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1130b656e5
This will make a number of things easier in the future, as well as (finally!) avoiding the Id-smashing problem which has plagued developers for so long. Boy, I'm glad we're not using sup anymore. This update would have been insane otherwise.
193 lines
5.4 KiB
Groff
193 lines
5.4 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 September 22, 1995
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.Dt SEMOP 2
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.Os FreeBSD
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.Sh NAME
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.Nm semop
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.Nd atomic array of operations on a semaphore set
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.Sh SYNOPSIS
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.Fd #include <sys/types.h>
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.Fd #include <sys/ipc.h>
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.Fd #include <sys/sem.h>
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.Ft int
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.Fn "semop" "int semid" "struct sembuf array[]" "unsigned nops"
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.Sh DESCRIPTION
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.Fn Semop
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atomically performs the array of operations indicated by
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.Fa array
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on the semaphore set indicated by
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.Fa semid .
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The length of
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.Fa array
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is indicated by
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.Fa nops .
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Each operation is encoded in a
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.Fa "struct sembuf" ,
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which is defined as follows:
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.Bd -literal
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.\"
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.\" From <sys/sem.h>
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.\"
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struct sembuf {
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u_short sem_num; /* semaphore # */
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short sem_op; /* semaphore operation */
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short sem_flg; /* operation flags */
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};
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.Ed
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.Pp
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For each element in
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.Fa array ,
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.Fa sem_op
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and
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.Fa sem_flg
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determine an operation to be performed on semaphore number
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.Fa sem_num
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in the set. The values SEM_UNDO and IPC_NOWAIT may be
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.Em OR Ns 'ed
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into the
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.Fa sem_flg
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member in order to modify the behavior of the given operation.
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.Pp
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The operation performed depends as follows on the value of
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.Fa sem_op :
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.\"
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.\" This section is based on the description of semop() in
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.\" Stevens, _Advanced Programming in the UNIX Environment_.
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.\"
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.Bl -bullet
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.It
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When
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.Fa sem_op
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is positive, the semaphore's value is incremented by
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.Fa sem_op Ns 's
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value. If SEM_UNDO is specified, the semaphore's adjust on exit
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value is decremented by
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.Fa sem_op Ns 's
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value. A positive value for
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.Fa sem_op
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generally corresponds to a process releasing a resource
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associated with the semaphore.
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.It
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The behavior when
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.Fa sem_op
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is negative depends on the current value of the semaphore:
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.Bl -bullet
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.It
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If the current value of the semaphore is greater than or equal to
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the absolute value of
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.Fa sem_op ,
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then the value is decremented by the absolute value of
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.Fa sem_op .
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If SEM_UNDO is specified, the semaphore's adjust on exit
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value is incremented by the absolute value of
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.Fa sem_op .
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.It
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If the current value of the semaphore is less than
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.Fa sem_op Ns 's
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value, one of the following happens:
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.\" XXX a *second* sublist?
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.Bl -bullet
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.It
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If IPC_NOWAIT was specified, then
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.Fn semop
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returns immediately with a return value of EAGAIN.
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.It
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If some other process has removed the semaphore with the IPC_RMID
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option of
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.Fn semctl ,
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then
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.Fn semop
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returns immediately with a return value of EINVAL.
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.It
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Otherwise, the calling process is put to sleep until the semaphore's
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value is greater than or equal to the absolute value of
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.Fa sem_op .
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When this condition becomes true, the semaphore's value is decremented
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by the absolute value of
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.Fa sem_op ,
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and the semaphore's adjust on exit value is incremented by the
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absolute value of
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.Fa sem_op .
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.El
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.Pp
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A negative value for
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.Fa sem_op
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generally means that a process is waiting for a resource to become
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available.
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.El
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.Pp
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.It
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When
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.Fa sem_op
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is zero, the process waits for the semaphore's value to become zero.
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If it is already zero, the call to
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.Fn semop
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can return immediately. Otherwise, the calling process is put to
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sleep until the semaphore's value becomes zero.
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.El
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.Pp
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For each semaphore a process has in use, the kernel maintains an
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`adjust on exit' value, as alluded to earlier. When a process
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exits, either voluntarily or involuntarily, the adjust on exit value
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for each semaphore is added to the semaphore's value. This can
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be used to insure that a resource is released if a process terminates
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unexpectedly.
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.Sh RETURN VALUES
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On success,
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.Fn semop
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returns 0; otherwise, -1 is returned and
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.Va errno
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is set to indicate the error.
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.Sh ERRORS
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.Fn Semop
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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No semaphore set corresponds to
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.Fa semid .
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.It Bq Er EACCES
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Permission denied due to mismatch between operation and mode of
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semaphore set.
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.It Bq Er EAGAIN
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The semaphore's value was less than
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.Fa sem_op ,
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and IPC_NOWAIT was specified.
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.It Bq Er E2BIG
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Too many operations were specified.
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.It Bq Er EFBIG
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.\"
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.\" I'd have thought this would be EINVAL, but the source says
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.\" EFBIG.
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.\"
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.Fa sem_num
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was not in the range of valid semaphores for the set.
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.Sh SEE ALSO
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.Xr semget 2 ,
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.Xr semctl 2
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