BRK(2) System Calls Manual BRK(2)
brk, sbrk - change data segment size
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
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
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
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
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).
OpenBSD 6.4 October 11, 2015 OpenBSD 6.4
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