brk, sbrk - change data segment size
#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
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.
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.
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)