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         xml:id="appendix.porting.internals" xreflabel="Portin Internals">

      ISO C++

      internals

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.)

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.

   Several libstdc++ source files unconditionally define the macro

_POSIX_SOURCE.  On many systems, defining this macro causes

large portions of the C library header files to be eliminated

at preprocessing time.  Therefore, you may have to #undef this

macro, or define other macros (like _LARGEFILE_SOURCE or

__EXTENSIONS__).  You won't know what macros to define or

undefine at this point; you'll have to try compiling the library and

seeing what goes wrong.  If you see errors about calling functions

that have not been declared, look in your C library headers to see if

the functions are declared there, and then figure out what macros you

need to define.  You will need to add them to the

CPLUSPLUS_CPP_SPEC macro in the GCC configuration file for your

target.  It will not work to simply define these macros in

os_defines.h.

   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.

   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.

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.

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;

     }

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;

     }

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).

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.