BRK(2) BSD System Calls Manual BRK(2)
brk, sbrk -- change data segment size
brk(const char *addr);
The brk and sbrk functions are historical curiosities left over from ear-
lier 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 seg-
ment. 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, e.g. ``qetext + rlp->rlim_max.'' (see end(3) for
the definition of etext).
Brk returns a pointer to the new end of memory if successful; otherwise
-1 with errno set to indicate why the allocation failed. The sbrk func-
tion returns a pointer to the base of the new storage if successful; oth-
erwise -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:
[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), malloc(3), mmap(2), end(3)
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.
A brk() function call appeared in Version 7 AT&T UNIX.
4th Berkeley Distribution December 11, 1993 4th Berkeley Distribution