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brk(2)                      BSD System Calls Manual                     brk(2)


     brk, sbrk -- change data segment size


     #include <unistd.h>

     void *
     brk(const void *addr);

     void *
     sbrk(int incr);


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


     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.


     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.


     execve(2), getrlimit(2), malloc(3), mmap(2), end(3)


     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.


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

4th Berkeley Distribution      December 11, 1993     4th Berkeley Distribution

Mac OS X 10.8 - Generated Sat Aug 25 05:51:19 CDT 2012
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