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15.2.1 Cross-Unix Function Calls
Functions not mentioned in POSIX.1 may not be available on all systems.
If you want to use one of these functions, you should normally check for
its presence by using `AC_CHECK_FUNCS' in your `configure.in'
script, and adapt to its absence if possible. Here is a list of some
popular functions which are available on many, but not all, modern Unix
systems:
alloca
- There are several portability issues with
alloca . See the
description of AC_FUNC_ALLOCA in the autoconf manual. Although
this function can be very convenient, it is normally best to avoid it in
highly portable code.
dlopen
- GNU libtool provides a portable alternate interface to
dlopen . See section 17. Dynamic Loading.
getline
- In some cases
fgets may be used as a fallback. In others, you
will need to provide your own version of this function.
getpagesize
- On some systems, the page size is available as the macro
PAGE_SIZE in the header file `sys/param.h'. On others, the
page size is available via the sysconf function. If none of
those work, you must generally simply guess a value such as
4096 .
gettimeofday
- When this is not available, fall back to a less precise function such as
time or ftime (which itself is not available on all
systems).
mmap
- In some cases you can use either
mmap or ordinary file I/O. In
others, a program which uses mmap will simply not be portable to
all Unix systems. Note that mmap is an optional part of the 1996
version of POSIX.1, so it is likely to be added to all Unix systems over
time.
ptrace
- Unix systems without
ptrace generally provide some other
mechanism for debugging subprocesses, such as `/proc'. However,
there is no widely portable method for controlling subprocesses, as
evidenced by the source code to the GNU debugger, gdb .
setuid
- Different Unix systems handle this differently. On some systems, any
program can switch between the effective user ID of the executable and
the real user ID. On others, switching to the real user ID is final;
some of those systems provide the
setreuid function instead to
switch the effective and real user ID. The effect when a program run by
the superuser calls setuid varies among systems.
snprintf
- If this is not available, then in some cases it will be reasonable to
simply use
sprintf , and in others you will need to write a little
routine to estimate the required length and allocate an appropriate
buffer before calling sprintf .
strcasecmp
strdup
strncasecmp
- You can normally provide your own version of these simple functions.
valloc
- When this is not available, just use
malloc instead.
vfork
- When this is not available, just use
fork instead.
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