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BRK(2)			    BSD	System Calls Manual			BRK(2)

NAME
     brk, sbrk -- change data segment size

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <unistd.h>

     int
     brk(void *addr);

     void *
     sbrk(intptr_t incr);

DESCRIPTION
     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 defini-
     tion of etext).

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; other-
     wise ((void *)-1) is returned and errno is	set to indicate	why the	allo-
     cation 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.

     [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
			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
     Note that mixing brk() and	sbrk() with malloc(3), free(3),	and similar
     functions may result in non-portable program behavior.  Caution is	ad-
     vised.

     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).

BSD				 July 12, 1999				   BSD

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY | BUGS

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