Linux Programmer’s Manual
alloca − allocate memory that is automatically freed
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()!
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, the