Node:Top, Next:Operating system

# Porting libstdc++-v3

This document explains how to port libstdc++-v3 (the GNU C++ library) to a new target.

In order to make the GNU C++ library (libstdc++-v3) work with a new target, you must edit some configuration files and provide some new header files.

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.

Here are the primary steps required to port the library:

- Operating system: Configuring for your operating system.
- Character types: Implementing character classification.
- Thread safety: Implementing atomic operations.
- Numeric limits: Implementing numeric limits.
- Libtool: Using libtool.
- GNU Free Documentation License: How you can copy and share this manual.

Node:Operating system, Next:Character types, Previous:Top, Up:Top

# 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'll have to change the `configure.target` 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. The switch statement switches on only the OS portion of the standard target triplet; e.g., the `solaris2.8` in `sparc-sun-solaris2.8`.

The first file to create in this directory, should be called `bits/os_defines.h`. This file contains basic macro definitions that are required to allow the C++ library to work with your C library. This file should provide macro definitions for `__off_t`, `__off64_t`, and `__ssize_t`. Typically, this just looks like:

    #define __off_t off_t
    #define __off64_t off64_t
    #define __ssize_t ssize_t

You don't have to provide these definitions if your system library already defines these types - but the only library known to provide these types is the GNU C Library, so you will almost certainly have to provide these macros. Note that this file does not have to include a header file that defines `off_t`, or the other types; you simply have to provide the macros.

In addition, several libstdc++-v3 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 two libstdc++-v3-specific macros which may be defined. `_G_USING_THUNKS` may be defined to 0 to express that the port doesn't use thunks (although it is unclear that this is still useful since libio support isn't currently working and the g++ v3 ABI invalidates the assumption that some ports don't use thunks). `_GLIBCPP_AVOID_FSEEK` may be defined if seeking on an interactive stream (or one hooked to a pipe) is not allowed by the OS. In this case, getc()/ungetc() will be used at some key locations in the library implementation instead of fseek(). Currently, the code path to avoid fseek() is only enabled when the seek size is 1 character away from the current stream position. This is known to improve *-unknown-freebsd*, sparc-sun-solaris2.* and *-*-mingw32*.

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

    #ifndef _GLIBCPP_OS_DEFINES
    #define _GLIBCPP_OS_DEFINES
    ...
    #endif

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

Node:Character types, Next:Thread safety, Previous:Operating system, Up:Top

# 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/bits`. However, these files will almost certainly need some modification.

The first file to write is `bits/ctype_base.h`. This file provides some very basic information about character classification. The libstdc++-v3 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 `bits/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 `bits/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 `bits/ctype_base.h` is included.

The next file to write is `bits/ctype_noninline.h`, which also does not require include guards. This file defines a few member functions that will be included in `include/bits/locale_facets.h`. 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 `bits/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;
    }

Node:Thread safety, Next:Numeric limits, Previous:Character types, Up:Top

# 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 `bits/atomicity.h`.

If you are using the assembly-language approach, put this code in `config/cpu/<chip>/bits/atomicity.h`, where chip is the name of your processor. In that case, edit the switch statement in `configure.target` to set the `cpu_include_dir`. In either case, set the switch statement that sets `ATOMICITYH` to be the directory containing `bits/atomicity.h`.

With those bits out of the way, you have to actually write `bits/atomicity.h` itself. This file should be wrapped in an include guard named `_BITS_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;
    }

Node:Numeric limits, Next:Libtool, Previous:Thread safety, Up:Top

# 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 `limits.h` in your CPU configuration directory (e.g., `config/cpu/i386/bits`) and then modify `configure.target` so that `LIMITSH` is set to the CPU directory (e.g., `config/cpu/i386`). Note that `LIMITSH` should not include the `bits` part of the directory name.

Node:Libtool, Next:GNU Free Documentation License, Previous:Numeric limits, Up:Top

# 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++-v3 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++-v3 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.

Node:GNU Free Documentation License, Previous:Libtool, Up:Top

# GNU Free Documentation License

Version 1.1, March 2000

    Copyright © 2000 Free Software Foundation, Inc.
    59 Temple Place, Suite 330, Boston, MA  02111-1307, USA

    Everyone is permitted to copy and distribute verbatim copies
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## ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

      Copyright (C)  year  your name.
      Permission is granted to copy, distribute and/or modify this document
      under the terms of the GNU Free Documentation License, Version 1.1
      or any later version published by the Free Software Foundation;
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If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

# Table of Contents

- Porting libstdc++-v3
- Operating system
- Character types
- Thread safety
- Numeric limits
- Libtool
- GNU Free Documentation License
  - ADDENDUM: How to use this License for your documents