BRK(2)

NAME
brk, sbrk - change data segment size

LIBRARY
libc

SYNOPSIS
#include <unistd.h>
int
brk(void *addr);
void *
sbrk(intptr_t incr);

DESCRIPTION
The brk and sbrk functions are legacy interfaces from before the advent of modern virtual memory management.

The brk() and sbrk() functions are used to change the amount of memory allocated in a process's data segment. They do this by moving the location of the “break”. The break is the first address after the end of the process's uninitialized data segment (also known as the “BSS”).

While the actual process data segment size maintained by the kernel will only grow or shrink in page sizes, these functions allow setting the break to unaligned values (i.e. it may point to any address inside the last page of the data segment).

The brk() function sets the break to addr.

The sbrk() function raises the break by at least incr bytes, thus allocating at least incr bytes of new memory in the data segment. If incr is negative, the break is lowered by incr bytes.

sbrk() returns the prior address of the break. The current value of the program break may be determined by calling sbrk(0). (See also end(3)).

The getrlimit(2) system call may be used to determine the maximum permissible size of the data segment; it will not be possible to set the break beyond the RLIMIT_DATA rlim_max value returned from a call to getrlimit(2), e.g. “etext + rlim.rlim_max”. (see end(3) for the definition of etext).

RETURN VALUES
brk() returns 0 if successful; otherwise -1 with errno set to indicate why the allocation failed.

The sbrk() function returns the prior break value if successful; otherwise ((void *)-1) is returned and errno is set to indicate why the allocation failed.

ERRORS
brk() or sbrk() will fail and no additional memory will be allocated if one of the following are true:

[ENOMEM]
    The limit, as set by setrlimit(2), was exceeded.

[ENOMEM]
    The maximum possible size of a data segment (compiled into the system) was exceeded.

[ENOMEM]
    Insufficient space existed in the swap area to support the expansion.

SEE ALSO
execve(2), getrlimit(2), mmap(2), end(3), free(3), malloc(3), sysconf(3)

BUGS
Note that mixing brk() and sbrk() with malloc(3), free(3), and similar functions may result in non-portable program behavior. Caution is advised.

Setting the break may fail due to a temporary lack of swap space. It is not possible to distinguish this from a failure caused by exceeding the maximum size of the data segment without consulting getrlimit(2).

HISTORY
A brk() function call appeared in AT&T UNIX.