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alloca(3)
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 allo‐
cated space. If the allocation causes stack overflow, program behavior
is undefined.
ATTRIBUTES
For an explanation of the terms used in this section, see attrib‐
utes(7). allbox; lb lb lb l l l. Interface Attribute Value T{ al‐
loca() T} 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 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.
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 ap‐
pear on the stack in the middle of the space for the function argu‐
ments.
SEE ALSO
brk(2), longjmp(3), malloc(3)
COLOPHON
This page is part of release 5.02 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2019-03-06 ALLOCA(3)