gcc/libstdc++-v3/porting.texi

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% File:   porting.texi
% Author: Mark Mitchell
% Date:   11/21/2000
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% Contents:
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% Copyright (c) 2000 by CodeSourcery, LLC.  All rights reserved. 
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\input texinfo

@c ---------------------------------------------------------------------
@c Prologue
@c ---------------------------------------------------------------------

@setfilename porting.info
@settitle Porting libstdc++-v3
@setchapternewpage odd

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

Copyright (c) 2000 Free Software Foundation, Inc.
@end ifinfo

@c ---------------------------------------------------------------------
@c Titlepage
@c ---------------------------------------------------------------------

@titlepage
@title Porting libstdc++-v3
@author Mark Mitchell
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 2000 Free Software Foundation, Inc.
@end titlepage

@c ---------------------------------------------------------------------
@c Top
@c ---------------------------------------------------------------------

@node Top
@top 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:

@menu
* Operating system::    Configuring for your operating system.
* Character types::     Implementing character classification.
* Thread safety::       Implementing atomic operations.
* Libtool::             Using libtool.
@end menu

@c ---------------------------------------------------------------------
@c Operating system
@c ---------------------------------------------------------------------

@node Operating system
@chapter 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 @file{config/os} hierarchy.  For example, the IRIX
configuration files are all in @file{config/os/irix}.  There is no set
way to organize the OS configuration directory.  For example,
@file{config/os/solaris/solaris-2.6} and
@file{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 @file{config/os/solaris/solaris-2.7}
directory.  The important information is that there needs to be a
directory under @file{config/os} to store the files for your operating
system.

You'll have to change the @file{configure.target} file to ensure that
your new directory is activated.  Look for the switch statement that
sets @code{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 @code{solaris2.8} in
@code{sparc-sun-solaris2.8}.

The first file to create in this directory, should be called
@file{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 @code{__off_t},
@code{__off64_t}, and @code{__ssize_t}.  Typically, this just looks
like:

@example
#define __off_t off_t
#define __off64_t off64_t
#define __ssize_t ssize_t
@end example

@noindent
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 @code{off_t}, or the other types; you simply
have to provide the macros.

In addition, several libstdc++-v3 source files unconditionally define the macro
@code{_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 @code{#undef} this macro, or define
other macros (like @code{_LARGEFILE_SOURCE} or @code{__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 should but in @file{bits/os_defines.h}
to make these declarations available.

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

@example
#ifndef _GLIBCPP_OS_DEFINES
#define _GLIBCPP_OS_DEFINES
...
#endif
@end example

We recommend copying an existing @file{bits/os_defines.h} to use as a
starting point.

@c ---------------------------------------------------------------------
@c Character types
@c ---------------------------------------------------------------------

@node Character types
@chapter Character types

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

The first file to write is @file{bits/ctype_base.h}.  This file provides
some very basic information about character classification.  The libstdc++-v3
library assumes that your C library implements @file{<ctype.h>} by using
a table (indexed by character code) containing integers, where each of
these integers is a bit-mask indicating whether the charcter is
upper-case, lower-case, alphabetic, etc.  The @file{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 @file{<ctype.h>} to figure out
how to define the values required by this file.

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

@example
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
  @};
@};
@end example

@noindent
The @code{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 @code{__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 @code{toupper} and
@code{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 @file{<ctype.h>}.  They can
be given symbolically (as above), or numerically, if you prefer.  You do
not have to include @file{<ctype.h>} in this header; it will always be
included before @file{bits/ctype_base.h} is included.

The next file to write is @file{bits/ctype_noninline.h}, which also does
not require include guards.  This file defines a few member functions
that will be included in @file{include/bits/locale_facets.h}.  The first
function that must be written is the @code{ctype<char>::ctype}
constructor.  Here is the IRIX example:

@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) 
  @{ @}
@end example

@noindent
There are two parts of this that you might choose to alter. The first,
and most important, is the line involving @code{__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 @code{_M_toupper} and
@code{_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:

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

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

@noindent
Your C library provides equivalents to IRIX's @code{_toupper} and
@code{_tolower}.  If you initialized @code{_M_toupper} and
@code{_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:

@example
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;
@}
@end example

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

In detail, the functions provided test characters for particular
properties; they are analagous to the functions like @code{isalpha} and
@code{islower} provided by the C library.

The first function is implemented like this on IRIX:

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

@noindent
The @code{_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:

@example
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;
@}
@end example

@noindent
This function is similar; it copies the masks for all the characters
from @code{__low} up until @code{__high} into the vector given by
@code{__vec}.

The last two functions again are entirely generic:

@example
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;
@}
@end example

@c ---------------------------------------------------------------------
@c Thread safety
@c ---------------------------------------------------------------------

@node Thread safety
@chapter 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 @file{bits/atomicity.h}.  

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

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

The type is @code{_Atomic_word}.  Here is the version used on IRIX:

@example
typedef long _Atomic_word;
@end example

@noindent
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:

@example
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;
@}
@end example

@c ---------------------------------------------------------------------
@c Libtool
@c ---------------------------------------------------------------------

@node Libtool
@chapter 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
@code{--tags CXX} option (the C++ definitions for libtool).  Therefore,
@file{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,
@file{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 @code{gcc -shared}.

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

@c ---------------------------------------------------------------------
@c Epilogue
@c ---------------------------------------------------------------------

@contents
@bye