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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
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 of this license document, but changing it is not allowed.
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