---
name: "brk"
section: "2"
platform: "macos"
package: "Libc"
version: "1752.120.2"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/brk"
---

# brk(2)

change data segment size

## NAME

**brk**, **sbrk** – change data segment size

## SYNOPSIS

**#include \<unistd.h\>**

*void* *\**\

**brk**(*const void \*addr*);

*void* *\**\

**sbrk**(*int incr*);

## DESCRIPTION

The brk and sbrk functions are historical curiosities left over from earlier days before the advent of virtual memory management. The **brk**() function sets the break or lowest address of a process's data segment (uninitialized data) to *addr* (immediately above bss). Data addressing is restricted between *addr* and the lowest stack pointer to the stack segment. Memory is allocated by *brk* in page size pieces; if *addr* is not evenly divisible by the system page size, it is increased to the next page boundary.

The current value of the program break is reliably returned by “sbrk(0)” (see also  [end(3)](/end/3.md)). The [getrlimit(2)](/getrlimit/2.md) system call may be used to determine the maximum permissible size of the *data* segment; it will not be possible to set the break beyond the *rlim\_max* value returned from a call to getrlimit, e.g. “qetext + rlp→rlim\_max.” (see [end(3)](/end/3.md) for the definition of *etext*).

## RETURN VALUES

**Brk** returns a pointer to the new end of memory if successful; otherwise -1 with *errno* set to indicate why the allocation failed. The **sbrk** function returns a pointer to the base of the new storage if successful; otherwise -1 with *errno* set to indicate why the allocation failed.

## ERRORS

Sbrk will fail and no additional memory will be allocated if one of the following are true:

- \[ENOMEM\]

  The limit, as set by [setrlimit(2)](/setrlimit/2.md), was exceeded.

- \[ENOMEM\]

  The maximum possible size of a data segment (compiled into the system) was exceeded.

- \[ENOMEM\]

  Insufficient space existed in the swap area to support the expansion.

## SEE ALSO

[execve(2)](/execve/2.md), [getrlimit(2)](/getrlimit/2.md), [malloc(3)](/malloc/3.md), [mmap(2)](/mmap/2.md), [end(3)](/end/3.md)

## BUGS

Setting the break may fail due to a temporary lack of swap space.  It is not possible to distinguish this from a failure caused by exceeding the maximum size of the data segment without consulting  getrlimit.

## HISTORY

A **brk**() function call appeared in  Version 7 AT&T UNIX.