.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "MSGSND" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" msgsnd 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
msgsnd \- XSI message send operation
.SH SYNOPSIS
.LP
\fB#include <sys/msg.h>
.br
.sp
int msgsnd(int\fP \fImsqid\fP\fB, const void *\fP\fImsgp\fP\fB, size_t\fP
\fImsgsz\fP\fB, int\fP \fImsgflg\fP\fB);
\fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fImsgsnd\fP() function operates on XSI message queues (see the
Base Definitions volume of IEEE\ Std\ 1003.1-2001,
Section 3.224, Message Queue). It is unspecified whether this function
interoperates with the realtime interprocess communication facilities
defined in \fIRealtime\fP .
.LP
The \fImsgsnd\fP() function shall send a message to the queue associated
with the message queue identifier specified by
\fImsqid\fP.
.LP
The application shall ensure that the argument \fImsgp\fP points to
a user-defined buffer that contains first a field of type
\fBlong\fP specifying the type of the message, and then a data portion
that holds the data bytes of the message. The structure
below is an example of what this user-defined buffer might look like:
.sp
.RS
.nf

\fBstruct mymsg {
    long   mtype;       /* Message type. */
    char   mtext[1];    /* Message text. */
}
\fP
.fi
.RE
.LP
The structure member \fImtype\fP is a non-zero positive type \fBlong\fP
that can be used by the receiving process for message
selection.
.LP
The structure member \fImtext\fP is any text of length \fImsgsz\fP
bytes. The argument \fImsgsz\fP can range from 0 to a
system-imposed maximum.
.LP
The argument \fImsgflg\fP specifies the action to be taken if one
or more of the following is true:
.IP " *" 3
The number of bytes already on the queue is equal to \fBmsg_qbytes\fP;
see \fI<sys/msg.h>\fP.
.LP
.IP " *" 3
The total number of messages on all queues system-wide is equal to
the system-imposed limit.
.LP
.LP
These actions are as follows:
.IP " *" 3
If (\fImsgflg\fP & IPC_NOWAIT) is non-zero, the message shall not
be sent and the calling thread shall return
immediately.
.LP
.IP " *" 3
If (\fImsgflg\fP & IPC_NOWAIT) is 0, the calling thread shall suspend
execution until one of the following occurs:
.RS
.IP " *" 3
The condition responsible for the suspension no longer exists, in
which case the message is sent.
.LP
.IP " *" 3
The message queue identifier \fImsqid\fP is removed from the system;
when this occurs, \fIerrno\fP shall be set equal to
[EIDRM] and -1 shall be returned.
.LP
.IP " *" 3
The calling thread receives a signal that is to be caught; in this
case the message is not sent and the calling thread resumes
execution in the manner prescribed in \fIsigaction\fP().
.LP
.RE
.LP
.LP
Upon successful completion, the following actions are taken with respect
to the data structure associated with \fImsqid\fP; see
\fI<sys/msg.h>\fP:
.IP " *" 3
\fBmsg_qnum\fP shall be incremented by 1.
.LP
.IP " *" 3
\fBmsg_lspid\fP shall be set equal to the process ID of the calling
process.
.LP
.IP " *" 3
\fBmsg_stime\fP shall be set equal to the current time.
.LP
.SH RETURN VALUE
.LP
Upon successful completion, \fImsgsnd\fP() shall return 0; otherwise,
no message shall be sent, \fImsgsnd\fP() shall return
-1, and \fIerrno\fP shall be set to indicate the error.
.SH ERRORS
.LP
The \fImsgsnd\fP() function shall fail if:
.TP 7
.B EACCES
Operation permission is denied to the calling process; see \fIXSI
Interprocess
Communication\fP .
.TP 7
.B EAGAIN
The message cannot be sent for one of the reasons cited above and
(\fImsgflg\fP & IPC_NOWAIT) is non-zero.
.TP 7
.B EIDRM
The message queue identifier \fImsqid\fP is removed from the system.
.TP 7
.B EINTR
The \fImsgsnd\fP() function was interrupted by a signal.
.TP 7
.B EINVAL
The value of \fImsqid\fP is not a valid message queue identifier,
or the value of \fImtype\fP is less than 1; or the value of
\fImsgsz\fP is less than 0 or greater than the system-imposed limit.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.SS Sending a Message
.LP
The following example sends a message to the queue identified by the
\fImsqid\fP argument (assuming that value has previously
been set). This call specifies that an error should be reported if
no message is available. The message size is calculated directly
using the \fIsizeof\fP operator.
.sp
.RS
.nf

\fB#include <sys/msg.h>
\&...
int result;
int msqid;
struct message {
    long type;
    char text[20];
} msg;
.sp

msg.type = 1;
strcpy(msg.text, "This is message 1");
\&...
result = msgsnd(msqid, (void *) &msg, sizeof(msg.text), IPC_NOWAIT);
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
The POSIX Realtime Extension defines alternative interfaces for interprocess
communication (IPC). Application developers who
need to use IPC should design their applications so that modules using
the IPC routines described in \fIXSI Interprocess Communication\fP
can be easily modified to use the alternative
interfaces.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIXSI Interprocess Communication\fP, \fIRealtime\fP, \fImq_close\fP(),
\fImq_getattr\fP(), \fImq_notify\fP(), \fImq_open\fP(), \fImq_receive\fP(),
\fImq_send\fP(), \fImq_setattr\fP(), \fImq_unlink\fP(), \fImsgctl\fP(),
\fImsgget\fP(),
\fImsgrcv\fP(), \fIsigaction\fP(), the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<sys/msg.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .