BRK(2) BSD Programmer's Manual BRK(2)NAME
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. ``etext + rlp->rlim_max.'' (see end(3) for
the definition of etext).
Brk returns 0 if successful; otherwise -1 with errno set to indicate why
the allocation failed. Sbrk returns a pointer to the base of the new
storage if successful; otherwise -1 with errno set to indicate why the
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 ex-
SEE ALSOexecve(2), getrlimit(2), malloc(3), end(3)BUGS
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 May 1, 1995 1