.\"	$OpenBSD: msgsnd.2,v 1.19 2014/12/10 19:19:00 schwarze Exp $
.\"	$NetBSD: msgsnd.2,v 1.2 1997/03/27 08:20:36 mikel Exp $
.\"
.\" Copyright (c) 1995 Frank van der Linden
.\" All rights reserved.
.\"
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.\" modification, are permitted provided that the following conditions
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.\"    must display the following acknowledgement:
.\"      This product includes software developed for the NetBSD Project
.\"      by Frank van der Linden
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.\"/
.Dd $Mdocdate: December 10 2014 $
.Dt MSGSND 2
.Os
.Sh NAME
.Nm msgsnd
.Nd send a message to a message queue
.Sh SYNOPSIS
.In sys/msg.h
.Ft int
.Fn msgsnd "int msqid" "const void *msgp" "size_t msgsz" "int msgflg"
.Sh DESCRIPTION
The
.Fn msgsnd
function sends a message to the message queue specified by
.Fa msqid .
.Fa msgp
points to a structure containing the message.
This structure should consist of the following members:
.Bd -literal -offset indent
long mtype;    /* message type */
char mtext[1]; /* body of message */
.Ed
.Pp
.Va mtype
is an integer greater than 0 that can be used for selecting messages (see
.Xr msgrcv 2 ) ;
.Va mtext
is an array of
.Fa msgsz
bytes, with a size between 0 and that of the system limit
.Pq Dv MSGMAX .
.Pp
If the number of bytes already on the message queue plus
.Fa msgsz
is bigger than the maximum number of bytes on the message queue
.Po Va msg_qbytes ,
 see
.Xr msgctl 2
.Pc ,
or the number of messages on all queues system-wide is already equal to
the system limit,
.Fa msgflg
determines the action of
.Fn msgsnd .
If
.Fa msgflg
has
.Dv IPC_NOWAIT
mask set in it, the call will return immediately.
If
.Fa msgflg
does not have
.Dv IPC_NOWAIT
set in it, the call will block until:
.Bl -bullet
.It
The condition which caused the call to block does no longer exist.
The message will be sent.
.It
The message queue is removed, in which case \-1 will be returned, and
.Va errno
is set to
.Er EINVAL .
.It
The caller catches a signal.
The call returns with
.Va errno
set to
.Er EINTR .
.El
.Pp
After a successful call, the data structure associated with the message
queue is updated in the following way:
.Bl -bullet
.It
.Va msg_cbytes
is incremented by the size of the message.
.It
.Va msg_qnum
is incremented by 1.
.It
.Va msg_lspid
is set to the pid of the calling process.
.It
.Va msg_stime
is set to the current time.
.El
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn msgsnd
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa msqid
is not a valid message queue identifier.
.Pp
.Fa msgsz
is greater than
.Va msg_qbytes .
.It Bq Er EACCES
The calling process does not have write access to the message queue.
.It Bq Er EAGAIN
There was no space for this message either on the queue, or in the whole
system, and
.Dv IPC_NOWAIT
was set in
.Fa msgflg .
.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 EIDRM
The message queue was removed while
.Fn msgsnd
was waiting for a resource to become available in order to deliver the
message.
.El
.Sh SEE ALSO
.Xr msgctl 2 ,
.Xr msgget 2 ,
.Xr msgrcv 2
.Sh HISTORY
Message queues appeared in
.At V.1 .