BRK(2)                    OpenBSD 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() 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 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
     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.  ``etext + rlp->rlim_max.'' (see end(3) for
     the definition of etext).

RETURN VALUES
     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()
     function returns a pointer to the base of the new storage if successful;
     otherwise -1 with errno set to indicate why the allocation failed.

ERRORS
     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
                   expansion.

SEE ALSO
     execve(2),  getrlimit(2),  mmap(2),  end(3),  malloc(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.

OpenBSD 2.6                    December 11, 1993                             1