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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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brk(2) System Calls Manual brk(2)
brk, sbrk - change data segment size
Standard C library (libc, -lc)
#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
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)).
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.
On success, brk() returns zero. On error, -1 is returned, and
errno 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 is set to ENOMEM.
None.
4.3BSD; SUSv1, marked LEGACY in SUSv2, removed in POSIX.1-2001.
Avoid using brk() and sbrk(): the malloc(3) 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, ssize_t, ptrdiff_t, intptr_t.
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.
execve(2), getrlimit(2), end(3), malloc(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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