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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 June 2, 2018 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. 54They are deprecated and not present on the arm64 or riscv architectures. 55The 56.Xr mmap 2 57interface should be used to allocate pages instead. 58.Ef 59.Pp 60The 61.Fn brk 62and 63.Fn sbrk 64functions are used to change the amount of memory allocated in a 65process's data segment. 66They do this by moving the location of the 67.Dq break . 68The break is the first address after the end of the process's 69uninitialized data segment (also known as the 70.Dq BSS ) . 71.Pp 72The 73.Fn brk 74function 75sets the break to 76.Fa addr . 77.Pp 78The 79.Fn sbrk 80function raises the break by 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.Sh NOTES 92While the actual process data segment size maintained by the kernel will only 93grow or shrink in page sizes, these functions allow setting the break 94to unaligned values (i.e., it may point to any address inside the last 95page of the data segment). 96.Pp 97The current value of the program break may be determined by calling 98.Fn sbrk 0 . 99See also 100.Xr end 3 . 101.Pp 102The 103.Xr getrlimit 2 104system call may be used to determine 105the maximum permissible size of the 106data segment. 107It will not be possible to set the break 108beyond 109.Dq Va etext No + Va rlim.rlim_max 110where the 111.Va rlim.rlim_max 112value is returned from a call to 113.Fn getrlimit RLIMIT_DATA &rlim . 114(See 115.Xr end 3 116for the definition of 117.Va etext ) . 118.Sh RETURN VALUES 119.Rv -std brk 120.Pp 121The 122.Fn sbrk 123function returns the prior break value if successful; 124otherwise the value 125.Po Vt "void *" Pc Ns \-1 126is returned and the global variable 127.Va errno 128is set to indicate the error. 129.Sh ERRORS 130The 131.Fn brk 132and 133.Fn sbrk 134functions 135will fail if: 136.Bl -tag -width Er 137.It Bq Er EINVAL 138The requested break value was beyond the beginning of the data segment. 139.It Bq Er ENOMEM 140The data segment size limit, as set by 141.Xr setrlimit 2 , 142was exceeded. 143.It Bq Er ENOMEM 144Insufficient space existed in the swap area 145to support the expansion of the data segment. 146.El 147.Sh SEE ALSO 148.Xr execve 2 , 149.Xr getrlimit 2 , 150.Xr mmap 2 , 151.Xr end 3 , 152.Xr free 3 , 153.Xr malloc 3 154.Sh HISTORY 155The 156.Fn brk 157function appeared in 158.At v7 . 159.Fx 11.0 160introduced the arm64 and riscv architectures which do not support 161.Fn brk 162or 163.Fn sbrk . 164.Sh BUGS 165Mixing 166.Fn brk 167or 168.Fn sbrk 169with 170.Xr malloc 3 , 171.Xr free 3 , 172or similar functions will result in non-portable program behavior. 173.Pp 174Setting the break may fail due to a temporary lack of 175swap space. 176It is not possible to distinguish this 177from a failure caused by exceeding the maximum size of 178the data segment without consulting 179.Xr getrlimit 2 . 180.Pp 181.Fn sbrk 182is sometimes used to monitor heap use by calling with an argument of 0. 183The result is unlikely to reflect actual utilization in combination with an 184.Xr mmap 2 185based malloc. 186.Pp 187.Fn brk 188and 189.Fn sbrk 190are not thread-safe. 191