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NetBSD 6.1.5 - man page for brk (netbsd section 2)

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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 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'').

     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 allocating 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 permissible 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
     definition 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; otherwise ((void *)-1) is
     returned and errno is set to indicate why the allocation 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 advised.

     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
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