brk(2) - Linux manual page (original) (raw)

brk(2) System Calls Manual brk(2)

NAME top

   brk, sbrk - change data segment size

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <unistd.h>**

   **int brk(void ***_addr_**);**
   **void *sbrk(intptr_t** _increment_**);**

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   **brk**(), **sbrk**():
       Since glibc 2.19:
           _DEFAULT_SOURCE
               || ((_XOPEN_SOURCE >= 500) &&
                   ! (_POSIX_C_SOURCE >= 200112L))
       From glibc 2.12 to glibc 2.19:
           _BSD_SOURCE || _SVID_SOURCE
               || ((_XOPEN_SOURCE >= 500) &&
                   ! (_POSIX_C_SOURCE >= 200112L))
       Before glibc 2.12:
           _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500

DESCRIPTION top

   **brk**() and **sbrk**() change the location of the _program break_, which
   defines the end of the process's data segment (i.e., the program
   break is the first location after the end of the uninitialized
   data segment).  Increasing the program break has the effect of
   allocating memory to the process; decreasing the break deallocates
   memory.

   **brk**() sets the end of the data segment to the value specified by
   _addr_, when that value is reasonable, the system has enough memory,
   and the process does not exceed its maximum data size (see
   [setrlimit(2)](../man2/setrlimit.2.html)).

   **sbrk**() increments the program's data space by _increment_ bytes.
   Calling **sbrk**() with an _increment_ of 0 can be used to find the
   current location of the program break.

RETURN VALUE top

   On success, **brk**() returns zero.  On error, -1 is returned, and
   _[errno](../man3/errno.3.html)_ is set to **ENOMEM**.

   On success, **sbrk**() returns the previous program break.  (If the
   break was increased, then this value is a pointer to the start of
   the newly allocated memory).  On error, _(void *) -1_ is returned,
   and _[errno](../man3/errno.3.html)_ is set to **ENOMEM**.

STANDARDS top

   None.

HISTORY top

   4.3BSD; SUSv1, marked LEGACY in SUSv2, removed in POSIX.1-2001.

NOTES top

   Avoid using **brk**() and **sbrk**(): the [malloc(3)](../man3/malloc.3.html) memory allocation
   package is the portable and comfortable way of allocating memory.

   Various systems use various types for the argument of **sbrk**().
   Common are _int_, _ssizet_, _ptrdifft_, _intptrt_.

C library/kernel differences The return value described above for brk() is the behavior provided by the glibc wrapper function for the Linux brk() system call. (On most other implementations, the return value from brk() is the same; this return value was also specified in SUSv2.) However, the actual Linux system call returns the new program break on success. On failure, the system call returns the current break. The glibc wrapper function does some work (i.e., checks whether the new break is less than addr) to provide the 0 and -1 return values described above.

   On Linux, **sbrk**() is implemented as a library function that uses
   the **brk**() system call, and does some internal bookkeeping so that
   it can return the old break value.

SEE ALSO top

   [execve(2)](../man2/execve.2.html), [getrlimit(2)](../man2/getrlimit.2.html), [end(3)](../man3/end.3.html), [malloc(3)](../man3/malloc.3.html)

COLOPHON top

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Linux man-pages 6.10 2024-07-23 brk(2)

Pages that refer to this page:getrlimit(2), mlock(2), mmap(2), mremap(2), PR_SET_MM_BRK(2const), PR_SET_MM_START_BRK(2const), PR_SET_TAGGED_ADDR_CTRL(2const), shmop(2), syscalls(2), alloca(3), end(3), malloc(3), malloc_hook(3), malloc_trim(3), mallopt(3), posix_memalign(3), proc_pid_stat(5), cpuset(7)