml version="1.0"?>
     <!DOCTYPE btrace
               PUBLIC "+//IDN gnu.org//DTD GDB Branch Trace V1.0//EN"
                      "http://sourceware.org/gdb/gdb-btrace.dtd">
     <btrace>
        block...
     </btrace>

   • A block of sequentially executed instructions starting at BEGIN and
     ending at END:

          <block begin="BEGIN" end="END"/>

   The formal DTD for the branch trace format is given below:

     <!ELEMENT btrace  (block* | pt) >
     <!ATTLIST btrace  version CDATA   #FIXED "1.0">

     <!ELEMENT block        EMPTY>
     <!ATTLIST block        begin  CDATA   #REQUIRED
                            end    CDATA   #REQUIRED>

     <!ELEMENT pt (pt-config?, raw?)>

     <!ELEMENT pt-config (cpu?)>

     <!ELEMENT cpu EMPTY>
     <!ATTLIST cpu vendor   CDATA #REQUIRED
                   family   CDATA #REQUIRED
                   model    CDATA #REQUIRED
                   stepping CDATA #REQUIRED>

     <!ELEMENT raw (#PCDATA)>


File: gdb.info,  Node: Branch Trace Configuration Format,  Prev: Branch Trace Format,  Up: Remote Protocol

E.21 Branch Trace Configuration Format
======================================

For each inferior thread, GDB can obtain the branch trace configuration
using the ‘qXfer:btrace-conf:read’ (*note qXfer btrace-conf read::)
packet.

   The configuration describes the branch trace format and configuration
settings for that format.  The following information is described:

‘bts’
     This thread uses the “Branch Trace Store” (BTS) format.
     ‘size’
          The size of the BTS ring buffer in bytes.
‘pt’
     This thread uses the “Intel Processor Trace” (Intel PT) format.
     ‘size’
          The size of the Intel PT ring buffer in bytes.

   GDB must be linked with the Expat library to support XML branch trace
configuration discovery.  *Note Expat::.

   The formal DTD for the branch trace configuration format is given
below:

     <!ELEMENT btrace-conf		(bts?, pt?)>
     <!ATTLIST btrace-conf		version	CDATA	#FIXED "1.0">

     <!ELEMENT bts	EMPTY>
     <!ATTLIST bts	size		CDATA	#IMPLIED>

     <!ELEMENT pt	EMPTY>
     <!ATTLIST pt	size		CDATA	#IMPLIED>
     <!ATTLIST pt	ptwrite		(yes | no)	#IMPLIED>
     <!ATTLIST pt	event-tracing	(yes | no)	#IMPLIED>


File: gdb.info,  Node: Agent Expressions,  Next: Target Descriptions,  Prev: Remote Protocol,  Up: Top

Appendix F The GDB Agent Expression Mechanism
*********************************************

In some applications, it is not feasible for the debugger to interrupt
the program's execution long enough for the developer to learn anything
helpful about its behavior.  If the program's correctness depends on its
real-time behavior, delays introduced by a debugger might cause the
program to fail, even when the code itself is correct.  It is useful to
be able to observe the program's behavior without interrupting it.

   Using GDB's ‘trace’ and ‘collect’ commands, the user can specify
locations in the program, and arbitrary expressions to evaluate when
those locations are reached.  Later, using the ‘tfind’ command, she can
examine the values those expressions had when the program hit the trace
points.  The expressions may also denote objects in memory -- structures
or arrays, for example -- whose values GDB should record; while visiting
a particular tracepoint, the user may inspect those objects as if they
were in memory at that moment.  However, because GDB records these
values without interacting with the user, it can do so quickly and
unobtrusively, hopefully not disturbing the program's behavior.

   When GDB is debugging a remote target, the GDB “agent” code running
on the target computes the values of the expressions itself.  To avoid
having a full symbolic expression evaluator on the agent, GDB translates
expressions in the source language into a simpler bytecode language, and
then sends the bytecode to the agent; the agent then executes the
bytecode, and records the values for GDB to retrieve later.

   The bytecode language is simple; there are forty-odd opcodes, the
bulk of which are the usual vocabulary of C operands (addition,
subtraction, shifts, and so on) and various sizes of literals and memory
reference operations.  The bytecode interpreter operates strictly on
machine-level values -- various sizes of integers and floating point
numbers -- and requires no information about types or symbols; thus, the
interpreter's internal data structures are simple, and each bytecode
requires only a few native machine instructions to implement it.  The
interpreter is small, and strict limits on the memory and time required
to evaluate an expression are easy to determine, making it suitable for
use by the debugging agent in real-time applications.

* Menu:

* General Bytecode Design::     Overview of the interpreter.
* Bytecode Descriptions::       What each one does.
* Using Agent Expressions::     How agent expressions fit into the big picture.
* Varying Target Capabilities:: How to discover what the target can do.
* Rationale::                   Why we did it this way.


File: gdb.info,  Node: General Bytecode Design,  Next: Bytecode Descriptions,  Up: Agent Expressions

F.1 General Bytecode Design
===========================

The agent represents bytecode expressions as an array of bytes.  Each
instruction is one byte long (thus the term “bytecode”).  Some
instructions are followed by operand bytes; for example, the ‘goto’
instruction is followed by a destination for the jump.

   The bytecode interpreter is a stack-based machine; most instructions
pop their operands off the stack, perform some operation, and push the
result back on the stack for the next instruction to consume.  Each
element of the stack may contain either a integer or a floating point
value; these values are as many bits wide as the largest integer that
can be directly manipulated in the source language.  Stack elements
carry no record of their type; bytecode could push a value as an
integer, then pop it as a floating point value.  However, GDB will not
generate code which does this.  In C, one might define the type of a
stack element as follows:
     union agent_val {
       LONGEST l;
       DOUBLEST d;
     };
where ‘LONGEST’ and ‘DOUBLEST’ are ‘typedef’ names for the largest
integer and floating point types on the machine.

   By the time the bytecode interpreter reaches the end of the
expression, the value of the expression should be the only value left on
the stack.  For tracing applications, ‘trace’ bytecodes in the
expression will have recorded the necessary data, and the value on the
stack may be discarded.  For other applications, like conditional
breakpoints, the value may be useful.

   Separate from the stack, the interpreter has two registers:
‘pc’
     The address of the next bytecode to execute.

‘start’
     The address of the start of the bytecode expression, necessary for
     interpreting the ‘goto’ and ‘if_goto’ instructions.

Neither of these registers is directly visible to the bytecode language
itself, but they are useful for defining the meanings of the bytecode
operations.

   There are no instructions to perform side effects on the running
program, or call the program's functions; we assume that these
expressions are only used for unobtrusive debugging, not for patching
the running code.

   Most bytecode instructions do not distinguish between the various
sizes of values, and operate on full-width values; the upper bits of the
values are simply ignored, since they do not usually make a difference
to the value computed.  The exceptions to this rule are:

memory reference instructions (‘ref’N)
     There are distinct instructions to fetch different word sizes from
     memory.  Once on the stack, however, the values are treated as
     full-size integers.  They may need to be sign-extended; the ‘ext’
     instruction exists for this purpose.

the sign-extension instruction (‘ext’ N)
     These clearly need to know which portion of their operand is to be
     extended to occupy the full length of the word.

   If the interpreter is unable to evaluate an expression completely for
some reason (a memory location is inaccessible, or a divisor is zero,
for example), we say that interpretation "terminates with an error".
This means that the problem is reported back to the interpreter's caller
in some helpful way.  In general, code using agent expressions should
assume that they may attempt to divide by zero, fetch arbitrary memory
locations, and misbehave in other ways.

   Even complicated C expressions compile to a few bytecode
instructions; for example, the expression ‘x + y * z’ would typically
produce code like the following, assuming that ‘x’ and ‘y’ live in
registers, and ‘z’ is a global variable holding a 32-bit ‘int’:
     reg 1
     reg 2
     const32 address of z
     ref32
     ext 32
     mul
     add
     end

   In detail, these mean:

‘reg 1’
     Push the value of register 1 (presumably holding ‘x’) onto the
     stack.

‘reg 2’
     Push the value of register 2 (holding ‘y’).

‘const32 address of z’
     Push the address of ‘z’ onto the stack.

‘ref32’
     Fetch a 32-bit word from the address at the top of the stack;
     replace the address on the stack with the value.  Thus, we replace
     the address of ‘z’ with ‘z’'s value.

‘ext 32’
     Sign-extend the value on the top of the stack from 32 bits to full
     length.  This is necessary because ‘z’ is a signed integer.

‘mul’
     Pop the top two numbers on the stack, multiply them, and push their
     product.  Now the top of the stack contains the value of the
     expression ‘y * z’.

‘add’
     Pop the top two numbers, add them, and push the sum.  Now the top
     of the stack contains the value of ‘x + y * z’.

‘end’
     Stop executing; the value left on the stack top is the value to be
     recorded.


File: gdb.info,  Node: Bytecode Descriptions,  Next: Using Agent Expressions,  Prev: General Bytecode Design,  Up: Agent Expressions

F.2 Bytecode Descriptions
=========================

Each bytecode description has the following form:

‘add’ (0x02): A B ⇒ A+B

     Pop the top two stack items, A and B, as integers; push their sum,
     as an integer.

   In this example, ‘add’ is the name of the bytecode, and ‘(0x02)’ is
the one-byte value used to encode the bytecode, in hexadecimal.  The
phrase "A B ⇒ A+B" shows the stack before and after the bytecode
executes.  Beforehand, the stack must contain at least two values, A and
B; since the top of the stack is to the right, B is on the top of the
stack, and A is underneath it.  After execution, the bytecode will have
popped A and B from the stack, and replaced them with a single value,
A+B.  There may be other values on the stack below those shown, but the
bytecode affects only those shown.

   Here is another example:

‘const8’ (0x22) N: ⇒ N
     Push the 8-bit integer constant N on the stack, without sign
     extension.

   In this example, the bytecode ‘const8’ takes an operand N directly
from the bytecode stream; the operand follows the ‘const8’ bytecode
itself.  We write any such operands immediately after the name of the
bytecode, before the colon, and describe the exact encoding of the
operand in the bytecode stream in the body of the bytecode description.

   For the ‘const8’ bytecode, there are no stack items given before the
⇒; this simply means that the bytecode consumes no values from the
stack.  If a bytecode consumes no values, or produces no values, the
list on either side of the ⇒ may be empty.

   If a value is written as A, B, or N, then the bytecode treats it as
an integer.  If a value is written is ADDR, then the bytecode treats it
as an address.

   We do not fully describe the floating point operations here; although
this design can be extended in a clean way to handle floating point
values, they are not of immediate interest to the customer, so we avoid
describing them, to save time.

‘float’ (0x01): ⇒

     Prefix for floating-point bytecodes.  Not implemented yet.

‘add’ (0x02): A B ⇒ A+B
     Pop two integers from the stack, and push their sum, as an integer.

‘sub’ (0x03): A B ⇒ A-B
     Pop two integers from the stack, subtract the top value from the
     next-to-top value, and push the difference.

‘mul’ (0x04): A B ⇒ A*B
     Pop two integers from the stack, multiply them, and push the
     product on the stack.  Note that, when one multiplies two N-bit
     numbers yielding another N-bit number, it is irrelevant whether the
     numbers are signed or not; the results are the same.

‘div_signed’ (0x05): A B ⇒ A/B
     Pop two signed integers from the stack; divide the next-to-top
     value by the top value, and push the quotient.  If the divisor is
     zero, terminate with an error.

‘div_unsigned’ (0x06): A B ⇒ A/B
     Pop two unsigned integers from the stack; divide the next-to-top
     value by the top value, and push the quotient.  If the divisor is
     zero, terminate with an error.

‘rem_signed’ (0x07): A B ⇒ A MODULO B
     Pop two signed integers from the stack; divide the next-to-top
     value by the top value, and push the remainder.  If the divisor is
     zero, terminate with an error.

‘rem_unsigned’ (0x08): A B ⇒ A MODULO B
     Pop two unsigned integers from the stack; divide the next-to-top
     value by the top value, and push the remainder.  If the divisor is
     zero, terminate with an error.

‘lsh’ (0x09): A B ⇒ A<<B
     Pop two integers from the stack; let A be the next-to-top value,
     and B be the top value.  Shift A left by B bits, and push the
     result.

‘rsh_signed’ (0x0a): A B ⇒ ‘(signed)’A>>B
     Pop two integers from the stack; let A be the next-to-top value,
     and B be the top value.  Shift A right by B bits, inserting copies
     of the top bit at the high end, and push the result.

‘rsh_unsigned’ (0x0b): A B ⇒ A>>B
     Pop two integers from the stack; let A be the next-to-top value,
     and B be the top value.  Shift A right by B bits, inserting zero
     bits at the high end, and push the result.

‘log_not’ (0x0e): A ⇒ !A
     Pop an integer from the stack; if it is zero, push the value one;
     otherwise, push the value zero.

‘bit_and’ (0x0f): A B ⇒ A&B
     Pop two integers from the stack, and push their bitwise ‘and’.

‘bit_or’ (0x10): A B ⇒ A|B
     Pop two integers from the stack, and push their bitwise ‘or’.

‘bit_xor’ (0x11): A B ⇒ A^B
     Pop two integers from the stack, and push their bitwise
     exclusive-‘or’.

‘bit_not’ (0x12): A ⇒ ~A
     Pop an integer from the stack, and push its bitwise complement.

‘equal’ (0x13): A B ⇒ A=B
     Pop two integers from the stack; if they are equal, push the value
     one; otherwise, push the value zero.

‘less_signed’ (0x14): A B ⇒ A<B
     Pop two signed integers from the stack; if the next-to-top value is
     less than the top value, push the value one; otherwise, push the
     value zero.

‘less_unsigned’ (0x15): A B ⇒ A<B
     Pop two unsigned integers from the stack; if the next-to-top value
     is less than the top value, push the value one; otherwise, push the
     value zero.

‘ext’ (0x16) N: A ⇒ A, sign-extended from N bits
     Pop an unsigned value from the stack; treating it as an N-bit
     twos-complement value, extend it to full length.  This means that
     all bits to the left of bit N-1 (where the least significant bit is
     bit 0) are set to the value of bit N-1.  Note that N may be larger
     than or equal to the width of the stack elements of the bytecode
     engine; in this case, the bytecode should have no effect.

     The number of source bits to preserve, N, is encoded as a single
     byte unsigned integer following the ‘ext’ bytecode.

‘zero_ext’ (0x2a) N: A ⇒ A, zero-extended from N bits
     Pop an unsigned value from the stack; zero all but the bottom N
     bits.

     The number of source bits to preserve, N, is encoded as a single
     byte unsigned integer following the ‘zero_ext’ bytecode.

‘ref8’ (0x17): ADDR ⇒ A
‘ref16’ (0x18): ADDR ⇒ A
‘ref32’ (0x19): ADDR ⇒ A
‘ref64’ (0x1a): ADDR ⇒ A
     Pop an address ADDR from the stack.  For bytecode ‘ref’N, fetch an
     N-bit value from ADDR, using the natural target endianness.  Push
     the fetched value as an unsigned integer.

     Note that ADDR may not be aligned in any particular way; the ‘refN’
     bytecodes should operate correctly for any address.

     If attempting to access memory at ADDR would cause a processor
     exception of some sort, terminate with an error.

‘ref_float’ (0x1b): ADDR ⇒ D
‘ref_double’ (0x1c): ADDR ⇒ D
‘ref_long_double’ (0x1d): ADDR ⇒ D
‘l_to_d’ (0x1e): A ⇒ D
‘d_to_l’ (0x1f): D ⇒ A
     Not implemented yet.

‘dup’ (0x28): A => A A
     Push another copy of the stack's top element.

‘swap’ (0x2b): A B => B A
     Exchange the top two items on the stack.

‘pop’ (0x29): A =>
     Discard the top value on the stack.

‘pick’ (0x32) N: A ... B => A ... B A
     Duplicate an item from the stack and push it on the top of the
     stack.  N, a single byte, indicates the stack item to copy.  If N
     is zero, this is the same as ‘dup’; if N is one, it copies the item
     under the top item, etc.  If N exceeds the number of items on the
     stack, terminate with an error.

‘rot’ (0x33): A B C => C A B
     Rotate the top three items on the stack.  The top item (c) becomes
     the third item, the next-to-top item (b) becomes the top item and
     the third item (a) from the top becomes the next-to-top item.

‘if_goto’ (0x20) OFFSET: A ⇒
     Pop an integer off the stack; if it is non-zero, branch to the
     given offset in the bytecode string.  Otherwise, continue to the
     next instruction in the bytecode stream.  In other words, if A is
     non-zero, set the ‘pc’ register to ‘start’ + OFFSET.  Thus, an
     offset of zero denotes the beginning of the expression.

     The OFFSET is stored as a sixteen-bit unsigned value, stored
     immediately following the ‘if_goto’ bytecode.  It is always stored
     most significant byte first, regardless of the target's normal
     endianness.  The offset is not guaranteed to fall at any particular
     alignment within the bytecode stream; thus, on machines where
     fetching a 16-bit on an unaligned address raises an exception, you
     should fetch the offset one byte at a time.

‘goto’ (0x21) OFFSET: ⇒
     Branch unconditionally to OFFSET; in other words, set the ‘pc’
     register to ‘start’ + OFFSET.

     The offset is stored in the same way as for the ‘if_goto’ bytecode.

‘const8’ (0x22) N: ⇒ N
‘const16’ (0x23) N: ⇒ N
‘const32’ (0x24) N: ⇒ N
‘const64’ (0x25) N: ⇒ N
     Push the integer constant N on the stack, without sign extension.
     To produce a small negative value, push a small twos-complement
     value, and then sign-extend it using the ‘ext’ bytecode.

     The constant N is stored in the appropriate number of bytes
     following the ‘const’B bytecode.  The constant N is always stored
     most significant byte first, regardless of the target's normal
     endianness.  The constant is not guaranteed to fall at any
     particular alignment within the bytecode stream; thus, on machines
     where fetching a 16-bit on an unaligned address raises an
     exception, you should fetch N one byte at a time.

‘reg’ (0x26) N: ⇒ A
     Push the value of register number N, without sign extension.  The
     registers are numbered following GDB's conventions.

     The register number N is encoded as a 16-bit unsigned integer
     immediately following the ‘reg’ bytecode.  It is always stored most
     significant byte first, regardless of the target's normal
     endianness.  The register number is not guaranteed to fall at any
     particular alignment within the bytecode stream; thus, on machines
     where fetching a 16-bit on an unaligned address raises an
     exception, you should fetch the register number one byte at a time.

‘getv’ (0x2c) N: ⇒ V
     Push the value of trace state variable number N, without sign
     extension.

     The variable number N is encoded as a 16-bit unsigned integer
     immediately following the ‘getv’ bytecode.  It is always stored
     most significant byte first, regardless of the target's normal
     endianness.  The variable number is not guaranteed to fall at any
     particular alignment within the bytecode stream; thus, on machines
     where fetching a 16-bit on an unaligned address raises an
     exception, you should fetch the register number one byte at a time.

‘setv’ (0x2d) N: V ⇒ V
     Set trace state variable number N to the value found on the top of
     the stack.  The stack is unchanged, so that the value is readily
     available if the assignment is part of a larger expression.  The
     handling of N is as described for ‘getv’.

‘trace’ (0x0c): ADDR SIZE ⇒
     Record the contents of the SIZE bytes at ADDR in a trace buffer,
     for later retrieval by GDB.

‘trace_quick’ (0x0d) SIZE: ADDR ⇒ ADDR
     Record the contents of the SIZE bytes at ADDR in a trace buffer,
     for later retrieval by GDB. SIZE is a single byte unsigned integer
     following the ‘trace’ opcode.

     This bytecode is equivalent to the sequence ‘dup const8 SIZE
     trace’, but we provide it anyway to save space in bytecode strings.

‘trace16’ (0x30) SIZE: ADDR ⇒ ADDR
     Identical to trace_quick, except that SIZE is a 16-bit big-endian
     unsigned integer, not a single byte.  This should probably have
     been named ‘trace_quick16’, for consistency.

‘tracev’ (0x2e) N: ⇒ A
     Record the value of trace state variable number N in the trace
     buffer.  The handling of N is as described for ‘getv’.

‘tracenz’ (0x2f) ADDR SIZE ⇒
     Record the bytes at ADDR in a trace buffer, for later retrieval by
     GDB. Stop at either the first zero byte, or when SIZE bytes have
     been recorded, whichever occurs first.

‘printf’ (0x34) NUMARGS STRING ⇒
     Do a formatted print, in the style of the C function ‘printf’).
     The value of NUMARGS is the number of arguments to expect on the
     stack, while STRING is the format string, prefixed with a two-byte
     length.  The last byte of the string must be zero, and is included
     in the length.  The format string includes escaped sequences just
     as it appears in C source, so for instance the format string
     ‘"\t%d\n"’ is six characters long, and the output will consist of a
     tab character, a decimal number, and a newline.  At the top of the
     stack, above the values to be printed, this bytecode will pop a
     "function" and "channel".  If the function is nonzero, then the
     target may treat it as a function and call it, passing the channel
     as a first argument, as with the C function ‘fprintf’.  If the
     function is zero, then the target may simply call a standard
     formatted print function of its choice.  In all, this bytecode pops
     2 + NUMARGS stack elements, and pushes nothing.

‘end’ (0x27): ⇒
     Stop executing bytecode; the result should be the top element of
     the stack.  If the purpose of the expression was to compute an
     lvalue or a range of memory, then the next-to-top of the stack is
     the lvalue's address, and the top of the stack is the lvalue's
     size, in bytes.


File: gdb.info,  Node: Using Agent Expressions,  Next: Varying Target Capabilities,  Prev: Bytecode Descriptions,  Up: Agent Expressions

F.3 Using Agent Expressions
===========================

Agent expressions can be used in several different ways by GDB, and the
debugger can generate different bytecode sequences as appropriate.

   One possibility is to do expression evaluation on the target rather
than the host, such as for the conditional of a conditional tracepoint.
In such a case, GDB compiles the source expression into a bytecode
sequence that simply gets values from registers or memory, does
arithmetic, and returns a result.

   Another way to use agent expressions is for tracepoint data
collection.  GDB generates a different bytecode sequence for collection;
in addition to bytecodes that do the calculation, GDB adds ‘trace’
bytecodes to save the pieces of memory that were used.

   • The user selects trace points in the program's code at which GDB
     should collect data.

   • The user specifies expressions to evaluate at each trace point.
     These expressions may denote objects in memory, in which case those
     objects' contents are recorded as the program runs, or computed
     values, in which case the values themselves are recorded.

   • GDB transmits the tracepoints and their associated expressions to
     the GDB agent, running on the debugging target.

   • The agent arranges to be notified when a trace point is hit.

   • When execution on the target reaches a trace point, the agent
     evaluates the expressions associated with that trace point, and
     records the resulting values and memory ranges.

   • Later, when the user selects a given trace event and inspects the
     objects and expression values recorded, GDB talks to the agent to
     retrieve recorded data as necessary to meet the user's requests.
     If the user asks to see an object whose contents have not been
     recorded, GDB reports an error.


File: gdb.info,  Node: Varying Target Capabilities,  Next: Rationale,  Prev: Using Agent Expressions,  Up: Agent Expressions

F.4 Varying Target Capabilities
===============================

Some targets don't support floating-point, and some would rather not
have to deal with ‘long long’ operations.  Also, different targets will
have different stack sizes, and different bytecode buffer lengths.

   Thus, GDB needs a way to ask the target about itself.  We haven't
worked out the details yet, but in general, GDB should be able to send
the target a packet asking it to describe itself.  The reply should be a
packet whose length is explicit, so we can add new information to the
packet in future revisions of the agent, without confusing old versions
of GDB, and it should contain a version number.  It should contain at
least the following information:

   • whether floating point is supported

   • whether ‘long long’ is supported

   • maximum acceptable size of bytecode stack

   • maximum acceptable length of bytecode expressions

   • which registers are actually available for collection

   • whether the target supports disabled tracepoints


File: gdb.info,  Node: Rationale,  Prev: Varying Target Capabilities,  Up: Agent Expressions

F.5 Rationale
=============

Some of the design decisions apparent above are arguable.

What about stack overflow/underflow?
     GDB should be able to query the target to discover its stack size.
     Given that information, GDB can determine at translation time
     whether a given expression will overflow the stack.  But this spec
     isn't about what kinds of error-checking GDB ought to do.

Why are you doing everything in LONGEST?

     Speed isn't important, but agent code size is; using LONGEST brings
     in a bunch of support code to do things like division, etc.  So
     this is a serious concern.

     First, note that you don't need different bytecodes for different
     operand sizes.  You can generate code without _knowing_ how big the
     stack elements actually are on the target.  If the target only
     supports 32-bit ints, and you don't send any 64-bit bytecodes,
     everything just works.  The observation here is that the MIPS and
     the Alpha have only fixed-size registers, and you can still get C's
     semantics even though most instructions only operate on full-sized
     words.  You just need to make sure everything is properly
     sign-extended at the right times.  So there is no need for 32- and
     64-bit variants of the bytecodes.  Just implement everything using
     the largest size you support.

     GDB should certainly check to see what sizes the target supports,
     so the user can get an error earlier, rather than later.  But this
     information is not necessary for correctness.

Why don't you have ‘>’ or ‘<=’ operators?
     I want to keep the interpreter small, and we don't need them.  We
     can combine the ‘less_’ opcodes with ‘log_not’, and swap the order
     of the operands, yielding all four asymmetrical comparison
     operators.  For example, ‘(x <= y)’ is ‘! (x > y)’, which is ‘! (y
     < x)’.

Why do you have ‘log_not’?
Why do you have ‘ext’?
Why do you have ‘zero_ext’?
     These are all easily synthesized from other instructions, but I
     expect them to be used frequently, and they're simple, so I include
     them to keep bytecode strings short.

     ‘log_not’ is equivalent to ‘const8 0 equal’; it's used in half the
     relational operators.

     ‘ext N’ is equivalent to ‘const8 S-N lsh const8 S-N rsh_signed’,
     where S is the size of the stack elements; it follows ‘refM’ and
     REG bytecodes when the value should be signed.  See the next
     bulleted item.

     ‘zero_ext N’ is equivalent to ‘constM MASK log_and’; it's used
     whenever we push the value of a register, because we can't assume
     the upper bits of the register aren't garbage.

Why not have sign-extending variants of the ‘ref’ operators?
     Because that would double the number of ‘ref’ operators, and we
     need the ‘ext’ bytecode anyway for accessing bitfields.

Why not have constant-address variants of the ‘ref’ operators?
     Because that would double the number of ‘ref’ operators again, and
     ‘const32 ADDRESS ref32’ is only one byte longer.

Why do the ‘refN’ operators have to support unaligned fetches?
     GDB will generate bytecode that fetches multi-byte values at
     unaligned addresses whenever the executable's debugging information
     tells it to.  Furthermore, GDB does not know the value the pointer
     will have when GDB generates the bytecode, so it cannot determine
     whether a particular fetch will be aligned or not.

     In particular, structure bitfields may be several bytes long, but
     follow no alignment rules; members of packed structures are not
     necessarily aligned either.

     In general, there are many cases where unaligned references occur
     in correct C code, either at the programmer's explicit request, or
     at the compiler's discretion.  Thus, it is simpler to make the GDB
     agent bytecodes work correctly in all circumstances than to make
     GDB guess in each case whether the compiler did the usual thing.

Why are there no side-effecting operators?
     Because our current client doesn't want them?  That's a cheap
     answer.  I think the real answer is that I'm afraid of implementing
     function calls.  We should re-visit this issue after the present
     contract is delivered.

Why aren't the ‘goto’ ops PC-relative?
     The interpreter has the base address around anyway for PC bounds
     checking, and it seemed simpler.

Why is there only one offset size for the ‘goto’ ops?
     Offsets are currently sixteen bits.  I'm not happy with this
     situation either:

     Suppose we have multiple branch ops with different offset sizes.
     As I generate code left-to-right, all my jumps are forward jumps
     (there are no loops in expressions), so I never know the target
     when I emit the jump opcode.  Thus, I have to either always assume
     the largest offset size, or do jump relaxation on the code after I
     generate it, which seems like a big waste of time.

     I can imagine a reasonable expression being longer than 256 bytes.
     I can't imagine one being longer than 64k.  Thus, we need 16-bit
     offsets.  This kind of reasoning is so bogus, but relaxation is
     pathetic.

     The other approach would be to generate code right-to-left.  Then
     I'd always know my offset size.  That might be fun.

Where is the function call bytecode?

     When we add side-effects, we should add this.

Why does the ‘reg’ bytecode take a 16-bit register number?

     Intel's IA-64 architecture has 128 general-purpose registers, and
     128 floating-point registers, and I'm sure it has some random
     control registers.

Why do we need ‘trace’ and ‘trace_quick’?
     Because GDB needs to record all the memory contents and registers
     an expression touches.  If the user wants to evaluate an expression
     ‘x->y->z’, the agent must record the values of ‘x’ and ‘x->y’ as
     well as the value of ‘x->y->z’.

Don't the ‘trace’ bytecodes make the interpreter less general?
     They do mean that the interpreter contains special-purpose code,
     but that doesn't mean the interpreter can only be used for that
     purpose.  If an expression doesn't use the ‘trace’ bytecodes, they
     don't get in its way.

Why doesn't ‘trace_quick’ consume its arguments the way everything else does?
     In general, you do want your operators to consume their arguments;
     it's consistent, and generally reduces the amount of stack
     rearrangement necessary.  However, ‘trace_quick’ is a kludge to
     save space; it only exists so we needn't write ‘dup const8 SIZE
     trace’ before every memory reference.  Therefore, it's okay for it
     not to consume its arguments; it's meant for a specific context in
     which we know exactly what it should do with the stack.  If we're
     going to have a kludge, it should be an effective kludge.

Why does ‘trace16’ exist?
     That opcode was added by the customer that contracted Cygnus for
     the data tracing work.  I personally think it is unnecessary;
     objects that large will be quite rare, so it is okay to use ‘dup
     const16 SIZE trace’ in those cases.

     Whatever we decide to do with ‘trace16’, we should at least leave
     opcode 0x30 reserved, to remain compatible with the customer who
     added it.


File: gdb.info,  Node: Target Descriptions,  Next: Operating System Information,  Prev: Agent Expressions,  Up: Top

Appendix G Target Descriptions
******************************

One of the challenges of using GDB to debug embedded systems is that
there are so many minor variants of each processor architecture in use.
It is common practice for vendors to start with a standard processor
core -- ARM, PowerPC, or MIPS, for example -- and then make changes to
adapt it to a particular market niche.  Some architectures have hundreds
of variants, available from dozens of vendors.  This leads to a number
of problems:

   • With so many different customized processors, it is difficult for
     the GDB maintainers to keep up with the changes.
   • Since individual variants may have short lifetimes or limited
     audiences, it may not be worthwhile to carry information about
     every variant in the GDB source tree.
   • When GDB does support the architecture of the embedded system at
     hand, the task of finding the correct architecture name to give the
     ‘set architecture’ command can be error-prone.

   To address these problems, the GDB remote protocol allows a target
system to not only identify itself to GDB, but to actually describe its
own features.  This lets GDB support processor variants it has never
seen before -- to the extent that the descriptions are accurate, and
that GDB understands them.

   GDB must be linked with the Expat library to support XML target
descriptions.  *Note Expat::.

* Menu:

* Retrieving Descriptions::         How descriptions are fetched from a target.
* Target Description Format::       The contents of a target description.
* Predefined Target Types::         Standard types available for target
                                    descriptions.
* Enum Target Types::               How to define enum target types.
* Standard Target Features::        Features GDB knows about.


File: gdb.info,  Node: Retrieving Descriptions,  Next: Target Description Format,  Up: Target Descriptions

G.1 Retrieving Descriptions
===========================

Target descriptions can be read from the target automatically, or
specified by the user manually.  The default behavior is to read the
description from the target.  GDB retrieves it via the remote protocol
using ‘qXfer’ requests (*note qXfer: General Query Packets.).  The ANNEX
in the ‘qXfer’ packet will be ‘target.xml’.  The contents of the
‘target.xml’ annex are an XML document, of the form described in *note
Target Description Format::.

   Alternatively, you can specify a file to read for the target
description.  If a file is set, the target will not be queried.  The
commands to specify a file are:

‘set tdesc filename PATH’
     Read the target description from PATH.

‘unset tdesc filename’
     Do not read the XML target description from a file.  GDB will use
     the description supplied by the current target.

‘show tdesc filename’
     Show the filename to read for a target description, if any.


File: gdb.info,  Node: Target Description Format,  Next: Predefined Target Types,  Prev: Retrieving Descriptions,  Up: Target Descriptions

G.2 Target Description Format
=============================

A target description annex is an XML (http://www.w3.org/XML/) document
which complies with the Document Type Definition provided in the GDB
sources in ‘gdb/features/gdb-target.dtd’.  This means you can use
generally available tools like ‘xmllint’ to check that your feature
descriptions are well-formed and valid.  However, to help people
unfamiliar with XML write descriptions for their targets, we also
describe the grammar here.

   Target descriptions can identify the architecture of the remote
target and (for some architectures) provide information about custom
register sets.  They can also identify the OS ABI of the remote target.
GDB can use this information to autoconfigure for your target, or to
warn you if you connect to an unsupported target.

   Here is a simple target description:

     <target version="1.0">
       <architecture>i386:x86-64</architecture>
     </target>

This minimal description only says that the target uses the x86-64
architecture.

   A target description has the following overall form, with [ ] marking
optional elements and ... marking repeatable elements.  The elements are
explained further below.

     <?xml version="1.0"?>
     <!DOCTYPE target SYSTEM "gdb-target.dtd">
     <target version="1.0">
       [ARCHITECTURE]
       [OSABI]
       [COMPATIBLE]
       [FEATURE...]
     </target>

The description is generally insensitive to whitespace and line breaks,
under the usual common-sense rules.  The XML version declaration and
document type declaration can generally be omitted (GDB does not require
them), but specifying them may be useful for XML validation tools.  The
‘version’ attribute for ‘<target>’ may also be omitted, but we recommend
including it; if future versions of GDB use an incompatible revision of
‘gdb-target.dtd’, they will detect and report the version mismatch.

G.2.1 Inclusion
---------------

It can sometimes be valuable to split a target description up into
several different annexes, either for organizational purposes, or to
share files between different possible target descriptions.  You can
divide a description into multiple files by replacing any element of the
target description with an inclusion directive of the form:

     <xi:include href="DOCUMENT"/>

When GDB encounters an element of this form, it will retrieve the named
XML DOCUMENT, and replace the inclusion directive with the contents of
that document.  If the current description was read using ‘qXfer’, then
so will be the included document; DOCUMENT will be interpreted as the
name of an annex.  If the current description was read from a file, GDB
will look for DOCUMENT as a file in the same directory where it found
the original description.

G.2.2 Architecture
------------------

An ‘<architecture>’ element has this form:

       <architecture>ARCH</architecture>

   ARCH is one of the architectures from the set accepted by ‘set
architecture’ (*note Specifying a Debugging Target: Targets.).

G.2.3 OS ABI
------------

This optional field was introduced in GDB version 7.0.  Previous
versions of GDB ignore it.

   An ‘<osabi>’ element has this form:

       <osabi>ABI-NAME</osabi>

   ABI-NAME is an OS ABI name from the same selection accepted by
‘set osabi’ (*note Configuring the Current ABI: ABI.).

G.2.4 Compatible Architecture
-----------------------------

This optional field was introduced in GDB version 7.0.  Previous
versions of GDB ignore it.

   A ‘<compatible>’ element has this form:

       <compatible>ARCH</compatible>

   ARCH is one of the architectures from the set accepted by ‘set
architecture’ (*note Specifying a Debugging Target: Targets.).

   A ‘<compatible>’ element is used to specify that the target is able
to run binaries in some other than the main target architecture given by
the ‘<architecture>’ element.  For example, on the Cell Broadband
Engine, the main architecture is ‘powerpc:common’ or ‘powerpc:common64’,
but the system is able to run binaries in the ‘spu’ architecture as
well.  The way to describe this capability with ‘<compatible>’ is as
follows:

       <architecture>powerpc:common</architecture>
       <compatible>spu</compatible>

G.2.5 Features
--------------

Each ‘<feature>’ describes some logical portion of the target system.
Features are currently used to describe available CPU registers and the
types of their contents.  A ‘<feature>’ element has this form:

     <feature name="NAME">
       [TYPE...]
       REG...
     </feature>

Each feature's name should be unique within the description.  The name
of a feature does not matter unless GDB has some special knowledge of
the contents of that feature; if it does, the feature should have its
standard name.  *Note Standard Target Features::.

G.2.6 Types
-----------

Any register's value is a collection of bits which GDB must interpret.
The default interpretation is a two's complement integer, but other
types can be requested by name in the register description.  Some
predefined types are provided by GDB (*note Predefined Target Types::),
and the description can define additional composite and enum types.

   Each type element must have an ‘id’ attribute, which gives a unique
(within the containing ‘<feature>’) name to the type.  Types must be
defined before they are used.

   Some targets offer vector registers, which can be treated as arrays
of scalar elements.  These types are written as ‘<vector>’ elements,
specifying the array element type, TYPE, and the number of elements,
COUNT:

     <vector id="ID" type="TYPE" count="COUNT"/>

   If a register's value is usefully viewed in multiple ways, define it
with a union type containing the useful representations.  The ‘<union>’
element contains one or more ‘<field>’ elements, each of which has a
NAME and a TYPE:

     <union id="ID">
       <field name="NAME" type="TYPE"/>
       ...
     </union>

   If a register's value is composed from several separate values,
define it with either a structure type or a flags type.  A flags type
may only contain bitfields.  A structure type may either contain only
bitfields or contain no bitfields.  If the value contains only
bitfields, its total size in bytes must be specified.

   Non-bitfield values have a NAME and TYPE.

     <struct id="ID">
       <field name="NAME" type="TYPE"/>
       ...
     </struct>

   Both NAME and TYPE values are required.  No implicit padding is
added.

   Bitfield values have a NAME, START, END and TYPE.

     <struct id="ID" size="SIZE">
       <field name="NAME" start="START" end="END" type="TYPE"/>
       ...
     </struct>

     <flags id="ID" size="SIZE">
       <field name="NAME" start="START" end="END" type="TYPE"/>
       ...
     </flags>

   The NAME value is required.  Bitfield values may be named with the
empty string, ‘""’, in which case the field is "filler" and its value is
not printed.  Not all bits need to be specified, so "filler" fields are
optional.

   The START and END values are required, and TYPE is optional.  The
field's START must be less than or equal to its END, and zero represents
the least significant bit.

   The default value of TYPE is ‘bool’ for single bit fields, and an
unsigned integer otherwise.

   Which to choose?  Structures or flags?

   Registers defined with ‘flags’ have these advantages over defining
them with ‘struct’:

   • Arithmetic may be performed on them as if they were integers.
   • They are printed in a more readable fashion.

   Registers defined with ‘struct’ have one advantage over defining them
with ‘flags’:

   • One can fetch individual fields like in ‘C’.

          (gdb) print $my_struct_reg.field3
          $1 = 42

G.2.7 Registers
---------------

Each register is represented as an element with this form:

     <reg name="NAME"
          bitsize="SIZE"
          [regnum="NUM"]
          [save-restore="SAVE-RESTORE"]
          [type="TYPE"]
          [group="GROUP"]/>

The components are as follows:

NAME
     The register's name; it must be unique within the target
     description.

BITSIZE
     The register's size, in bits.

REGNUM
     The register's number.  If omitted, a register's number is one
     greater than that of the previous register (either in the current
     feature or in a preceding feature); the first register in the
     target description defaults to zero.  This register number is used
     to read or write the register; e.g. it is used in the remote ‘p’
     and ‘P’ packets, and registers appear in the ‘g’ and ‘G’ packets in
     order of increasing register number.

SAVE-RESTORE
     Whether the register should be preserved across inferior function
     calls; this must be either ‘yes’ or ‘no’.  The default is ‘yes’,
     which is appropriate for most registers except for some system
     control registers; this is not related to the target's ABI.

TYPE
     The type of the register.  It may be a predefined type, a type
     defined in the current feature, or one of the special types ‘int’
     and ‘float’.  ‘int’ is an integer type of the correct size for
     BITSIZE, and ‘float’ is a floating point type (in the
     architecture's normal floating point format) of the correct size
     for BITSIZE.  The default is ‘int’.

GROUP
     The register group to which this register belongs.  It can be one
     of the standard register groups ‘general’, ‘float’, ‘vector’ or an
     arbitrary string.  Group names should be limited to alphanumeric
     characters.  If a group name is made up of multiple words the words
     may be separated by hyphens; e.g. ‘special-group’ or
     ‘ultra-special-group’.  If no GROUP is specified, GDB will not
     display the register in ‘info registers’.


File: gdb.info,  Node: Predefined Target Types,  Next: Enum Target Types,  Prev: Target Description Format,  Up: Target Descriptions

G.3 Predefined Target Types
===========================

Type definitions in the self-description can build up composite types
from basic building blocks, but can not define fundamental types.
Instead, standard identifiers are provided by GDB for the fundamental
types.  The currently supported types are:

‘bool’
     Boolean type, occupying a single bit.

‘int8’
‘int16’
‘int24’
‘int32’
‘int64’
‘int128’
     Signed integer types holding the specified number of bits.

‘uint8’
‘uint16’
‘uint24’
‘uint32’
‘uint64’
‘uint128’
     Unsigned integer types holding the specified number of bits.

‘code_ptr’
‘data_ptr’
     Pointers to unspecified code and data.  The program counter and any
     dedicated return address register may be marked as code pointers;
     printing a code pointer converts it into a symbolic address.  The
     stack pointer and any dedicated address registers may be marked as
     data pointers.

‘ieee_half’
     Half precision IEEE floating point.

‘ieee_single’
     Single precision IEEE floating point.

‘ieee_double’
     Double precision IEEE floating point.

‘bfloat16’
     The 16-bit “brain floating point” format used e.g. by x86 and ARM.

‘arm_fpa_ext’
     The 12-byte extended precision format used by ARM FPA registers.

‘i387_ext’
     The 10-byte extended precision format used by x87 registers.

‘i386_eflags’
     32bit EFLAGS register used by x86.

‘i386_mxcsr’
     32bit MXCSR register used by x86.


File: gdb.info,  Node: Enum Target Types,  Next: Standard Target Features,  Prev: Predefined Target Types,  Up: Target Descriptions

G.4 Enum Target Types
=====================

Enum target types are useful in ‘struct’ and ‘flags’ register
descriptions.  *Note Target Description Format::.

   Enum types have a name, size and a list of name/value pairs.

     <enum id="ID" size="SIZE">
       <evalue name="NAME" value="VALUE"/>
       ...
     </enum>

   Enums must be defined before they are used.

     <enum id="levels_type" size="4">
       <evalue name="low" value="0"/>
       <evalue name="high" value="1"/>
     </enum>
     <flags id="flags_type" size="4">
       <field name="X" start="0"/>
       <field name="LEVEL" start="1" end="1" type="levels_type"/>
     </flags>
     <reg name="flags" bitsize="32" type="flags_type"/>

   Given that description, a value of 3 for the ‘flags’ register would
be printed as:

     (gdb) info register flags
     flags 0x3 [ X LEVEL=high ]


File: gdb.info,  Node: Standard Target Features,  Prev: Enum Target Types,  Up: Target Descriptions

G.5 Standard Target Features
============================

A target description must contain either no registers or all the
target's registers.  If the description contains no registers, then GDB
will assume a default register layout, selected based on the
architecture.  If the description contains any registers, the default
layout will not be used; the standard registers must be described in the
target description, in such a way that GDB can recognize them.

   This is accomplished by giving specific names to feature elements
which contain standard registers.  GDB will look for features with those
names and verify that they contain the expected registers; if any known
feature is missing required registers, or if any required feature is
missing, GDB will reject the target description.  You can add additional
registers to any of the standard features -- GDB will display them just
as if they were added to an unrecognized feature.

   This section lists the known features and their expected contents.
Sample XML documents for these features are included in the GDB source
tree, in the directory ‘gdb/features’.

   Names recognized by GDB should include the name of the company or
organization which selected the name, and the overall architecture to
which the feature applies; so e.g. the feature containing ARM core
registers is named ‘org.gnu.gdb.arm.core’.

   The names of registers are not case sensitive for the purpose of
recognizing standard features, but GDB will only display registers using
the capitalization used in the description.

* Menu:

* AArch64 Features::
* Alpha Features::
* ARC Features::
* ARM Features::
* i386 Features::
* LoongArch Features::
* MicroBlaze Features::
* MIPS Features::
* M68K Features::
* NDS32 Features::
* OpenRISC 1000 Features::
* PowerPC Features::
* RISC-V Features::
* RX Features::
* S/390 and System z Features::
* Sparc Features::
* TIC6x Features::


File: gdb.info,  Node: AArch64 Features,  Next: Alpha Features,  Up: Standard Target Features

G.5.1 AArch64 Features
----------------------

G.5.1.1 AArch64 core registers feature
......................................

The ‘org.gnu.gdb.aarch64.core’ feature is required for AArch64 targets.
It must contain the following:

   − ‘x0’ through ‘x30’, the general purpose registers, with size of 64
     bits.  Register ‘x30’ is also known as the “link register”, or
     ‘lr’.
   − ‘sp’, the stack pointer register or ‘x31’.  It is 64 bits in size
     and has a type of ‘data_ptr’.
   − ‘pc’, the program counter register.  It is 64 bits in size and has
     a type of ‘code_ptr’.
   − ‘cpsr’, the current program status register.  It is 32 bits in size
     and has a custom flags type.

   The semantics of the individual flags and fields in ‘cpsr’ can change
as new architectural features are added.  The current layout can be
found in the aarch64-core.xml file.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.1.2 AArch64 floating-point registers feature
................................................

The ‘org.gnu.gdb.aarch64.fpu’ feature is optional.  If present, it must
contain the following registers:

   − ‘v0’ through ‘v31’, the vector registers with size of 128 bits.
     The type is a custom vector type.
   − ‘fpsr’, the floating-point status register.  It is 32 bits in size
     and has a custom flags type.
   − ‘fpcr’, the floating-point control register.  It is 32 bits in size
     and has a custom flags type.

   The semantics of the individual flags and fields in ‘fpsr’ and ‘fpcr’
can change as new architectural features are added.

   The types for the vector registers, ‘fpsr’ and ‘fpcr’ registers can
be found in the aarch64-fpu.xml file.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.1.3 AArch64 SVE registers feature
.....................................

The ‘org.gnu.gdb.aarch64.sve’ feature is optional.  If present, it means
the target supports the Scalable Vector Extension and must contain the
following registers:

   − ‘z0’ through ‘z31’, the scalable vector registers.  Their sizes are
     variable and a multiple of 128 bits up to a maximum of 2048 bit.
     Their type is a custom union type that helps visualize different
     sizes of sub-vectors.
   − ‘fpsr’, the floating-point status register.  It is 32 bits in size
     and has a custom flags type.
   − ‘fpcr’, the floating-point control register.  It is 32 bits in size
     and has a custom flags type.
   − ‘p0’ through ‘p15’, the predicate registers.  Their sizes are
     variable, based on the current vector length, and a multiple of 16
     bits.  Their types are a custom union to help visualize
     sub-elements.
   − ‘ffr’, the First Fault register.  It has a variable size based on
     the current vector length and is a multiple of 16 bits.  The type
     is the same as the predicate registers.
   − ‘vg’, the vector granule.  It represents the number of 64 bits
     chunks in a ‘z’ register.  It is closely associated with the
     current vector length.  It has a type of ‘int’.

   When GDB sees the SVE feature, it will assume the Scalable Vector
Extension is supported, and will adjust the sizes of the ‘z’, ‘p’ and
‘ffr’ registers accordingly, based on the value of ‘vg’.

   GDB will also create pseudo-registers equivalent to the ‘v’ vector
registers from the ‘org.gnu.gdb.aarch64.fpu’ feature.

   The first 128 bits of the ‘z’ registers overlap the 128 bits of the
‘v’ registers, so changing one will trigger a change to the other.

   For the types of the ‘z’, ‘p’ and ‘ffr’ registers, please check the
aarch64-sve.c file.  No XML file is available for this feature because
it is dynamically generated based on the current vector length.

   The semantics of the individual flags and fields in ‘fpsr’ and ‘fpcr’
can change as new architectural features are added.

   The types for the ‘fpsr’ and ‘fpcr’ registers can be found in the
aarch64-sve.c file, and should match what is described in
aarch64-fpu.xml.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.1.4 AArch64 Pointer Authentication registers feature
........................................................

The ‘org.gnu.gdb.aarch64.pauth’ optional feature was introduced so GDB
could detect support for the Pointer Authentication extension.  If
present, it must contain one of two possible register sets.

   Pointer Authentication masks for user-mode:

   − ‘pauth_dmask’, the user-mode pointer authentication mask for data
     pointers.  It is 64 bits in size.
   − ‘pauth_cmask’, the user-mode pointer authentication mask for code
     pointers.  It is 64 bits in size.

   Pointer Authentication masks for user-mode and kernel-mode:

   − ‘pauth_dmask’, the user-mode pointer authentication mask for data
     pointers.  It is 64 bits in size.
   − ‘pauth_cmask’, the user-mode pointer authentication mask for code
     pointers.  It is 64 bits in size.
   − ‘pauth_dmask_high’, the kernel-mode pointer authentication mask for
     data pointers.  It is 64 bits in size.
   − ‘pauth_cmask_high’, the kernel-mode pointer authentication mask for
     code pointers.  It is 64 bits in size.

   If GDB sees any of the two sets of registers in this feature, it will
assume the target is capable of signing pointers.  If so, GDB will
decorate backtraces with a ‘[PAC]’ marker alongside a function that has
a signed link register value that needs to be unmasked/decoded.

   GDB will also use the masks to remove non-address bits from pointers.

   Extra registers are allowed in this feature, but they will not affect
GDB.

   Please note the ‘org.gnu.gdb.aarch64.pauth’ feature string is
deprecated and must only be used for backwards compatibility with older
releases of GDB and ‘gdbserver’.  Targets that support Pointer
Authentication must advertise such capability by using the
‘org.gnu.gdb.aarch64.pauth_v2’ feature string instead.

   The ‘org.gnu.gdb.aarch64.pauth_v2’ feature has the exact same
contents as feature ‘org.gnu.gdb.aarch64.pauth’.

   The reason for having feature ‘org.gnu.gdb.aarch64.pauth_v2’ is a bug
in previous versions of GDB (versions 9, 10, 11 and 12).  This bug
caused GDB to crash whenever the target reported support for Pointer
Authentication (using feature string ‘org.gnu.gdb.aarch64.pauth’) and
also reported additional system registers that were not accounted for by
GDB.  This is more common when using emulators and on bare-metal
debugging scenarios.

   It can also happen if a newer gdbserver is used with an old GDB that
has the bug.  In such a case, the newer gdbserver might report Pointer
Authentication support via the ‘org.gnu.gdb.aarch64.pauth’ feature
string and also report additional registers the older GDB does not know
about, potentially leading to a crash.

G.5.1.5 AArch64 TLS registers feature
.....................................

The ‘org.gnu.gdb.aarch64.tls’ optional feature was introduced to expose
the TLS registers to GDB.  If present, it must contain either one of the
following register sets.

   Only ‘tpidr’:

   − ‘tpidr’, the software thread id register.  It is 64 bits in size
     and has a type of ‘data_ptr’.

   Both ‘tpidr’ and ‘tpidr2’.

   − ‘tpidr’, the software thread id register.  It is 64 bits in size
     and has a type of ‘data_ptr’.
   − ‘tpidr2’, the second software thread id register.  It is 64 bits in
     size and has a type of ‘data_ptr’.  It may be used in the future
     alongside the Scalable Matrix Extension for a lazy restore scheme.

   If GDB sees this feature, it will attempt to find one of the
variations of the register set.  If ‘tpidr2’ is available, GDB may act
on it to display additional data in the future.

   There is no XML for this feature as the presence of ‘tpidr2’ is
determined dynamically at runtime.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.1.6 AArch64 MTE registers feature
.....................................

The ‘org.gnu.gdb.aarch64.mte’ optional feature was introduced so GDB
could detect support for the Memory Tagging Extension and control memory
tagging settings.  If present, this feature must have the following
register:

   − ‘tag_ctl’, the tag control register.  It is 64 bits in size and has
     a type of ‘uint64’.

   Memory Tagging detection is done via a runtime check though, so the
presence of this feature and register is not enough to enable memory
tagging support.

   This restriction may be lifted in the future.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.1.7 AArch64 SME registers feature
.....................................

The ‘org.gnu.gdb.aarch64.sme’ feature is optional.  If present, it
should contain registers ‘ZA’, ‘SVG’ and ‘SVCR’.  *Note AArch64 SME::.

   − ‘ZA’ is a register represented by a 2-dimensional array of bytes,
     with each dimension having size SVL.  *Note svl::.

   − ‘SVG’ is a 64-bit register containing the value of SVG.  *Note
     svg::.

   − ‘SVCR’ is a 64-bit status pseudo-register with two valid bits.  Bit
     0 (SM) shows whether the streaming SVE mode is enabled or disabled.
     Bit 1 (ZA) shows whether the ‘ZA’ register state is active (in use)
     or not.  *Note aarch64 sme svcr::.

     The rest of the unused bits of the ‘SVCR’ pseudo-register is
     undefined and reserved.  Such bits should not be used and may be
     defined by future extensions of the architecture.

   Extra registers are allowed in this feature, but they will not affect
GDB.

   The ‘org.gnu.gdb.aarch64.sme’ feature is required when the target
also reports support for the ‘org.gnu.gdb.aarch64.sme2’ feature.

G.5.1.8 AArch64 SME2 registers feature
......................................

The ‘org.gnu.gdb.aarch64.sme2’ feature is optional.  If present, then
the ‘org.gnu.gdb.aarch64.sme’ feature must also be present.  The
‘org.gnu.gdb.aarch64.sme2’ feature should contain the following: *Note
AArch64 SME2::.

   − ‘ZT0’ is a register of 512 bits (64 bytes).  It is defined as a
     vector of bytes.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.1.9 AArch64 GCS registers feature
.....................................

The ‘org.gnu.gdb.aarch64.gcs’ feature is optional.  If present, it means
the target supports Guarded Control Stacks and must contain the
following register:

   − ‘gcspr’, which points to the thread's Guarded Control Stack.  It is
     64 bits in size and has a type of ‘data_ptr’.

   The ‘org.gnu.gdb.aarch64.gcs.linux’ feature is optional.  If present,
then the ‘org.gnu.gdb.aarch64.gcs’ feature must also be present.  The
‘org.gnu.gdb.aarch64.gcs.linux’ feature represents facilities provided
by the Linux kernel for GCS support and should contain the following:

   − ‘gcs_features_enabled’ shows the features currently enabled via the
     prctl or ptrace system calls.  It is represented as if it were a
     64-bit register with a custom flags type.

   − ‘gcs_features_locked’ shows the features currently locked via the
     prctl or ptrace system calls.  It is represented as if it were a
     64-bit register with a custom flags type.

   The custom flags type allows GDB to print a human-friendly
representation of the contents of ‘gcs_features_enabled’ and
‘gcs_features_locked’ and should contain:

   − ‘PR_SHADOW_STACK_ENABLE’

   − ‘PR_SHADOW_STACK_WRITE’

   − ‘PR_SHADOW_STACK_PUSH’

   For further information, see the documentation in the Linux kernel.

   Extra registers are allowed in these features, but they will not
affect GDB.

G.5.1.10 AArch64 FPMR feature
.............................

The ‘org.gnu.gdb.aarch64.fpmr’ optional feature was introduced so GDB
could detect support for the Floating Point Mode Register extension.  If
present, this feature must have the following register:

   − ‘fpmr’, the floating point mode register.  It is represented as if
     it were a 64-bit register with a custom flags type.

   The custom flags type allows GDB to print a human-friendly
representation of the contents of ‘fpmr’ should contain:

   − ‘F8S1’, which is either ‘E5M2’ or ‘E4M3’

   − ‘F8S2’, which is either ‘E5M2’ or ‘E4M3’

   − ‘F8D’, which is either ‘E5M2’ or ‘E4M3’

   − ‘OSM’, which is either ‘Inf’ or ‘MaxNormal’

   − ‘OSC’, which is either ‘Inf/NaN’ or ‘MaxNormal’

   − ‘LSCALE’, which is a 7-bit unsigned value

   − ‘NSCALE’, which is actually an 8-bit signed value but represented
     as an 8-bit unsigned value

   − ‘LSCALE2’, which is a 6-bit unsigned value

   Extra registers are allowed in this feature, but they will not affect
GDB.

   For more information about FPMR, please refer to the official
architecture registers documentation.


File: gdb.info,  Node: Alpha Features,  Next: ARC Features,  Prev: AArch64 Features,  Up: Standard Target Features

G.5.2 Alpha Features
--------------------

The ‘org.gnu.gdb.alpha.core’ feature is required for Alpha targets.  It
must contain the following 64-bit registers; note that GDB uses the
software names for Alpha registers:

   − ‘v0’: function return value
   − ‘t0’ through ‘t12’: temporary registers
   − ‘s0’ through ‘s5’: saved registers
   − ‘fp’: frame pointer
   − ‘a0’ through ‘a5’: argument registers
   − ‘ra’: return address
   − ‘at’: assembler temporary register
   − ‘gp’: global pointer
   − ‘sp’: stack pointer
   − ‘zero’: always zero
   − ‘f0’ through ‘f30’: floating-point registers
   − ‘fpcr’: floating-point control register
   − ‘pc’: program counter
   − ‘’: an anonymous register for historical purpose
   − ‘unique’: PALcode memory slot


File: gdb.info,  Node: ARC Features,  Next: ARM Features,  Prev: Alpha Features,  Up: Standard Target Features

G.5.3 ARC Features
------------------

ARC processors are so configurable that even core registers and their
numbers are not predetermined completely.  Moreover, _flags_ and _PC_
registers, which are important to GDB, are not "core" registers in ARC.
Therefore, there are two features that their presence is mandatory:
‘org.gnu.gdb.arc.core’ and ‘org.gnu.gdb.arc.aux’.

   The ‘org.gnu.gdb.arc.core’ feature is required for all targets.  It
must contain registers:

   − ‘r0’ through ‘r25’ for normal register file targets.
   − ‘r0’ through ‘r3’, and ‘r10’ through ‘r15’ for reduced register
     file targets.
   − ‘gp’, ‘fp’, ‘sp’, ‘r30’(1), ‘blink’, ‘lp_count’, ‘pcl’.

   In case of an ARCompact target (ARCv1 ISA), the
‘org.gnu.gdb.arc.core’ feature may contain registers ‘ilink1’ and
‘ilink2’.  While in case of ARC EM and ARC HS targets (ARCv2 ISA),
register ‘ilink’ may be present.  The difference between ARCv1 and ARCv2
is the naming of registers _29th_ and _30th_.  They are called ‘ilink1’
and ‘ilink2’ for ARCv1 and are optional.  For ARCv2, they are called
‘ilink’ and ‘r30’ and only ‘ilink’ is optional.  The optionality of
‘ilink*’ registers is because of their inaccessibility during user space
debugging sessions.

   Extension core registers ‘r32’ through ‘r59’ are optional and their
existence depends on the configuration.  When debugging GNU/Linux
applications, i.e. user space debugging, these core registers are not
available.

   The ‘org.gnu.gdb.arc.aux’ feature is required for all ARC targets.
Here is the list of registers pertinent to this feature:

   − mandatory: ‘pc’ and ‘status32’.
   − optional: ‘lp_start’, ‘lp_end’, and ‘bta’.

   ---------- Footnotes ----------

   (1) Not necessary for ARCv1.


File: gdb.info,  Node: ARM Features,  Next: i386 Features,  Prev: ARC Features,  Up: Standard Target Features

G.5.4 ARM Features
------------------

G.5.4.1 Core register set for non-M-profile
...........................................

The ‘org.gnu.gdb.arm.core’ feature is required for non-M-profile ARM
targets.  It must contain the following registers:

   − ‘r0’ through ‘r12’.  The general purpose registers.  They are 32
     bits in size and have a type of ‘uint32’.
   − ‘sp’, the stack pointer register, also known as ‘r13’.  It is 32
     bits in size and has a type of ‘data_ptr’.
   − ‘lr’, the link register.  It is 32 bits in size.
   − ‘pc’, the program counter register.  It is 32 bit in size and of
     type ‘code_ptr’.
   − ‘cpsr’, the current program status register containing all the
     status bits.  It is 32 bits in size.  Historically this register
     was hardwired to number 25, but debugging stubs that report XML do
     not need to use this number anymore.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.4.2 Core register set for M-profile
.......................................

For M-profile targets (e.g. Cortex-M3), the ‘org.gnu.gdb.arm.core’
feature is replaced by ‘org.gnu.gdb.arm.m-profile’, and it is a required
feature.  It must contain the following registers:

   − ‘r0’ through ‘r12’, the general purpose registers.  They have a
     size of 32 bits and a type of ‘uint32’.
   − ‘sp’, the stack pointer register, also known as ‘r13’.  It has a
     size of 32 bits and a type of ‘data_ptr’.
   − ‘lr’, the link register.  It has a size of 32 bits.
   − ‘pc’, the program counter register.  It has a size of 32 bits and a
     type of ‘code_ptr’.
   − ‘xpsr’, the program status register containing all the status bits.
     It has a size of 32 bits.  Historically this register was hardwired
     to number 25, but debugging stubs that report XML do not need to
     use this number anymore.

   Upon seeing this feature, GDB will acknowledge that it is dealing
with an M-profile target.  This means GDB will use hooks and
configurations that are meaningful to M-profiles.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.4.3 FPA registers feature (obsolete)
........................................

The ‘org.gnu.gdb.arm.fpa’ feature is obsolete and should not be
advertised by debugging stubs anymore.  It used to advertise registers
for the old FPA architecture that has long been discontinued in
toolchains.

   It is kept in GDB for backward compatibility purposes so older
debugging stubs that don't support XML target descriptions still work
correctly.  One such example is the KGDB debugging stub from Linux or
BSD kernels.

   The description below is for historical purposes only.  This feature
used to contain the following registers:

   − ‘f0’ through ‘f8’.  The floating point registers.  They are 96 bits
     in size and of type ‘arm_fpa_ext’.  ‘f0’ is pinned to register
     number 16.
   − ‘fps’, the status register.  It has a size of 32 bits.

G.5.4.4 M-profile Vector Extension (MVE)
........................................

Also known as Helium, the M-profile Vector Extension is advertised via
the optional ‘org.gnu.gdb.arm.m-profile-mve’ feature.

   It must contain the following:

   − ‘vpr’, the vector predication status and control register.  It is
     32 bits in size and has a custom flags type.  The flags type is
     laid out in a way that exposes the ‘P0’ field from bits 0 to 15,
     the ‘MASK01’ field from bits 16 to 19 and the ‘MASK23’ field from
     bits 20 to 23.

     Bits 24 through 31 are reserved.

   When this feature is available, GDB will synthesize the ‘p0’
pseudo-register from ‘vpr’ contents.

   This feature must only be advertised if the target is M-profile.
Advertising this feature for targets that are not M-profile may cause
GDB to assume the target is M-profile when it isn't.

   If the ‘org.gnu.gdb.arm.vfp’ feature is available alongside the
‘org.gnu.gdb.arm.m-profile-mve’ feature, GDB will synthesize the ‘q’
pseudo-registers from ‘d’ register contents.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.4.5 XScale iwMMXt feature
.............................

The XScale ‘org.gnu.gdb.xscale.iwmmxt’ feature is optional.  If present,
it must contain the following:

   − ‘wR0’ through ‘wR15’, registers with size 64 bits and a custom type
     ‘iwmmxt_vec64i’.  ‘iwmmxt_vec64i’ is a union of four other types:
     ‘uint64’, a 2-element vector of ‘uint32’, a 4-element vector of
     ‘uint16’ and a 8-element vector of ‘uint8’.
   − ‘wCGR0’ through ‘wCGR3’, registers with size 32 bits and type
     ‘int’.

   The following registers are optional:

   − ‘wCID’, register with size of 32 bits and type ‘int’.
   − ‘wCon’, register with size 32 bits and type ‘int’.
   − ‘wCSSF’, register with size 32 bits and type ‘int’.
   − ‘wCASF’, register with size 32 bit and type ‘int’.

   This feature should only be reported if the target is XScale.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.4.6 Vector Floating-Point (VFP) feature
...........................................

The ‘org.gnu.gdb.arm.vfp’ feature is optional.  If present, it should
contain one of two possible sets of values depending on whether VFP
version 2 or VFP version 3 is in use.

   For VFP v2:

   − ‘d0’ through ‘d15’.  The double-precision registers.  They are 64
     bits in size and have type ‘ieee_double’.
   − ‘fpscr’, the floating-point status and control register.  It has a
     size of 32 bits and a type of ‘int’.

   For VFP v3:

   − ‘d0’ through ‘d31’.  The double-precision registers.  They are 64
     bits in size and have type ‘ieee_double’.
   − ‘fpscr’, the floating-point status and control register.  It has a
     size of 32 bits and a type of ‘int’.

   If this feature is available, GDB will synthesize the
single-precision floating-point registers from halves of the
double-precision registers as pseudo-registers.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.4.7 NEON architecture feature
.................................

The ‘org.gnu.gdb.arm.neon’ feature is optional.  It does not need to
contain registers; it instructs GDB to display the VFP double-precision
registers as vectors and to synthesize the quad-precision registers from
pairs of double-precision registers.  If this feature is present,
‘org.gnu.gdb.arm.vfp’ must also be present and include 32
double-precision registers.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.4.8 M-profile Pointer Authentication and Branch Target Identification feature
.................................................................................

The ‘org.gnu.gdb.arm.m-profile-pacbti’ feature is optional, and
acknowledges support for the ARMv8.1-m PACBTI extensions.

   This feature doesn't contain any required registers, and it only
serves as a hint to GDB that the debugging stub supports the ARMv8.1-m
PACBTI extensions.

   When GDB sees this feature, it will track return address signing
states and will decorate backtraces using the [PAC] marker, similar to
AArch64's PAC extension.  *Note AArch64 PAC::.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.4.9 M-profile system registers feature
..........................................

The ‘org.gnu.gdb.arm.m-system’ optional feature was introduced as a way
to inform GDB about additional system registers.

   At the moment this feature must contain the following:

   − ‘msp’, the main stack pointer register.  It is 32 bits in size with
     type ‘data_ptr’.
   − ‘psp’, the process stack pointer register.  It is 32 bits in size
     with type ‘data_ptr’.

   This feature must only be advertised for M-profile targets.  When GDB
sees this feature, it will attempt to track the values of ‘msp’ and
‘psp’ across frames.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.4.10 M-profile Security Extensions feature
..............................................

The ‘org.gnu.gdb.arm.secext’ optional feature was introduced so GDB
could better support the switching of stack pointers and secure states
in the Security Extensions.

   At the moment this feature must contain the following:

   − ‘msp_ns’, the main stack pointer register (non-secure state).  It
     is 32 bits in size with type ‘data_ptr’.
   − ‘psp_ns’, the process stack pointer register (non-secure state).
     It is 32 bits in size with type ‘data_ptr’.
   − ‘msp_s’, the main stack pointer register (secure state).  It is 32
     bits in size with type ‘data_ptr’.
   − ‘psp_s’, the process stack pointer register (secure state).  It is
     32 bits in size with type ‘data_ptr’.

   When GDB sees this feature, it will attempt to track the values of
all 4 stack pointers across secure state transitions, potentially
improving unwinding when applications switch between security states.

   Extra registers are allowed in this feature, but they will not affect
GDB.

G.5.4.11 TLS registers feature
..............................

The optional ‘org.gnu.gdb.arm.tls’ feature contains TLS registers.

   Currently it contains the following:

   − ‘tpidruro’, the user read-only thread id register.  It is 32 bits
     in size and has type ‘data_ptr’.

   At the moment GDB looks for this feature, but doesn't do anything
with it other than displaying it.

   Extra registers are allowed in this feature, but they will not affect
GDB.

