210 lines
5.5 KiB
Groff
210 lines
5.5 KiB
Groff
.\" $NetBSD: msgrcv.2,v 1.9 1999/08/25 20:30:05 thorpej Exp $
|
|
.\"
|
|
.\" Copyright (c) 1995 Frank van der Linden
|
|
.\" All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\" 3. All advertising materials mentioning features or use of this software
|
|
.\" must display the following acknowledgement:
|
|
.\" This product includes software developed for the NetBSD Project
|
|
.\" by Frank van der Linden
|
|
.\" 4. The name of the author may not be used to endorse or promote products
|
|
.\" derived from this software without specific prior written permission
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
|
|
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
|
|
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
|
|
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
|
|
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
|
|
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
|
|
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
|
|
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
|
|
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
|
|
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
|
|
.\"
|
|
.Dd August 25, 1999
|
|
.Dt MSGRCV 2
|
|
.Os
|
|
.Sh NAME
|
|
.Nm msgrcv
|
|
.Nd receive a message from a message queue
|
|
.Sh SYNOPSIS
|
|
.Fd #include <sys/msg.h>
|
|
.Ft ssize_t
|
|
.Fn msgrcv "int msqid" "void *msgp" "size_t msgsz" "long msgtyp" "int msgflg"
|
|
.Sh DESCRIPTION
|
|
The
|
|
.Fn msgrcv
|
|
function receives a message from the message queue specified in
|
|
.Fa msqid ,
|
|
and places it into the user-defined structure pointed to by
|
|
.Fa msgp .
|
|
This structure must contain a first field of type
|
|
.Sy long
|
|
that will indicate the user-defined type of the message. The remaining
|
|
fields will contain the contents of the message. The following is
|
|
an example of what this user-defined structure might look like:
|
|
.Bd -literal
|
|
struct mymsg {
|
|
long mtype; /* message type */
|
|
char mtext[1]; /* body of message */
|
|
};
|
|
.Ed
|
|
.Pp
|
|
.Va mtype
|
|
is an integer greater than 0 that can be used to select messages.
|
|
.Va mtext
|
|
is an array of bytes, with size up to the system limit
|
|
.Dv MSGMAX .
|
|
.Pp
|
|
The value of
|
|
.Fa msgtyp
|
|
has one of the following meanings:
|
|
.Bl -bullet
|
|
.It
|
|
.Fa msgtyp
|
|
is greater than 0. The first message of type
|
|
.Fa msgtyp
|
|
will be received.
|
|
.It
|
|
.Fa msgtyp
|
|
is equal to 0. The first message on the queue will be received.
|
|
.It
|
|
.Fa msgtyp
|
|
is less than 0. The first message of the lowest message type that is
|
|
less than or equal to the absolute value of
|
|
.Fa msgtyp
|
|
will be received.
|
|
.El
|
|
.Pp
|
|
.Fa msgsz
|
|
specifies the maximum length of the requested message. If the received
|
|
message has a length greater than
|
|
.Fa msgsz
|
|
it will be silently truncated if the
|
|
.Dv MSG_NOERROR
|
|
flag is set in
|
|
.Fa msgflg ,
|
|
otherwise an error will be returned.
|
|
.Pp
|
|
If no matching message is present on the message queue specified by
|
|
.Fa msqid ,
|
|
the behaviour of
|
|
.Fn msgrcv
|
|
depends on whether the
|
|
.Dv IPC_NOWAIT
|
|
flag is set in
|
|
.Fa msgflg
|
|
or not. If
|
|
.Dv IPC_NOWAIT
|
|
is set, then
|
|
.Fn msgrcv
|
|
will immediately return a value of -1 and set
|
|
.Va errno
|
|
to
|
|
.Er EAGAIN .
|
|
If
|
|
.Dv IPC_NOWAIT
|
|
is not set, the calling process will block until:
|
|
.Bl -bullet
|
|
.It
|
|
A message of the requested type becomes available on the message queue.
|
|
.It
|
|
The message queue is removed, in which case -1 will be returned and
|
|
.Va errno
|
|
set to
|
|
.Er EINVAL .
|
|
.It
|
|
A signal is received and caught. -1 is returned and
|
|
.Va errno
|
|
is set to
|
|
.Er EINTR .
|
|
.El
|
|
.Pp
|
|
If a message is successfully received, the data structure associated with
|
|
.Fa msqid
|
|
is updated as follows:
|
|
.Bl -bullet
|
|
.It
|
|
.Va msg_lrpid
|
|
is set to the pid of the caller.
|
|
.It
|
|
.Va msg_lrtime
|
|
is set to the current time.
|
|
.It
|
|
.Va msg_qnum
|
|
is decremented by 1.
|
|
.El
|
|
.Sh RETURN VALUES
|
|
Upon successful completion,
|
|
.Fn msgrcv
|
|
returns the number of bytes received into the
|
|
.Va mtext
|
|
field of the structure pointed to by
|
|
.Fa msgp .
|
|
Otherwise, -1 is returned, and
|
|
.Va errno
|
|
set to indicate the error.
|
|
.Sh ERRORS
|
|
.Fn msgrcv
|
|
will fail if:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EINVAL
|
|
.Fa msqid
|
|
is not a valid message queue identifier
|
|
.Pp
|
|
The message queue was removed while
|
|
.Fn msgrcv
|
|
was waiting for a message of the requested type to become available in it.
|
|
.Pp
|
|
.Fa msgsz
|
|
is less than 0.
|
|
.It Bq Er E2BIG
|
|
A matching message was received, but its size was greater than
|
|
.Fa msgsz
|
|
and the
|
|
.Dv MSG_NOERROR
|
|
flag was not set in
|
|
.Fa msgflg .
|
|
.It Bq Er EACCES
|
|
The calling process does not have read access to the message queue.
|
|
.It Bq Er EFAULT
|
|
.Fa msgp
|
|
points to an invalid address.
|
|
.It Bq Er EINTR
|
|
The system call was interrupted by the delivery of a signal.
|
|
.It Bq Er EAGAIN
|
|
There is no message of the requested type available on the message queue,
|
|
and
|
|
.Dv IPC_NOWAIT
|
|
is set in
|
|
.Fa msgflg .
|
|
.It Bq Er EIDRM
|
|
The message queue identifier
|
|
.Fa msqid
|
|
is removed from the system.
|
|
.It Bq Er ENOMSG
|
|
The queue does not contain a message of the desired type and
|
|
.Dv IPC_NOWAIT
|
|
is set.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr msgctl 2 ,
|
|
.Xr msgget 2 ,
|
|
.Xr msgsnd 2
|
|
.Sh STANDARDS
|
|
The
|
|
.Nm
|
|
system call conforms to
|
|
.St -xsh5 .
|
|
.Sh HISTORY
|
|
Message queues appeared in the first release of
|
|
.At V .
|