alloca
—
memory allocator
Standard C Library (libc, -lc)
#include <stdlib.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 on return.
The alloca
() function returns a pointer to the beginning
of the allocated space. If the allocation failed, a
NULL
pointer is returned.
Few limitations can be mentioned:
- The
alloca
() function is not part of any C
standard and its use is not portable.
- The
alloca
() function should be supplied by the
compiler because the compiler is allowed to make assumptions about the
stack and frame pointers. The libc alloca
()
implementation cannot account for those assumptions. While there is a
machine dependent implementation of alloca
() in
libc, its use is discouraged and in most cases it will not work. Using
this implementation will produce linker warnings.
- The
alloca
() function is unsafe because it cannot
ensure that the pointer returned points to a valid and usable block of
memory. The allocation made may exceed the bounds of the stack, or even go
further into other objects in memory, and alloca
()
cannot determine such an error. For that all
alloca
() allocations should be bounded and limited
to a small size.
- Since
alloca
() modifies the stack at runtime and
the stack usage of each function frame cannot be predicted, it makes many
compiler security features (such as
cc(1)
-fstack-protector
) useless for the calling
function. See security(7)
for a discussion.