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144 lines
3.9 KiB
Groff
144 lines
3.9 KiB
Groff
.\"
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.\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>
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.\"
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.\" 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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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd July 3, 1995
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.Dt SHMGET 2
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.Os
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.Sh NAME
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.Nm shmget
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.Nd obtain a shared memory identifier
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In machine/param.h
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.In sys/types.h
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.In sys/ipc.h
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.In sys/shm.h
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.Ft int
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.Fn shmget "key_t key" "size_t size" "int flag"
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.Sh DESCRIPTION
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Based on the values of
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.Fa key
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and
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.Fa flag ,
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.Fn shmget
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returns the identifier of a newly created or previously existing shared
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memory segment.
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.\"
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.\" The following bit about keys and modes also applies to semaphores
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.\" and message queues.
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.\"
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The key
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is analogous to a filename: it provides a handle that names an
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IPC object. There are three ways to specify a key:
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.Bl -bullet
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.It
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IPC_PRIVATE may be specified, in which case a new IPC object
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will be created.
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.It
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An integer constant may be specified. If no IPC object corresponding
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to
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.Fa key
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is specified and the IPC_CREAT bit is set in
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.Fa flag ,
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a new one will be created.
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.It
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The
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.Xr ftok 3
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may be used to generate a key from a pathname.
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.El
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.Pp
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The mode of a newly created IPC object is determined by
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.Em OR Ns 'ing
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the following constants into the
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.Fa flag
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argument:
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.Bl -tag -width XSHM_WXX6XXX
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.It Dv SHM_R
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Read access for user.
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.It Dv SHM_W
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Write access for user.
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.It Dv ( SHM_R>>3 )
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Read access for group.
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.It Dv ( SHM_W>>3 )
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Write access for group.
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.It Dv ( SHM_R>>6 )
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Read access for other.
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.It Dv ( SHM_W>>6 )
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Write access for other.
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.El
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.\"
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.\" XXX - we should also mention how uid, euid, and gid affect ownership
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.\" and use
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.\"
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.\" end section about keys and modes
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.\"
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.Pp
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When creating a new shared memory segment,
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.Fa size
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indicates the desired size of the new segment in bytes. The size
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of the segment may be rounded up to a multiple convenient to the
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kernel (i.e., the page size).
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.Sh RETURN VALUES
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Upon successful completion,
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.Fn shmget
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returns the positive integer identifier of a shared memory segment.
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Otherwise, -1 is returned and
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.Va errno
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set to indicate the error.
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.Sh ERRORS
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The
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.Fn shmget
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system call
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will fail if:
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.Bl -tag -width Er
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.\"
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.\" XXX What about ipcperm failing?
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.\"
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.It Bq Er EINVAL
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Size specified is greater than the size of the previously existing segment.
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Size specified is less than the system imposed minimum, or greater than
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the system imposed maximum.
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.It Bq Er ENOENT
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No shared memory segment was found matching
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.Fa key ,
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and IPC_CREAT was not specified.
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.It Bq Er ENOSPC
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The kernel was unable to allocate enough memory to
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satisfy the request.
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.It Bq Er EEXIST
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IPC_CREAT and IPC_EXCL were specified, and a shared memory segment
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corresponding to
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.Fa key
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already exists.
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.El
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.Sh "SEE ALSO"
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.Xr shmat 2 ,
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.Xr shmctl 2 ,
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.Xr shmdt 2 ,
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.Xr ftok 3
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