1.\" $NetBSD: brk.2,v 1.27 2002/02/08 01:28:16 ross 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. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)brk.2 8.4 (Berkeley) 5/1/95 35.\" 36.Dd July 12, 1999 37.Dt BRK 2 38.Os 39.Sh NAME 40.Nm brk , 41.Nm sbrk 42.Nd change data segment size 43.Sh LIBRARY 44.Lb libc 45.Sh SYNOPSIS 46.Fd #include \*[Lt]unistd.h\*[Gt] 47.Ft int 48.Fn brk "void *addr" 49.Ft void * 50.Fn sbrk "intptr_t incr" 51.Sh DESCRIPTION 52.Bf -symbolic 53The brk and sbrk functions are legacy interfaces from before the 54advent of modern virtual memory management. 55.Ef 56.Pp 57The 58.Fn brk 59and 60.Fn sbrk 61functions are used to change the amount of memory allocated in a 62process's data segment. They do this by moving the location of the 63.Dq break . 64The break is the first address after the end of the process's 65uninitialized data segment (also known as the 66.Dq BSS ) . 67.Pp 68While the actual process data segment size maintained by the kernel will only 69grow or shrink in page sizes, these functions allow setting the break 70to unaligned values (i.e. it may point to any address inside the last 71page of the data segment). 72.Pp 73The 74.Fn brk 75function sets the break to 76.Fa addr . 77.Pp 78The 79.Fn sbrk 80function raises the break by at least 81.Fa incr 82bytes, thus allocating at least 83.Fa incr 84bytes of new memory in the data segment. 85If 86.Fa incr 87is negative, 88the break is lowered by 89.Fa incr 90bytes. 91.Pp 92.Fn sbrk 93returns the prior address of the break. 94The current value of the program break may be determined by calling 95.Fn sbrk 0 . 96(See also 97.Xr end 3 ) . 98.Pp 99The 100.Xr getrlimit 2 101system call may be used to determine 102the maximum permissible size of the 103.Em data 104segment; 105it will not be possible to set the break 106beyond the 107.Dv RLIMIT_DATA 108.Em rlim_max 109value returned from a call to 110.Xr getrlimit 2 , 111e.g. 112.Dq etext + rlim.rlim_max . 113(see 114.Xr end 3 115for the definition of 116.Em etext ) . 117.Sh RETURN VALUES 118.Fn brk 119returns 0 if successful; 120otherwise -1 with 121.Va errno 122set to indicate why the allocation failed. 123.Pp 124The 125.Fn sbrk 126function returns the prior break value if successful; 127otherwise ((void *)-1) is returned and 128.Va errno 129is set to indicate why the allocation failed. 130.Sh ERRORS 131.Fn brk 132or 133.Fn sbrk 134will fail and no additional memory will be allocated if 135one of the following are true: 136.Bl -tag -width Er 137.It Bq Er ENOMEM 138The limit, as set by 139.Xr setrlimit 2 , 140was exceeded. 141.It Bq Er ENOMEM 142The maximum possible size of a data segment (compiled into the 143system) was exceeded. 144.It Bq Er ENOMEM 145Insufficient space existed in the swap area 146to support the expansion. 147.El 148.Sh SEE ALSO 149.Xr execve 2 , 150.Xr getrlimit 2 , 151.Xr mmap 2 , 152.Xr end 3 , 153.Xr free 3 , 154.Xr malloc 3 , 155.Xr sysconf 3 156.Sh HISTORY 157A 158.Fn brk 159function call appeared in 160.At v7 . 161.Sh BUGS 162Note that 163mixing 164.Fn brk 165and 166.Fn sbrk 167with 168.Xr malloc 3 , 169.Xr free 3 , 170and similar functions may result in non-portable program 171behavior. Caution is advised. 172.Pp 173Setting the break may fail due to a temporary lack of swap space. 174It is not possible to distinguish this from a failure caused by 175exceeding the maximum size of the data segment without consulting 176.Xr getrlimit 2 . 177