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.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| alloca () |
Thread safety |
MT-Safe |
This function is not in POSIX.1.
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).
The space allocated by
alloca() is
not automatically deallocated
if the pointer that refers to it simply goes out of scope.
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
-std=c11 option is given
and the header
<alloca.h> is not included. Otherwise, (without an -ansi or
-std=c* option) the glibc version of
<stdlib.h> includes
<alloca.h> and that contains the lines:
#ifdef __GNUC__
#define alloca(size) __builtin_alloca (size)
#endif
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)