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BRK(2) System Calls Manual BRK(2)

brk, sbrk —
change data segment size

#include <unistd.h>

int
brk(void *addr);

void *
sbrk(int incr);

The brk() and sbrk() functions are historical curiosities left over from earlier days before the advent of virtual memory management.
The brk() function sets the break or lowest address of a process's data segment (uninitialized data) to addr (immediately above bss). Data addressing is restricted between addr and the lowest stack pointer to the stack segment. Memory is allocated by brk() in page size pieces; if addr is not evenly divisible by the system page size, it is increased to the next page boundary.

The current value of the program break is reliably returned by “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 rlim_max value returned from a call to getrlimit(2), e.g., ‘etext + rlp->rlim_max’ (see end(3) for the definition of etext).

The brk() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The sbrk() function returns a pointer to the base of the new storage if successful; otherwise -1 with errno set to indicate why the allocation failed.

sbrk() will fail and no additional memory will be allocated if one of the following are true:
[]
The limit, as set by setrlimit(2), was exceeded.
[]
The maximum possible size of a data segment (compiled into the system) was exceeded.
[]
Insufficient space existed in the swap area to support the expansion.

execve(2), getrlimit(2), mmap(2), end(3), malloc(3)

A brk() function call appeared in Version 7 AT&T UNIX.

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).
October 11, 2015 OpenBSD-current