1.\" $NetBSD: alloca.3,v 1.16 2012/10/24 22:56:27 wiz Exp $ 2.\" 3.\" Copyright (c) 1980, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" from: @(#)alloca.3 8.1 (Berkeley) 6/4/93 31.\" 32.Dd October 24, 2012 33.Dt ALLOCA 3 34.Os 35.Sh NAME 36.Nm alloca 37.Nd memory allocator 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In stdlib.h 42.Ft void * 43.Fn alloca "size_t size" 44.Sh DESCRIPTION 45The 46.Fn alloca 47function 48allocates 49.Fa size 50bytes of space in the stack frame of the caller. 51This temporary space is automatically freed on 52return. 53.Sh RETURN VALUES 54The 55.Fn alloca 56function returns a pointer to the beginning of the allocated space. 57If the allocation failed, a 58.Dv NULL 59pointer is returned. 60.Sh SEE ALSO 61.Xr cc 1 , 62.Xr brk 2 , 63.Xr calloc 3 , 64.Xr getpagesize 3 , 65.Xr malloc 3 , 66.Xr realloc 3 , 67.Xr security 7 68.Sh CAVEATS 69Few limitations can be mentioned: 70.Bl -bullet 71.It 72The 73.Fn alloca 74function is not part of any C standard and its use is not portable. 75.It 76The 77.Fn alloca 78function should be supplied by the compiler because the compiler is allowed to 79make assumptions about the stack and frame pointers. 80The libc 81.Fn alloca 82implementation cannot account for those assumptions. 83While there is a 84machine dependent implementation of 85.Fn alloca 86in libc, its use is discouraged and in most cases it will not work. 87Using this implementation will produce linker warnings. 88.It 89The 90.Fn alloca 91function is unsafe because it cannot ensure that the pointer 92returned points to a valid and usable block of memory. 93The allocation made may exceed the bounds of the stack, or even go 94further into other objects in memory, and 95.Fn alloca 96cannot determine such an error. 97For that all 98.Fn alloca 99allocations should be bounded and limited to a small size. 100.It 101Since 102.Fn alloca 103modifies the stack at runtime and the stack usage of each function frame 104cannot be predicted, it makes many compiler security features 105(such as 106.Xr cc 1 107.Fl fstack-protector ) 108useless for the calling function. 109See 110.Xr security 7 111for a discussion. 112.El 113.\" .Sh HISTORY 114.\" The 115.\" .Fn alloca 116.\" function appeared in 117.\" .Bx ?? . 118.\" The function appeared in 32v, pwb and pwb.2 and in 3bsd 4bsd 119.\" The first man page (or link to a man page that I can find at the 120.\" moment is 4.3... 121