alloca - allocate memory that is automatically freed
Standard C library (
libc,
-lc)
#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.
alloca() originates from PWB and 32V, and appears in all their
derivatives.
The
alloca() function is machine- and compiler-dependent. Because it
allocates from the stack, it's faster than
malloc(3) and
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()!
By necessity,
alloca() is a compiler built-in, also known as
__builtin_alloca(). By default, modern compilers automatically
translate all uses of
alloca() into the built-in, but this is forbidden
if standards conformance is requested (
-ansi,
-std=c*), in which
case
<alloca.h> is required, lest a symbol dependency be emitted.
The fact that
alloca() is a built-in means it is impossible to take its
address or to change its behavior by linking with a different library.
Variable length arrays (VLAs) are part of the C99 standard, optional since C11,
and can be used for a similar purpose. However, they do not port to standard
C++, and, being variables, live in their block scope and don't have an
allocator-like interface, making them unfit for implementing functionality
like
strdupa(3).
Due to the nature of the stack, it is impossible to check if the allocation
would overflow the space available, and, hence, neither is indicating an
error. (However, the program is likely to receive a
SIGSEGV signal if
it attempts to access unavailable 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)