\" $NetBSD: alloca.3,v 1.16 2012/10/24 22:56:27 wiz Exp $

\" $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...