BRK(2) | System Calls Manual | BRK(2) |
brk
, sbrk
—
#include <unistd.h>
int
brk
(void
*addr);
void *
sbrk
(intptr_t
incr);
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.
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.
brk
() or sbrk
() will fail and no
additional memory will be allocated if one of the following are true:
ENOMEM
]brk
() function call appeared in
Version 7 AT&T UNIX.
August 27, 2016 | NetBSD 9.0 |