BRK(2) BSD Programmer's Manual BRK(2)NAME
brk, sbrk - change data segment size
SYNOPSIS
#include <unistd.h>
char *
brk(const char *addr);
char *
sbrk(int incr);
DESCRIPTION
The brk() function sets the break or lowest address of a process's data
segment (uninitialized data) to addr (immediately above bss). Data ad-
dressing 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 sbrk() function adjusts the break (up or down) by the amount speci-
fied by incr and returns the previous value of the break.
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).
Application program's should generally not use these interfaces, they
should use malloc(3) instead.
RETURN VALUES
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
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 ex-
pansion.
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.
HISTORY
A brk function call appeared in Version 7 AT&T UNIX.
4th Berkeley Distribution May 1, 1995 1
