sbrk(2) - NetBSD Manual Pages

BRK(2)                    NetBSD Programmer's Manual                    BRK(2)


NAME

     brk, sbrk - change data segment size


SYNOPSIS

     #include <unistd.h>

     int
     brk(void *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 RLIMIT_DATA rlim_max value returned from a
     call to getrlimit(2), e.g.  ``etext + rlim.rlim_max''.  (see end(3) for
     the definition of etext).


RETURN VALUES

     brk() returns 0 if successful; otherwise -1 with errno set to indicate
     why the allocation failed.  The sbrk() function returns the prior break
     value if successful; otherwise ((void *)-1) is returned and errno is 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
                   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(2).


HISTORY

     A brk() function call appeared in Version 7 AT&T UNIX.

NetBSD 1.4                        May 1, 1995                                1