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+.\"	$NetBSD: brk.2,v 1.39 2019/09/07 19:32:11 wiz Exp $
+.\"
+.\" Copyright (c) 1980, 1991, 1993
+.\"	The Regents of the University of California.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"     @(#)brk.2	8.4 (Berkeley) 5/1/95
+.\"
+.Dd September 7, 2019
+.Dt BRK 2
+.Os
+.Sh NAME
+.Nm brk ,
+.Nm sbrk
+.Nd change data segment size
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In unistd.h
+.Ft int
+.Fn brk "void *addr"
+.Ft void *
+.Fn sbrk "intptr_t incr"
+.Sh DESCRIPTION
+.Bf -symbolic
+The
+.Fn brk
+and
+.Fn sbrk
+functions are legacy interfaces from before the
+advent of modern virtual memory management.
+.Fn brk
+is subject to removal and
+.Fn sbrk
+is destined for full compat, where the
+system call will exist in the kernel, but no longer be exposed.
+.Ef
+.Pp
+The
+.Fn brk
+and
+.Fn sbrk
+functions are used to change the amount of memory allocated in a
+process's data segment.
+They do this by moving the address at which the process's heap ends.
+This address is known as the
+.Dq break .
+.Pp
+The
+.Fn brk
+function sets the break to
+.Fa addr .
+.Pp
+The
+.Fn sbrk
+function changes the break by
+.Fa incr
+bytes.
+If
+.Fa incr
+is positive, this allocates
+.Fa incr
+bytes of new memory in the data segment.
+If
+.Fa incr
+is negative,
+this releases the corresponding number of bytes.
+.Pp
+While the break may be set to any address, actual allocation takes
+place in page-sized quantities.
+For allocation and access control purposes the address of the break is
+always rounded up to the next page boundary.
+Thus, changes to the break that do not cross a page boundary have no
+material effect.
+Any new pages that are allocated, however, always appear freshly
+zeroed.
+.Pp
+The
+.Xr getrlimit 2
+system call may be used to determine
+the maximum permissible size of the
+.Em data
+segment;
+it will not be possible to set the break so that the sum of the heap
+size and the data segment is greater than the
+.Dv RLIMIT_DATA
+.Em rlim_max
+value returned from a call to
+.Xr getrlimit 2 .
+One can use the
+.Dq _etext
+symbol to find the end of the program text and thus the beginning of
+the data segment.
+.\" XXX is that always true? there are platforms where there's a fairly
+.\" large unmapped gap between text and data, plus using etext doesn't
+.\" take into account read-only data, which is probably (or should be)
+.\" billed against text size and not data size.
+See
+.Xr end 3
+regarding
+.Dq _etext .
+.Pp
+Historically and in
+.Nx
+the heap immediately follows the data segment, and in fact is
+considered part of it.
+Thus the initial break is the first address after the end of the
+process's uninitialized data (also known as the
+.Dq BSS ) .
+This address is provided by the linker as
+.Dq _end ;
+see
+.Xr end 3 .
+.Pp
+There exist implementations in the wild where this is not the case,
+however, or where the initial break is rounded up to a page boundary,
+or other minor variations, so the recommended more-portable way to
+retrieve the initial break is by calling
+.Fn sbrk 0
+at program startup.
+(This returns the current break without changing it.)
+.Pp
+In any event, the break may not be set to an address below its initial
+position.
+.Pp
+Note that ordinary application code should use
+.Xr malloc 3
+and related functions to allocate memory, or
+.Xr mmap 2
+for lower-level page-granularity control.
+While the
+.Fn brk
+and/or
+.Fn sbrk
+functions exist in most Unix-like environments, their semantics
+sometimes vary subtly and their use is not particularly portable.
+Also, one must take care not to mix calls to
+.Xr malloc 3
+or related functions with calls to
+.Fn brk
+or
+.Fn sbrk
+as this will ordinarily confuse
+.Xr malloc 3 ;
+this can be difficult to accomplish given that many things in the
+C library call
+.Xr malloc 3
+themselves.
+.Sh RETURN VALUES
+.Fn brk
+returns 0 if successful;
+otherwise \-1 with
+.Va errno
+set to indicate why the allocation failed.
+.Pp
+The
+.Fn sbrk
+function returns the prior break value if successful;
+otherwise ((void *)\-1) is returned and
+.Va errno
+is set to indicate why the allocation failed.
+.Sh ERRORS
+.Fn brk
+or
+.Fn sbrk
+will fail and no additional memory will be allocated if
+one of the following are true:
+.Bl -tag -width Er
+.It Bq Er ENOMEM
+The limit, as set by
+.Xr setrlimit 2 ,
+was exceeded;
+or the maximum possible size of a data segment (compiled into the
+system) was exceeded;
+or insufficient space existed in the swap area
+to support the expansion.
+.El
+.Sh SEE ALSO
+.Xr execve 2 ,
+.Xr getrlimit 2 ,
+.Xr mmap 2 ,
+.Xr end 3 ,
+.Xr free 3 ,
+.Xr malloc 3 ,
+.Xr sysconf 3
+.Sh HISTORY
+An
+.Fn sbrk
+function call appeared in
+.At v4 .
+A
+.Fn brk
+function call appeared in
+.At v6 .
+.Sh BUGS
+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
+.Xr getrlimit 2 .