MirBSD manpage: brk(2), sbrk(2)

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


     brk, sbrk - change data segment size


     #include <unistd.h>

     brk(void *addr);

     void *
     sbrk(intptr_t 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 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(2), e.g., "etext + rlp->rlim_max" (see end(3) for the defini-
     tion 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


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


     The brk() and sbrk() function calls disappeared in Version 3 of the
     Single UNIX Specification ("SUSv3"), and this implementation honours
     Version 2 of the Single UNIX Specification ("SUSv2").


     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).

MirBSD #10-current            December 11, 1993                              1

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