219 lines
5.6 KiB
Groff
219 lines
5.6 KiB
Groff
.\" $NetBSD: msgrcv.2,v 1.19 2009/01/28 08:57:02 wiz Exp $
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.\"
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.\" Copyright (c) 1995 Frank van der Linden
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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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.\" 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 for the NetBSD Project
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.\" by Frank van der Linden
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.\" 4. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``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 AUTHOR 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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.Dd January 26, 2009
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.Dt MSGRCV 2
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.Os
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.Sh NAME
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.Nm msgrcv
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.Nd receive a message from a message queue
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/msg.h
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.Ft ssize_t
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.Fn msgrcv "int msqid" "void *msgp" "size_t msgsz" "long msgtyp" "int msgflg"
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.Sh DESCRIPTION
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The
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.Fn msgrcv
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function receives a message from the message queue specified in
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.Fa msqid ,
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and places it into the user-defined structure pointed to by
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.Fa msgp .
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This structure must contain a first field of type
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.Sy long
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that will indicate the user-defined type of the message.
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The remaining fields will contain the contents of the message.
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The following is an example of what this user-defined structure might
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look like:
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.Bd -literal
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struct mymsg {
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long mtype; /* message type */
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char mtext[1]; /* body of message */
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};
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.Ed
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.Pp
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.Va mtype
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is an integer greater than 0 that can be used to select messages.
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.Va mtext
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is an array of bytes, with size up to the system limit
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.Dv MSGMAX .
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.Pp
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The value of
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.Fa msgtyp
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has one of the following meanings:
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.Bl -bullet
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.It
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.Fa msgtyp
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is greater than 0.
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The first message of type
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.Fa msgtyp
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will be received.
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.It
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.Fa msgtyp
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is equal to 0.
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The first message on the queue will be received.
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.It
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.Fa msgtyp
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is less than 0.
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The first message of the lowest message type that is
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less than or equal to the absolute value of
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.Fa msgtyp
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will be received.
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.El
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.Pp
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.Fa msgsz
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specifies the maximum length of the requested message.
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If the received message has a length greater than
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.Fa msgsz
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it will be silently truncated if the
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.Dv MSG_NOERROR
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flag is set in
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.Fa msgflg ,
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otherwise an error will be returned.
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.Pp
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If no matching message is present on the message queue specified by
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.Fa msqid ,
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the behaviour of
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.Fn msgrcv
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depends on whether the
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.Dv IPC_NOWAIT
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flag is set in
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.Fa msgflg
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or not.
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If
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.Dv IPC_NOWAIT
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is set, then
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.Fn msgrcv
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will immediately return a value of \-1 and set
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.Va errno
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to
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.Er EAGAIN .
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If
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.Dv IPC_NOWAIT
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is not set, the calling process will block until:
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.Bl -bullet
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.It
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A message of the requested type becomes available on the message queue.
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.It
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The message queue is removed, in which case \-1 will be returned and
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.Va errno
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set to
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.Er EIDRM .
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.It
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A signal is received and caught.
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\-1 is returned and
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.Va errno
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is set to
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.Er EINTR .
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.El
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.Pp
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If a message is successfully received, the data structure associated with
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.Fa msqid
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is updated as follows:
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.Bl -bullet
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.It
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.Va msg_lrpid
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is set to the pid of the caller.
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.It
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.Va msg_lrtime
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is set to the current time.
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.It
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.Va msg_qnum
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is decremented by 1.
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.El
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.Sh RETURN VALUES
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Upon successful completion,
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.Fn msgrcv
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returns the number of bytes received into the
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.Va mtext
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field of the structure pointed to by
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.Fa msgp .
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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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.Fn msgrcv
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will fail if:
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.Bl -tag -width Er
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.It Bq Er E2BIG
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A matching message was received, but its size was greater than
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.Fa msgsz
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and the
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.Dv MSG_NOERROR
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flag was not set in
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.Fa msgflg .
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.It Bq Er EACCES
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The calling process does not have read access to the message queue.
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.It Bq Er EAGAIN
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There is no message of the requested type available on the message queue,
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and
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.Dv IPC_NOWAIT
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is set in
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.Fa msgflg .
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.It Bq Er EFAULT
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.Fa msgp
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points to an invalid address.
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.It Bq Er EIDRM
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The message queue identifier
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.Fa msqid
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is removed from the system.
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.It Bq Er EINTR
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The system call was interrupted by the delivery of a signal.
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.It Bq Er EINVAL
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.Fa msqid
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is not a valid message queue identifier
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.Pp
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The message queue was removed while
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.Fn msgrcv
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was waiting for a message of the requested type to become available in it.
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.Pp
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.Fa msgsz
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is greater than
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.Dv SSIZE_MAX .
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.It Bq Er ENOMSG
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The queue does not contain a message of the desired type and
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.Dv IPC_NOWAIT
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is set.
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.El
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.Sh SEE ALSO
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.Xr msgctl 2 ,
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.Xr msgget 2 ,
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.Xr msgsnd 2
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.Sh STANDARDS
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The
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.Nm
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system call conforms to
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.St -xsh5 .
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.Sh HISTORY
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Message queues appeared in the first release of
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.At V .
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