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BRK(2)			  FreeBSD 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 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'').

     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 permis-
     sible 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; other-
     wise 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.

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).

FreeBSD	9.3			 July 12, 1999			   FreeBSD 9.3

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

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