man3/alloca.3

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.\"     @(#)alloca.3	5.1 (Berkeley) 5/2/91
.\"
.\" Converted Mon Nov 29 11:05:55 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Tue Oct 22 23:41:56 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2002-07-17, aeb
.\" 2008-01-24, mtk:
.\"     Various rewrites and additions (notes on longjmp() and SIGSEGV).
.\"     Weaken warning against use of alloca() (as per Debian bug 461100).
.\"
.TH ALLOCA 3 2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
alloca \- allocate memory that is automatically freed
.SH SYNOPSIS
.nf
.B #include <alloca.h>
.PP
.BI "void *alloca(size_t " size );
.fi
.SH DESCRIPTION
The
.BR alloca ()
function allocates
.I size
bytes of space in the stack frame of the caller.
This temporary space is
automatically freed when the function that called
.BR alloca ()
returns to its caller.
.SH RETURN VALUE
The
.BR alloca ()
function returns a pointer to the beginning of the allocated space.
If the allocation causes stack overflow, program behavior is undefined.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR alloca ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
This function is not in POSIX.1.
.PP
There is evidence that the
.BR 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.
.SH NOTES
The
.BR alloca ()
function is machine- and compiler-dependent.
For certain applications,
its use can improve efficiency compared to the use of
.BR malloc (3)
plus
.BR free (3).
In certain cases,
it can also simplify memory deallocation in applications that use
.BR longjmp (3)
or
.BR siglongjmp (3).
Otherwise, its use is discouraged.
.PP
Because the space allocated by
.BR alloca ()
is allocated within the stack frame,
that space is automatically freed if the function return
is jumped over by a call to
.BR longjmp (3)
or
.BR siglongjmp (3).
.PP
The space allocated by
.BR alloca ()
is
.I not
automatically deallocated if the pointer that refers to it
simply goes out of scope.
.PP
Do not attempt to
.BR free (3)
space allocated by
.BR alloca ()!
.SS Notes on the GNU version
Normally,
.BR gcc (1)
translates calls to
.BR alloca ()
with inlined code.
This is not done when either the
.IR "\-ansi" ,
.IR "\-std=c89" ,
.IR "\-std=c99" ,
or the
.IR "\-std=c11"
option is given
.BR and
the header
.I <alloca.h>
is not included.
Otherwise, (without an \-ansi or \-std=c* option) the glibc version of
.I <stdlib.h>
includes
.I <alloca.h>
and that contains the lines:
.PP
.in +4n
.EX
#ifdef  __GNUC__
#define alloca(size)   __builtin_alloca (size)
#endif
.EE
.in
.PP
with messy consequences if one has a private version of this function.
.PP
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.
.PP
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.
.SH 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
.B SIGSEGV
signal if it attempts to access the unallocated space.)
.PP
On many systems
.BR alloca ()
cannot be used inside the list of arguments of a function call, because
the stack space reserved by
.BR alloca ()
would appear on the stack in the middle of the space for the
function arguments.
.SH SEE ALSO
.BR brk (2),
.BR longjmp (3),
.BR malloc (3)