NAME
brk
, sbrk
—
change data segment size
LIBRARY
library “libc”
SYNOPSIS
#include
<unistd.h>
int
brk
(void
*addr);
void *
sbrk
(intptr_t
incr);
DESCRIPTION
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).