BRK(2)                        System Calls Manual                       BRK(2)


     brk, sbrk - change data segment size


     #include <unistd.h>

     brk(void *addr);

     void *
     sbrk(int incr);


     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
     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(2), e.g., `etext + rlp->rlim_max' (see end(3) for the
     definition of etext).


     The brk() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the

     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


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


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


     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).

OpenBSD 6.4                    October 11, 2015                    OpenBSD 6.4

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