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.