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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
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ALLOCA(3) Linux Programmer's Manual ALLOCA(3)
alloca - allocate memory that is automatically freed
#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.
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).
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.
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.
brk(2), longjmp(3), malloc(3)
This page is part of release 5.13 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 2021-03-22 ALLOCA(3)
Pages that refer to this page: malloc(3), strdup(3)
Copyright and license for this manual page
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