186 lines
4.9 KiB
Groff
186 lines
4.9 KiB
Groff
.\" $NetBSD: brk.2,v 1.22 1999/08/16 14:10:06 is 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. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. 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 July 12, 1999
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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 SYNOPSIS
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.Fd #include <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. They do this by moving the location of the
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.Dq break .
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The break is the first address after the end of the process's
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uninitialized data segment (also known as the
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.Dq BSS ) .
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.Pp
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In
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.Nx ,
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the break must be at an even page boundary. If
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.Fn brk
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or
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.Fn sbrk
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are asked for a non-integral number of pages, the address of the break
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will be rounded up to the next highest page boundary.
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Note that this page-aligning behavior is not standardized and
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is thus not necessarily present in other systems.
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(The size of a system page can be determined by calling
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.Fn sysconf _SC_PAGESIZE .
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See
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.Xr sysconf 3
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for details.)
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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 raises the break by at least
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.Fa incr
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bytes, thus allocating at least
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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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the break is lowered by
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.Fa incr
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bytes.
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.Pp
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.Fn sbrk
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returns the prior address of the break.
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The current value of the program break may be determined by calling
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.Fn sbrk 0 .
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(See also
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.Xr end 3 ) .
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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
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beyond 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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e.g.
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.Dq etext + rlim.rlim_max .
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(see
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.Xr end 3
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for the definition of
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.Em etext ) .
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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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.It Bq Er ENOMEM
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The maximum possible size of a data segment (compiled into the
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system) was exceeded.
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.It Bq Er ENOMEM
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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 BUGS
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Note that
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mixing
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.Fn brk
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and
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.Fn sbrk
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with
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.Xr malloc 3 ,
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.Xr free 3 ,
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and similar functions may result in non-portable program
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behavior. Caution is advised.
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.Pp
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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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.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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