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BRK(2) | System Calls Manual | BRK(2) |
NAME¶
brk
, sbrk
—
change data segment size
LIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include
<sys/types.h>
#include
<unistd.h>
int
brk
(const
void *addr);
void *
sbrk
(intptr_t
incr);
DESCRIPTION¶
The
The brk
() and
sbrk
() functions are legacy interfaces from
before the advent of modern virtual memory management.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 location of the “break”. The break is the first address
after the end of the process's uninitialized data segment (also known as the
“BSS”).
The brk
() function sets the break to
addr.
The sbrk
() function raises the break by
incr bytes, thus allocating at least
incr bytes of new memory in the data segment.
If incr is negative, the break is lowered by
incr bytes.
NOTES¶
While the actual process data segment size maintained by the kernel will only grow or shrink in page sizes, these functions allow setting the break to unaligned values (i.e., it may point to any address inside the last page of the data segment). The current value of the program break may be determined by callingsbrk
(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
“etext +
rlim.rlim_max” where the
rlim.rlim_max value is returned from a call
to
getrlimit
(RLIMIT_DATA,
&rlim). (See
end(3) for the definition of
etext).
RETURN VALUES¶
Thebrk
() 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 the prior break
value if successful; otherwise the value (void
*)-1 is returned and the global variable
errno is set to indicate the error.
ERRORS¶
Thebrk
() and
sbrk
() functions will fail if:
- [
EINVAL
] - The requested break value was beyond the beginning of the data segment.
- [
ENOMEM
] - The data segment size limit, as set by setrlimit(2), was exceeded.
- [
ENOMEM
] - Insufficient space existed in the swap area to support the expansion of the data segment.
SEE ALSO¶
execve(2), getrlimit(2), mmap(2), end(3), free(3), malloc(3)HISTORY¶
Thebrk
() function appeared in
Version 7 AT&T UNIX.
BUGS¶
Mixingbrk
() or
sbrk
() with
malloc(3), free(3),
or similar functions will result in non-portable program behavior.
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).July 12, 1999 | Debian |