sbrk(2) - NetBSD Manual Pages

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BRK(2)                    NetBSD System Calls Manual                    BRK(2)


NAME

     brk, sbrk -- change data segment size


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <unistd.h>

     int
     brk(void *addr);

     void *
     sbrk(intptr_t incr);


DESCRIPTION

     The brk and sbrk functions are legacy interfaces from before the advent
     of modern virtual memory management.

     The brk() and sbrk() functions are used to change the amount of memory
     allocated in a process's data segment.  They do this by moving the loca-
     tion of the ``break''.  The break is the first address after the end of
     the process's uninitialized data segment (also known as the ``BSS'').

     While the actual process data segment size maintained by the kernel will
     only grow or shrink in page sizes, these functions allow setting the
     break to unaligned values (i.e. it may point to any address inside the
     last page of the data segment).

     The brk() function sets the break to addr.

     The sbrk() function raises the break by at least incr bytes, thus allo-
     cating at least incr bytes of new memory in the data segment.  If incr is
     negative, the break is lowered by incr bytes.

     sbrk() returns the prior address of the break.  The current value of the
     program break may be determined by calling sbrk(0).  (See also end(3)).

     The getrlimit(2) system call may be used to determine the maximum permis-
     sible 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 defi-
     nition 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; other-
     wise ((void *)-1) is returned and errno is set to indicate why the allo-
     cation 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), end(3), free(3), malloc(3), sysconf(3)


HISTORY

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


BUGS

     Note that mixing brk() and sbrk() with malloc(3), free(3), and similar
     functions may result in non-portable program behavior.  Caution is
     advised.

     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).

NetBSD 4.0                       July 12, 1999                      NetBSD 4.0