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ALLOCA(3) Linux Programmer's Manual ALLOCA(3)
alloca - allocate memory that is automatically freed
#include <alloca.h>
void *alloca(size_t size);
The alloca() function allocates size bytes of space in the stack frame of the
caller. This temporary space is automatically freed when the function that
called alloca() returns to its caller.
The alloca() function returns a pointer to the beginning of the allocated
space. If the allocation causes stack overflow, program behavior is
undefined.
This function is not in POSIX.1-2001.
There is evidence that the 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.
The alloca() function is machine- and compiler-dependent. For certain
applications, its use can improve efficiency compared to the use of malloc(3)
plus free(3). In certain cases, it can also simplify memory deallocation in
applications that use longjmp(3) or siglongjmp(3). Otherwise, its use is
discouraged.
Because the space allocated by alloca() is allocated within the stack frame,
that space is automatically freed if the function return is jumped over by a
call to longjmp(3) or siglongjmp(3).
Do not attempt to free(3) space allocated by alloca()!
Normally, gcc(1) translates calls to alloca() with inlined code. This is not
done when either the -ansi, -std=c89, -std=c99, or the -fno-builtin option is
given (and the header <alloca.h> is not included). But beware! By default
the glibc version of <stdlib.h> includes <alloca.h> and that contains the
line:
#define alloca(size) __builtin_alloca (size)
with messy consequences if one has a private version of this function.
The fact that the code is inlined means that it is impossible to take the
address of this function, or to change its behavior by linking with a
different library.
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.
There is no error indication if the stack frame cannot be extended. (However,
after a failed allocation, the program is likely to receive a SIGSEGV signal
if it attempts to access the unallocated space.)
On many systems alloca() cannot be used inside the list of arguments of a
function call, because the stack space reserved by alloca() would appear on
the stack in the middle of the space for the function arguments.
brk(2), longjmp(3), malloc(3)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2008-01-24 ALLOCA(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface