NAME
brk,
sbrk —
change
data segment size
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int
brk(
void
*addr);
void *
sbrk(
intptr_t
incr);
DESCRIPTION
The brk and sbrk
functions are legacy interfaces from before the advent of modern virtual
memory management.
The
brk() and
sbrk() functions are used to
change the amount of memory allocated in a process's data segment. They do
this by moving the address at which the process's heap ends. This address is
known as the “break”.
The
brk() function sets the break to
addr.
The
sbrk() function changes the break by
incr bytes. If
incr is positive,
this allocates
incr bytes of new memory in the data
segment. If
incr is negative, this releases the
corresponding number of bytes.
While the break may be set to any address, actual allocation takes place in
page-sized quantities. For allocation and access control purposes the address
of the break is always rounded up to the next page boundary. Thus, changes to
the break that do not cross a page boundary have no material effect. Any new
pages that are allocated, however, always appear freshly zeroed.
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 so
that the sum of the heap size and the data segment is greater than the
RLIMIT_DATA
rlim_max value returned
from a call to
getrlimit(2).
One can use the “_etext” symbol to find the end of the program
text and thus the beginning of the data segment. See
end(3) regarding
“_etext”.
Historically and in
NetBSD the heap immediately follows
the data segment, and in fact is considered part of it. Thus the initial break
is the first address after the end of the process's uninitialized data (also
known as the “BSS”). This address is provided by the linker as
“_end”; see
end(3).
There exist implementations in the wild where this is not the case, however, or
where the initial break is rounded up to a page boundary, or other minor
variations, so the recommended more-portable way to retrieve the initial break
is by calling
sbrk(
0) at program
startup. (This returns the current break without changing it.)
In any event, the break may not be set to an address below its initial position.
Note that ordinary application code should use
malloc(3) and related functions
to allocate memory, or
mmap(2) for
lower-level page-granularity control. While the
brk() and/or
sbrk() functions exist in most Unix-like environments, their
semantics sometimes vary subtly and their use is not particularly portable.
Also, one must take care not to mix calls to
malloc(3) or related functions
with calls to
brk() or
sbrk() as this will
ordinarily confuse
malloc(3);
this can be difficult to accomplish given that many things in the C library
call
malloc(3) themselves.
RETURN VALUES
brk() returns 0 if successful; otherwise -1 with
errno set to indicate why the allocation failed.
The
sbrk() function returns the prior break value if
successful; otherwise ((void *)-1) is returned and
errno
is set to indicate why the allocation failed.
ERRORS
brk() or
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; or the maximum possible size of a data segment (compiled into
the system) was exceeded; or insufficient space existed in the swap area
to support the expansion.
SEE ALSO
execve(2),
getrlimit(2),
mmap(2),
end(3),
free(3),
malloc(3),
sysconf(3)
HISTORY
A
brk() function call appeared in
Version 7 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).