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: src/lib/libc/sys/brk.2,v 1.13.2.10 2002/03/04 12:00:31 dwmalone Exp $ 30.\" 31.Dd March 1, 2019 32.Dt SBRK 2 33.Os 34.Sh NAME 35.Nm sbrk 36.Nd change data segment size (obsolete) 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In sys/types.h 41.In unistd.h 42.Ft void * 43.Fn sbrk "intptr_t incr" 44.Sh DESCRIPTION 45.Bf -symbolic 46The 47.Fn sbrk 48function is a legacy interface from before the 49advent of modern virtual memory management. 50.Fn sbrk 51has only limited functionality due to having to play nice with 52modern system calls such as 53.Xr mmap 2 . 54.Ef 55.Pp 56The 57.Fn sbrk 58function is used to change the amount of memory allocated in a 59process's data segment. 60It does this by moving the location of the 61.Dq break . 62The break is the first address after the end of the process's 63uninitialized data segment (also known as the 64.Dq BSS ) . 65.Pp 66The break range is limited by the 67.Dv RLIMIT_DATA 68resource limit applied to the process. 69.Pp 70The 71.Fn sbrk 72function raises the break by 73.Fa incr 74bytes, returning a pointer to the base of the new memory. 75Any attempt to lower the break point will return 76.Po Vt "void *" Pc Ns \-1 77and set errno to 78.Er EOPNOTSUPP . 79.Sh NOTES 80While the actual process data segment size maintained by the kernel will only 81grow or shrink in page sizes, this function allows setting the break 82to unaligned values (i.e., it may point to any address inside the last 83page of the data segment). 84.Pp 85The current value of the program break may be determined by calling 86.Fn sbrk 0 . 87.Pp 88The 89.Fn sbrk 90function is thread-safe. 91See also 92.Xr end 3 . 93.Pp 94The 95.Xr getrlimit 2 96system call may be used to determine 97the maximum permissible size of the 98data segment. 99It will not be possible to set the break 100beyond 101.Dq Va etext No + Va rlim.rlim_max 102where the 103.Va rlim.rlim_max 104value is returned from a call to 105.Fn getrlimit RLIMIT_DATA &rlim . 106(See 107.Xr end 3 108for the definition of 109.Va etext ) . 110.Sh RETURN VALUES 111The 112.Fn sbrk 113function returns the prior break pointer if successful; 114otherwise the value 115.Po Vt "void *" Pc Ns \-1 116is returned and the global variable 117.Va errno 118is set to indicate the error. 119.Sh ERRORS 120.Fn sbrk 121will fail if: 122.Bl -tag -width Er 123.It Bq Er EINVAL 124The requested break value was beyond the beginning of the data segment. 125.It Bq Er ENOMEM 126The data segment size limit, as set by 127.Xr setrlimit 2 , 128was exceeded. 129.It Bq Er ENOMEM 130Insufficient space existed in the swap area 131to support the expansion of the data segment. 132.It Bq Er EOPNOTSUPP 133An attempt has been made to perform an action that is no longer supported 134by this function. 135.El 136.Sh SEE ALSO 137.Xr execve 2 , 138.Xr getrlimit 2 , 139.Xr mmap 2 , 140.Xr end 3 , 141.Xr free 3 , 142.Xr malloc 3 143.Sh BUGS 144Mixing 145.Fn sbrk 146with 147.Xr malloc 3 , 148.Xr free 3 , 149or similar functions will result in non-portable program behavior. 150.Pp 151Setting the break may fail due to a temporary lack of 152swap space. 153It is not possible to distinguish this 154from a failure caused by exceeding the maximum size of 155the data segment without consulting 156.Xr getrlimit 2 . 157