210 lines
5.9 KiB
Groff
210 lines
5.9 KiB
Groff
.\" $NetBSD: brk.2,v 1.34 2016/08/28 05:07:50 wiz Exp $
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.\"
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.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)brk.2 8.4 (Berkeley) 5/1/95
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.\"
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.Dd August 27, 2016
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.Dt BRK 2
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.Os
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.Sh NAME
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.Nm brk ,
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.Nm sbrk
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.Nd change data segment size
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In unistd.h
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.Ft int
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.Fn brk "void *addr"
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.Ft void *
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.Fn sbrk "intptr_t incr"
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.Sh DESCRIPTION
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.Bf -symbolic
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The brk and sbrk functions are legacy interfaces from before the
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advent of modern virtual memory management.
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.Ef
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.Pp
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The
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.Fn brk
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and
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.Fn sbrk
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functions are used to change the amount of memory allocated in a
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process's data segment.
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They do this by moving the address at which the process's heap ends.
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This address is known as the
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.Dq break .
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.Pp
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The
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.Fn brk
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function sets the break to
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.Fa addr .
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.Pp
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The
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.Fn sbrk
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function changes the break by
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.Fa incr
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bytes.
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If
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.Fa incr
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is positive, this allocates
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.Fa incr
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bytes of new memory in the data segment.
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If
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.Fa incr
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is negative,
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this releases the corresponding number of bytes.
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.Pp
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While the break may be set to any address, actual allocation takes
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place in page-sized quantities.
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For allocation and access control purposes the address of the break is
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always rounded up to the next page boundary.
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Thus, changes to the break that do not cross a page boundary have no
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material effect.
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Any new pages that are allocated, however, always appear freshly
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zeroed.
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.Pp
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The
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.Xr getrlimit 2
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system call may be used to determine
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the maximum permissible size of the
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.Em data
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segment;
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it will not be possible to set the break so that the sum of the heap
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size and the data segment is greater than the
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.Dv RLIMIT_DATA
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.Em rlim_max
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value returned from a call to
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.Xr getrlimit 2 .
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One can use the
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.Dq _etext
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symbol to find the end of the program text and thus the beginning of
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the data segment.
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.\" XXX is that always true? there are platforms where there's a fairly
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.\" large unmapped gap between text and data, plus using etext doesn't
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.\" take into account read-only data, which is probably (or should be)
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.\" billed against text size and not data size.
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See
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.Xr end 3
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regarding
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.Dq _etext .
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.Pp
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Historically and in
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.Nx
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the heap immediately follows the data segment, and in fact is
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considered part of it.
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Thus the initial break is the first address after the end of the
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process's uninitialized data (also known as the
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.Dq BSS ) .
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This address is provided by the linker as
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.Dq _end ;
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see
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.Xr end 3 .
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.Pp
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There exist implementations in the wild where this is not the case,
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however, or where the initial break is rounded up to a page boundary,
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or other minor variations, so the recommended more-portable way to
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retrieve the initial break is by calling
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.Fn sbrk 0
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at program startup.
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(This returns the current break without changing it.)
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.Pp
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In any event, the break may not be set to an address below its initial
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position.
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.Pp
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Note that ordinary application code should use
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.Xr malloc 3
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and related functions to allocate memory, or
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.Xr mmap 2
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for lower-level page-granularity control.
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While the
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.Fn brk
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and/or
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.Fn sbrk
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functions exist in most Unix-like environments, their semantics
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sometimes vary subtly and their use is not particularly portable.
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Also, one must take care not to mix calls to
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.Xr malloc 3
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or related functions with calls to
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.Fn brk
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or
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.Fn sbrk
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as this will ordinarily confuse
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.Xr malloc 3 ;
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this can be difficult to accomplish given that many things in the
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C library call
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.Xr malloc 3
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themselves.
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.Sh RETURN VALUES
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.Fn brk
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returns 0 if successful;
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otherwise \-1 with
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.Va errno
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set to indicate why the allocation failed.
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.Pp
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The
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.Fn sbrk
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function returns the prior break value if successful;
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otherwise ((void *)\-1) is returned and
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.Va errno
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is set to indicate why the allocation failed.
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.Sh ERRORS
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.Fn brk
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or
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.Fn sbrk
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will fail and no additional memory will be allocated if
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one of the following are true:
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.Bl -tag -width Er
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.It Bq Er ENOMEM
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The limit, as set by
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.Xr setrlimit 2 ,
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was exceeded;
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or the maximum possible size of a data segment (compiled into the
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system) was exceeded;
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or insufficient space existed in the swap area
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to support the expansion.
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.El
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.Sh SEE ALSO
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.Xr execve 2 ,
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.Xr getrlimit 2 ,
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.Xr mmap 2 ,
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.Xr end 3 ,
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.Xr free 3 ,
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.Xr malloc 3 ,
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.Xr sysconf 3
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.Sh HISTORY
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A
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.Fn brk
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function call appeared in
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.At v7 .
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.Sh BUGS
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Setting the break may fail due to a temporary lack of swap space.
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It is not possible to distinguish this from a failure caused by
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exceeding the maximum size of the data segment without consulting
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.Xr getrlimit 2 .
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