man-pages/man3/alloca.3

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.\"     @(#)alloca.3	5.1 (Berkeley) 5/2/91
.\"
.\" Converted Mon Nov 29 11:05:55 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Tue Oct 22 23:41:56 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2002-07-17, aeb
.\"
.TH ALLOCA 3  2002-07-17 "GNU" "Linux Programmer's Manual"
.SH NAME
alloca \- memory allocator
.SH SYNOPSIS
.B #include <alloca.h>
.sp
.BI "void *alloca(size_t " size );
.SH DESCRIPTION
The
.B alloca
function allocates
.I size
bytes of space in the stack frame of the caller.  This temporary space is
automatically freed when the function that called
.B alloca
returns to its caller.
.SH "RETURN VALUE"
The
.B alloca
function returns a pointer to the beginning of the allocated space.
If the allocation causes stack overflow, program behaviour is undefined.
.SH "CONFORMING TO"
There is evidence that the
.B alloca
function appeared in 32v, pwb, pwb.2, 3bsd, and 4bsd.  There is a man page
for it in 4.3BSD.  Linux uses the GNU version.
This function is not in POSIX or SUSv3.
.SH "NOTES ON THE GNU VERSION"
Normally,
.B gcc
translates calls to
.B alloca
by inlined code. This is not done when either the \-ansi or
the \-fno\-builtin option is given. But beware! By default
the glibc version of
.I <stdlib.h>
includes
.I <alloca.h>
and that contains the line
.RS
# define alloca(size)   __builtin_alloca (size)
.RE
with messy consequences if one has a private version of this function.
.LP
The fact that the code is inlined, means that it is impossible
to take the address of this function, or to change its behaviour
by linking with a different library.
.LP
The inlined code often consists of a single instruction adjusting
the stack pointer, and does not check for stack overflow.
Thus, there is no NULL error return.
.SH BUGS
The
.B alloca
function is machine and compiler dependent. On many systems
its implementation is buggy. Its use is discouraged.
.LP
On many systems
.B alloca
cannot be used inside the list of arguments of a function call, because
the stack space reserved by
.B alloca
would appear on the stack in the middle of the space for the
function arguments.
.SH "SEE ALSO"
.BR brk (2),
.BR pagesize (2),
.BR calloc (3),
.BR malloc (3),
.BR realloc (3)