NAME
brk, sbrk —
change data segment size
SYNOPSIS
#include
<unistd.h>
void *
brk(const
void *addr);
void *
sbrk(int
incr);
DESCRIPTION
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, 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
function returns 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 expansion.
SEE ALSO
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.