1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)brk.2 8.4 (Berkeley) 5/1/95 29.\" $FreeBSD$ 30.\" 31.Dd December 15, 2015 32.Dt BRK 2 33.Os 34.Sh NAME 35.Nm brk , 36.Nm sbrk 37.Nd change data segment size 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In unistd.h 42.Ft int 43.Fn brk "const void *addr" 44.Ft void * 45.Fn sbrk "intptr_t incr" 46.Sh DESCRIPTION 47.Bf -symbolic 48The 49.Fn brk 50and 51.Fn sbrk 52functions are legacy interfaces from before the 53advent of modern virtual memory management. 54.Ef 55.Pp 56The 57.Fn brk 58and 59.Fn sbrk 60functions are used to change the amount of memory allocated in a 61process's data segment. 62They 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 68The 69.Fn brk 70function 71sets the break to 72.Fa addr . 73.Pp 74The 75.Fn sbrk 76function raises the break by 77.Fa incr 78bytes, thus allocating at least 79.Fa incr 80bytes of new memory in the data segment. 81If 82.Fa incr 83is negative, 84the break is lowered by 85.Fa incr 86bytes. 87.Sh NOTES 88While the actual process data segment size maintained by the kernel will only 89grow or shrink in page sizes, these functions allow setting the break 90to unaligned values (i.e., it may point to any address inside the last 91page of the data segment). 92.Pp 93The current value of the program break may be determined by calling 94.Fn sbrk 0 . 95See also 96.Xr end 3 . 97.Pp 98The 99.Xr getrlimit 2 100system call may be used to determine 101the maximum permissible size of the 102data segment. 103It will not be possible to set the break 104beyond 105.Dq Va etext No + Va rlim.rlim_max 106where the 107.Va rlim.rlim_max 108value is returned from a call to 109.Fn getrlimit RLIMIT_DATA &rlim . 110(See 111.Xr end 3 112for the definition of 113.Va etext ) . 114.Sh RETURN VALUES 115.Rv -std brk 116.Pp 117The 118.Fn sbrk 119function returns the prior break value if successful; 120otherwise the value 121.Po Vt "void *" Pc Ns \-1 122is returned and the global variable 123.Va errno 124is set to indicate the error. 125.Sh ERRORS 126The 127.Fn brk 128and 129.Fn sbrk 130functions 131will fail if: 132.Bl -tag -width Er 133.It Bq Er EINVAL 134The requested break value was beyond the beginning of the data segment. 135.It Bq Er ENOMEM 136The data segment size limit, as set by 137.Xr setrlimit 2 , 138was exceeded. 139.It Bq Er ENOMEM 140Insufficient space existed in the swap area 141to support the expansion of the data segment. 142.El 143.Sh SEE ALSO 144.Xr execve 2 , 145.Xr getrlimit 2 , 146.Xr mmap 2 , 147.Xr end 3 , 148.Xr free 3 , 149.Xr malloc 3 150.Sh HISTORY 151The 152.Fn brk 153function appeared in 154.At v7 . 155.Sh BUGS 156Mixing 157.Fn brk 158or 159.Fn sbrk 160with 161.Xr malloc 3 , 162.Xr free 3 , 163or similar functions will result in non-portable program behavior. 164.Pp 165Setting the break may fail due to a temporary lack of 166swap space. 167It is not possible to distinguish this 168from a failure caused by exceeding the maximum size of 169the data segment without consulting 170.Xr getrlimit 2 . 171