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.\"	$NetBSD: msgsnd.2,v 1.1 1995/10/16 23:49:24 jtc Exp $
.\"
.\" Copyright (c) 1995 Frank van der Linden
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.\"      by Frank van der Linden
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.\" $FreeBSD: release/9.1.0/lib/libc/sys/msgsnd.2 235580 2012-05-18 01:04:03Z gjb $
.\"
.Dd July 9, 2009
.Dt MSGSND 2
.Os
.Sh NAME
.Nm msgsnd
.Nd send a message to a message queue
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/ipc.h
.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 in
.Fa msqid .
The
.Fa msgp
argument
points to a structure containing the message.
This structure should
consist of the following members:
.Bd -literal
    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.
The argument
.Fa msgsz
can range from 0 to a system-imposed maximum,
.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
.Pf ( Va msg_qbytes ,
see
.Xr msgctl 2 ) ,
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 msgsnd
.Sh ERRORS
The
.Fn msgsnd
function
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa msqid
argument
is not a valid message queue identifier.
.Pp
The message queue was removed while
.Fn msgsnd
was waiting for a resource to become available in order to deliver the
message.
.Pp
The
.Fa msgsz
argument
is greater than
.Va msg_qbytes .
.Pp
The
.Fa mtype
argument
is not greater than 0.
.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
The
.Fa msgp
argument
points to an invalid address.
.It Bq Er EINTR
The system call was interrupted by the delivery of a signal.
.El
.Sh HISTORY
Message queues appeared in the first release of AT&T Unix System V.
.Sh BUGS
.Nx
and
.Fx
do not define the
.Er EIDRM
error value, which should be used
in the case of a removed message queue.