BRK
Section: Linux Programmer's Manual (2)
Updated: 2016-03-15
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NAME
brk, sbrk - change data segment size
SYNOPSIS
#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 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
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.
RETURN VALUE
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.
CONFORMING TO
4.3BSD; SUSv1, marked LEGACY in SUSv2, removed in POSIX.1-2001.
NOTES
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.
SEE ALSO
execve(2),
getrlimit(2),
end(3),
malloc(3)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- CONFORMING TO
-
- NOTES
-
- C library/kernel differences
-
- SEE ALSO
-
- COLOPHON
-
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