BRK(2) MachTen Programmer’s Manual BRK(2)

NAME
brk, sbrk - change data segment size

SYNOPSIS
#include <sys/types.h>

char
*brk(const char *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 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).

RETURN VALUES
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 re-
turns a pointer to the base of the new storage if successful; otherwise
-1 with errno set to indicate why the allocation failed.

ERRORS
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 ex-
pansion.

SEE ALSO
execve(2), getrlimit(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.

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

4th Berkeley Distribution December 11, 1993 1