xref: /netbsd-src/lib/libc/stdlib/alloca.3 (revision 76b8df0e8c557dc0500196e809acfe7294f78b6d)

.\"	$NetBSD: alloca.3,v 1.16 2012/10/24 22:56:27 wiz Exp $
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.\"     from: @(#)alloca.3	8.1 (Berkeley) 6/4/93
.\"
.Dd October 24, 2012
.Dt ALLOCA 3
.Os
.Sh NAME
.Nm alloca
.Nd memory allocator
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft void *
.Fn alloca "size_t size"
.Sh DESCRIPTION
The
.Fn alloca
function
allocates
.Fa size
bytes of space in the stack frame of the caller.
This temporary space is automatically freed on
return.
.Sh RETURN VALUES
The
.Fn alloca
function returns a pointer to the beginning of the allocated space.
If the allocation failed, a
.Dv NULL
pointer is returned.
.Sh SEE ALSO
.Xr cc 1 ,
.Xr brk 2 ,
.Xr calloc 3 ,
.Xr getpagesize 3 ,
.Xr malloc 3 ,
.Xr realloc 3 ,
.Xr security 7
.Sh CAVEATS
Few limitations can be mentioned:
.Bl -bullet
.It
The
.Fn alloca
function is not part of any C standard and its use is not portable.
.It
The
.Fn alloca
function should be supplied by the compiler because the compiler is allowed to
make assumptions about the stack and frame pointers.
The libc
.Fn alloca
implementation cannot account for those assumptions.
While there is a
machine dependent implementation of
.Fn alloca
in libc, its use is discouraged and in most cases it will not work.
Using this implementation will produce linker warnings.
.It
The
.Fn alloca
function is unsafe because it cannot ensure that the pointer
returned points to a valid and usable block of memory.
The allocation made may exceed the bounds of the stack, or even go
further into other objects in memory, and
.Fn alloca
cannot determine such an error.
For that all
.Fn alloca
allocations should be bounded and limited to a small size.
.It
Since
.Fn alloca
modifies the stack at runtime and the stack usage of each function frame
cannot be predicted, it makes many compiler security features
(such as
.Xr cc 1
.Fl fstack-protector )
useless for the calling function.
See
.Xr security 7
for a discussion.
.El
.\" .Sh HISTORY
.\" The
.\" .Fn alloca
.\" function appeared in
.\" .Bx ?? .
.\" The function appeared in 32v, pwb and pwb.2 and in 3bsd 4bsd
.\" The first man page (or link to a man page that I can find at the
.\" moment is 4.3...