BRK(2)

NAME
brk, sbrk - change data segment size

SYNOPSIS
#include <sys/types.h>
char
*brk(const char *addr);
char *
*sbrk(int incr);

DESCRIPTION
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 (unilitialized 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, e.g. “qetext + rlp→rlim_max.” (see end(3) for the definition of etext).

RETURN VALUES
Brk returns 0 if successful; -1 if the process requests more memory than than allowed by the system limit. The sbrk function returns 0 if successful, otherwise the error EOPNOTSUPP is returned.

SEE ALSO
execve(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.