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)):
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
.
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.