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BRK(2) NetBSD System Calls Manual BRK(2)
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brk, sbrk -- change data segment size
Standard C Library (libc, -lc)
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 loca-
tion of the ``break''. The break is the first address after the end of
the process's uninitialized data segment (also known as the ``BSS'').
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 brk() function sets the break to addr.
The sbrk() function raises the break by at least incr bytes, thus allo-
cating at least incr bytes of new memory in the data segment. If incr is
negative, the break is lowered by incr bytes.
sbrk() returns the prior address of the break. 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 permis-
sible size of the data segment; it will not be possible to set the break
beyond the RLIMIT_DATA rlim_max value returned from a call to
getrlimit(2), e.g. ``etext + rlim.rlim_max''. (see end(3) for the defi-
nition of etext).
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; other-
wise ((void *)-1) is returned and errno is set to indicate why the allo-
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.
[ENOMEM] The maximum possible size of a data segment (compiled
into the system) was exceeded.
[ENOMEM] Insufficient space existed in the swap area to support
execve(2), getrlimit(2), mmap(2), end(3), free(3), malloc(3), sysconf(3)
A brk() function call appeared in Version 7 AT&T UNIX.
Note that mixing brk() and sbrk() with malloc(3), free(3), and similar
functions may result in non-portable program behavior. Caution is
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).
NetBSD 6.0 July 12, 1999 NetBSD 6.0