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ALLOCA(3) Linux Programmer's Manual ALLOCA(3)
NAME
alloca - allocate memory that is automatically freed
SYNOPSIS
#include <alloca.h>
void *alloca(size_t size);
DESCRIPTION
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.
RETURN VALUE
The alloca() function returns a pointer to the beginning of the allocated space. If the allocation causes stack overflow, program behavior is undefined.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
│alloca() │ Thread safety │ MT-Safe │
└──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘
CONFORMING TO
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.
NOTES
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()!
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 -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 li‐
brary.
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.
BUGS
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.
SEE ALSO
brk(2), longjmp(3), malloc(3)
GNU 2021-03-22 ALLOCA(3)