NAME
brk, sbrk —
change data segment size
LIBRARY
library “libc”
SYNOPSIS
#include
<unistd.h>
int
brk(const
void *addr);
void *
sbrk(intptr_t
incr);
DESCRIPTION
brk()
and sbrk() functions are legacy interfaces from before
the advent of modern virtual memory management. They are deprecated and not
present on the arm64 or riscv architectures. The
mmap(2) interface should be used to allocate pages instead.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
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 calling
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 “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
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 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
The brk() 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
The brk() function appeared in
Version 7 AT&T UNIX.
FreeBSD 11.0 introduced the arm64 and riscv
architectures which do not support brk() or
sbrk().
BUGS
Mixing brk() 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).
sbrk() is sometimes used to monitor heap
use by calling with an argument of 0. The result is unlikely to reflect
actual utilization in combination with an
mmap(2) based malloc.
brk() and sbrk()
are not thread-safe.