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.
RETURN VALUE
The
alloca()
function returns a pointer to the beginning of the allocated space.
If the allocation causes stack overflow, program behavior is undefined.
CONFORMING TO
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.
NOTES
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()!
Notes on the GNU Version
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.
BUGS
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.
This page is part of release 3.14 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/.
