NAME
brk, sbrk —
change data segment size
SYNOPSIS
#include
<unistd.h>
int
brk(void
*addr);
void *
sbrk(int
incr);
DESCRIPTION
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).
RETURN VALUES
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
error.
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
HISTORY
A predecessor break() appeared in
Version 1 AT&T UNIX. The
sbrk() function call first appeared in
Version 4 AT&T UNIX and
brk() in Version 6 AT&T
UNIX.
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(2).