# Porting libstdc++-v3

Node: Top, Next: Operating system, Up: (dir)

## 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. Unless this is done, libstdc++-v3 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.)

Here are the primary steps required to port the library:

Node: Operating system, Next: CPU, 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 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. 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 a few libstdc++-v3-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.

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

## 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.

Node: Character types, Next: Thread safety, Previous: CPU, 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`. 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++-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 `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_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 `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 `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;
         }

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 `cpu_limits.h` in your CPU configuration directory (see CPU).

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 1.2, November 2002

         Copyright © 2000,2001,2002 Free Software Foundation, Inc.
         59 Temple Place, Suite 330, Boston, MA  02111-1307, USA

         Everyone is permitted to copy and distribute verbatim copies
         of this license document, but changing it is not allowed.

0. PREAMBLE

   The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

   This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

   We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

   This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you". You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

   A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

   A "Secondary Section" is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

   The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.

   The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.

   A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not "Transparent" is called "Opaque".

   Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.

   The "Title Page" means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, "Title Page" means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.

   A section "Entitled XYZ" means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as "Acknowledgements", "Dedications", "Endorsements", or "History".) To "Preserve the Title" of such a section when you modify the Document means that it remains a section "Entitled XYZ" according to this definition.

   The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.

2. VERBATIM COPYING

   You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

   You may also lend copies, under the same conditions stated above, and you may publicly display copies.

3. COPYING IN QUANTITY

   If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

   If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

   If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

   It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

   You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

   1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
   2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
   3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
   4. Preserve all the copyright notices of the Document.
   5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
   6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
   7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.
   8. Include an unaltered copy of this License.
   9. Preserve the section Entitled "History", Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled "History" in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
   10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
   11. For any section Entitled "Acknowledgements" or "Dedications", Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
   12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
   13. Delete any section Entitled "Endorsements". Such a section may not be included in the Modified Version.
   14. Do not retitle any existing section to be Entitled "Endorsements" or to conflict in title with any Invariant Section.
   15. Preserve any Warranty Disclaimers.

   If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

   You may add a section Entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties--for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

   You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

   The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

   You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

   The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

   In the combination, you must combine any sections Entitled "History" in the various original documents, forming one section Entitled "History"; likewise combine any sections Entitled "Acknowledgements", and any sections Entitled "Dedications". You must delete all sections Entitled "Endorsements."

6. COLLECTIONS OF DOCUMENTS

   You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

   You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

   A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an "aggregate" if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

   If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

8. TRANSLATION

   Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warrany Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

   If a section in the Document is Entitled "Acknowledgements", "Dedications", or "History", the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

9. TERMINATION

   You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

10. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

### 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 1.2
           or any later version published by the Free Software Foundation;
           with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
           A copy of the license is included in the section entitled ``GNU
           Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the "with...Texts." line with this:

             with the Invariant Sections being list their titles, with
             the Front-Cover Texts being list, and with the Back-Cover Texts
             being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

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
- CPU
- Character types
- Thread safety
- Numeric limits
- Libtool
- GNU Free Documentation License
  - ADDENDUM: How to use this License for your documents