sbrk(2) - NetBSD Manual Pages

Command:
Section:
Arch:
Collection:
>>>
BRK(2)                    NetBSD 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 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 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 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 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.


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),  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      December 11, 1993                             1