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Porting to New Hardware or Operating Systems
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## Porting to New Hardware or Operating Systems

This document explains how to port libstdc++ (the GNU C++ library) to a new target.

In order to make the GNU C++ library (libstdc++) work with a new target, you must edit some configuration files and provide some new header files. Unless this is done, libstdc++ will use generic settings which may not be correct for your target; even if they are correct, they will likely be inefficient.

Before you get started, make sure that you have a working C library on your target. The C library need not precisely comply with any particular standard, but should generally conform to the requirements imposed by the ANSI/ISO standard.

In addition, you should try to verify that the C++ compiler generally works. It is difficult to test the C++ compiler without a working library, but you should at least try some minimal test cases.

(Note that what we think of as a "target," the library refers to as a "host." The comment at the top of `configure.ac` explains why.)

### Operating System

If you are porting to a new operating system (as opposed to a new chip using an existing operating system), you will need to create a new directory in the `config/os` hierarchy. For example, the IRIX configuration files are all in `config/os/irix`. There is no set way to organize the OS configuration directory. For example, `config/os/solaris/solaris-2.6` and `config/os/solaris/solaris-2.7` are used as configuration directories for these two versions of Solaris. On the other hand, both Solaris 2.7 and Solaris 2.8 use the `config/os/solaris/solaris-2.7` directory. The important information is that there needs to be a directory under `config/os` to store the files for your operating system.

You might have to change the `configure.host` file to ensure that your new directory is activated. Look for the switch statement that sets `os_include_dir`, and add a pattern to handle your operating system if the default will not suffice. The switch statement switches on only the OS portion of the standard target triplet; e.g., the `solaris2.8` in `sparc-sun-solaris2.8`. If the new directory is named after the OS portion of the triplet (the default), then nothing needs to be changed.

The first file to create in this directory, should be called `os_defines.h`. This file contains basic macro definitions that are required to allow the C++ library to work with your C library.

At this time, there are a few libstdc++-specific macros which may be defined:

`_GLIBCXX_USE_C99_CHECK` may be defined to 1 to check C99 function declarations (which are not covered by specialization below) found in system headers against versions found in the library headers derived from the standard.

`_GLIBCXX_USE_C99_DYNAMIC` may be defined to an expression that yields 0 if and only if the system headers are exposing proper support for C99 functions (which are not covered by specialization below). If defined, it must be 0 while bootstrapping the compiler/rebuilding the library.

`_GLIBCXX_USE_C99_LONG_LONG_CHECK` may be defined to 1 to check the set of C99 long long function declarations found in system headers against versions found in the library headers derived from the standard.

`_GLIBCXX_USE_C99_LONG_LONG_DYNAMIC` may be defined to an expression that yields 0 if and only if the system headers are exposing proper support for the set of C99 long long functions. If defined, it must be 0 while bootstrapping the compiler/rebuilding the library.

`_GLIBCXX_USE_C99_FP_MACROS_DYNAMIC` may be defined to an expression that yields 0 if and only if the system headers are exposing proper support for the related set of macros. If defined, it must be 0 while bootstrapping the compiler/rebuilding the library.

`_GLIBCXX_USE_C99_FLOAT_TRANSCENDENTALS_CHECK` may be defined to 1 to check the related set of function declarations found in system headers against versions found in the library headers derived from the standard.

`_GLIBCXX_USE_C99_FLOAT_TRANSCENDENTALS_DYNAMIC` may be defined to an expression that yields 0 if and only if the system headers are exposing proper support for the related set of functions. If defined, it must be 0 while bootstrapping the compiler/rebuilding the library.

`_GLIBCXX_NO_OBSOLETE_ISINF_ISNAN_DYNAMIC` may be defined to an expression that yields 0 if and only if the system headers are exposing non-standard `isinf(double)` and `isnan(double)` functions in the global namespace. Those functions should be detected automatically by the `configure` script when libstdc++ is built but if their presence depends on compilation flags or other macros the static configuration can be overridden.

Finally, you should bracket the entire file in an include-guard, like this:

    #ifndef _GLIBCXX_OS_DEFINES
    #define _GLIBCXX_OS_DEFINES
    ...
    #endif

We recommend copying an existing `os_defines.h` to use as a starting point.

### CPU

If you are porting to a new chip (as opposed to a new operating system running on an existing chip), you will need to create a new directory in the `config/cpu` hierarchy. Much like the Operating system setup, there are no strict rules on how to organize the CPU configuration directory, but careful naming choices will allow the configury to find your setup files without explicit help.

We recommend that for a target triplet `<CPU>-<vendor>-<OS>`, you name your configuration directory `config/cpu/<CPU>`. If you do this, the configury will find the directory by itself. Otherwise you will need to edit the `configure.host` file and, in the switch statement that sets `cpu_include_dir`, add a pattern to handle your chip.

Note that some chip families share a single configuration directory, for example, `alpha`, `alphaev5`, and `alphaev6` all use the `config/cpu/alpha` directory, and there is an entry in the `configure.host` switch statement to handle this.

The `cpu_include_dir` sets default locations for the files controlling Thread safety and Numeric limits, if the defaults are not appropriate for your chip.

### Character Types

The library requires that you provide three header files to implement character classification, analogous to that provided by the C libraries `<ctype.h>` header. You can model these on the files provided in `config/os/generic`. However, these files will almost certainly need some modification.

The first file to write is `ctype_base.h`. This file provides some very basic information about character classification. The libstdc++ library assumes that your C library implements `<ctype.h>` by using a table (indexed by character code) containing integers, where each of these integers is a bit-mask indicating whether the character is upper-case, lower-case, alphabetic, etc. The `ctype_base.h` file gives the type of the integer, and the values of the various bit masks. You will have to peer at your own `<ctype.h>` to figure out how to define the values required by this file.

The `ctype_base.h` header file does not need include guards. It should contain a single `struct` definition called `ctype_base`. This `struct` should contain two type declarations, and one enumeration declaration, like this example, taken from the IRIX configuration:

      struct ctype_base
         {
           typedef unsigned int 	mask;
           typedef int* 		__to_type;

           enum
           {
    	 space = _ISspace,
    	 print = _ISprint,
    	 cntrl = _IScntrl,
    	 upper = _ISupper,
    	 lower = _ISlower,
    	 alpha = _ISalpha,
    	 digit = _ISdigit,
    	 punct = _ISpunct,
    	 xdigit = _ISxdigit,
    	 alnum = _ISalnum,
    	 graph = _ISgraph
           };
         };

The `mask` type is the type of the elements in the table. If your C library uses a table to map lower-case numbers to upper-case numbers, and vice versa, you should define `__to_type` to be the type of the elements in that table. If you don't mind taking a minor performance penalty, or if your library doesn't implement `toupper` and `tolower` in this way, you can pick any pointer-to-integer type, but you must still define the type.

The enumeration should give definitions for all the values in the above example, using the values from your native `<ctype.h>`. They can be given symbolically (as above), or numerically, if you prefer. You do not have to include `<ctype.h>` in this header; it will always be included before `ctype_base.h` is included.

The next file to write is `ctype_configure_char.cc`. The first function that must be written is the `ctype<char>::ctype` constructor. Here is the IRIX example:

    ctype<char>::ctype(const mask* __table = 0, bool __del = false,
    	   size_t __refs = 0)
           : _Ctype_nois<char>(__refs), _M_del(__table != 0 && __del),
    	 _M_toupper(NULL),
    	 _M_tolower(NULL),
    	 _M_ctable(NULL),
    	 _M_table(!__table
    		  ? (const mask*) (__libc_attr._ctype_tbl->_class + 1)
    		  : __table)
           { }

There are two parts of this that you might choose to alter. The first, and most important, is the line involving `__libc_attr`. That is IRIX system-dependent code that gets the base of the table mapping character codes to attributes. You need to substitute code that obtains the address of this table on your system. If you want to use your operating system's tables to map upper-case letters to lower-case, and vice versa, you should initialize `_M_toupper` and `_M_tolower` with those tables, in similar fashion.

Now, you have to write two functions to convert from upper-case to lower-case, and vice versa. Here are the IRIX versions:

         char
         ctype<char>::do_toupper(char __c) const
         { return _toupper(__c); }

         char
         ctype<char>::do_tolower(char __c) const
         { return _tolower(__c); }

Your C library provides equivalents to IRIX's `_toupper` and `_tolower`. If you initialized `_M_toupper` and `_M_tolower` above, then you could use those tables instead.

Finally, you have to provide two utility functions that convert strings of characters. The versions provided here will always work - but you could use specialized routines for greater performance if you have machinery to do that on your system:

         const char*
         ctype<char>::do_toupper(char* __low, const char* __high) const
         {
           while (__low < __high)
    	 {
    	   *__low = do_toupper(*__low);
    	   ++__low;
    	 }
           return __high;
         }

         const char*
         ctype<char>::do_tolower(char* __low, const char* __high) const
         {
           while (__low < __high)
    	 {
    	   *__low = do_tolower(*__low);
    	   ++__low;
    	 }
           return __high;
         }

You must also provide the `ctype_inline.h` file, which contains a few more functions. On most systems, you can just copy `config/os/generic/ctype_inline.h` and use it on your system.

In detail, the functions provided test characters for particular properties; they are analogous to the functions like `isalpha` and `islower` provided by the C library.

The first function is implemented like this on IRIX:

         bool
         ctype<char>::
         is(mask __m, char __c) const throw()
         { return (_M_table)[(unsigned char)(__c)] & __m; }

The `_M_table` is the table passed in above, in the constructor. This is the table that contains the bitmasks for each character. The implementation here should work on all systems.

The next function is:

         const char*
         ctype<char>::
         is(const char* __low, const char* __high, mask* __vec) const throw()
         {
           while (__low < __high)
    	 *__vec++ = (_M_table)[(unsigned char)(*__low++)];
           return __high;
         }

This function is similar; it copies the masks for all the characters from `__low` up until `__high` into the vector given by `__vec`.

The last two functions again are entirely generic:

         const char*
         ctype<char>::
         scan_is(mask __m, const char* __low, const char* __high) const throw()
         {
           while (__low < __high && !this->is(__m, *__low))
    	 ++__low;
           return __low;
         }

         const char*
         ctype<char>::
         scan_not(mask __m, const char* __low, const char* __high) const throw()
         {
           while (__low < __high && this->is(__m, *__low))
    	 ++__low;
           return __low;
         }

### Thread Safety

The C++ library string functionality requires a couple of atomic operations to provide thread-safety. If you don't take any special action, the library will use stub versions of these functions that are not thread-safe. They will work fine, unless your applications are multi-threaded.

If you want to provide custom, safe, versions of these functions, there are two distinct approaches. One is to provide a version for your CPU, using assembly language constructs. The other is to use the thread-safety primitives in your operating system. In either case, you make a file called `atomicity.h`, and the variable `ATOMICITYH` must point to this file.

If you are using the assembly-language approach, put this code in `config/cpu/<chip>/atomicity.h`, where chip is the name of your processor (see CPU). No additional changes are necessary to locate the file in this case; `ATOMICITYH` will be set by default.

If you are using the operating system thread-safety primitives approach, you can also put this code in the same CPU directory, in which case no more work is needed to locate the file. For examples of this approach, see the `atomicity.h` file for IRIX or IA64.

Alternatively, if the primitives are more closely related to the OS than they are to the CPU, you can put the `atomicity.h` file in the Operating system directory instead. In this case, you must edit `configure.host`, and in the switch statement that handles operating systems, override the `ATOMICITYH` variable to point to the appropriate `os_include_dir`. For examples of this approach, see the `atomicity.h` file for AIX.

With those bits out of the way, you have to actually write `atomicity.h` itself. This file should be wrapped in an include guard named `_GLIBCXX_ATOMICITY_H`. It should define one type, and two functions.

The type is `_Atomic_word`. Here is the version used on IRIX:

    typedef long _Atomic_word;

This type must be a signed integral type supporting atomic operations. If you're using the OS approach, use the same type used by your system's primitives. Otherwise, use the type for which your CPU provides atomic primitives.

Then, you must provide two functions. The bodies of these functions must be equivalent to those provided here, but using atomic operations:

         static inline _Atomic_word
         __attribute__ ((__unused__))
         __exchange_and_add (_Atomic_word* __mem, int __val)
         {
           _Atomic_word __result = *__mem;
           *__mem += __val;
           return __result;
         }

         static inline void
         __attribute__ ((__unused__))
         __atomic_add (_Atomic_word* __mem, int __val)
         {
           *__mem += __val;
         }

### Numeric Limits

The C++ library requires information about the fundamental data types, such as the minimum and maximum representable values of each type. You can define each of these values individually, but it is usually easiest just to indicate how many bits are used in each of the data types and let the library do the rest. For information about the macros to define, see the top of `include/bits/std_limits.h`.

If you need to define any macros, you can do so in `os_defines.h`. However, if all operating systems for your CPU are likely to use the same values, you can provide a CPU-specific file instead so that you do not have to provide the same definitions for each operating system. To take that approach, create a new file called `cpu_limits.h` in your CPU configuration directory (see CPU).

### Libtool

The C++ library is compiled, archived and linked with libtool. Explaining the full workings of libtool is beyond the scope of this document, but there are a few, particular bits that are necessary for porting.

Some parts of the libstdc++ library are compiled with the libtool `--tags CXX` option (the C++ definitions for libtool). Therefore, `ltcf-cxx.sh` in the top-level directory needs to have the correct logic to compile and archive objects equivalent to the C version of libtool, `ltcf-c.sh`. Some libtool targets have definitions for C but not for C++, or C++ definitions which have not been kept up to date.

The C++ run-time library contains initialization code that needs to be run as the library is loaded. Often, that requires linking in special object files when the C++ library is built as a shared library, or taking other system-specific actions.

The libstdc++ library is linked with the C version of libtool, even though it is a C++ library. Therefore, the C version of libtool needs to ensure that the run-time library initializers are run. The usual way to do this is to build the library using `gcc -shared`.

If you need to change how the library is linked, look at `ltcf-c.sh` in the top-level directory. Find the switch statement that sets `archive_cmds`. Here, adjust the setting for your operating system.

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