.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "MSGRCV" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" msgrcv 
.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
msgrcv \- XSI message receive operation
.SH SYNOPSIS
.LP
\fB#include <sys/msg.h>
.br
.sp
ssize_t msgrcv(int\fP \fImsqid\fP\fB, void *\fP\fImsgp\fP\fB, size_t\fP
\fImsgsz\fP\fB, long\fP
\fImsgtyp\fP\fB,
.br
\ \ \ \ \ \  int\fP \fImsgflg\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fImsgrcv\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 \fImsgrcv\fP() function shall read a message from the queue associated
with the message queue identifier specified by
\fImsqid\fP and place it in the user-defined buffer pointed to by
\fImsgp\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 the received message's type as
specified by the sending process.
.LP
The structure member \fImtext\fP is the text of the message.
.LP
The argument \fImsgsz\fP specifies the size in bytes of \fImtext\fP.
The received message shall be truncated to \fImsgsz\fP
bytes if it is larger than \fImsgsz\fP and (\fImsgflg\fP & MSG_NOERROR)
is non-zero. The truncated part of the message shall
be lost and no indication of the truncation shall be given to the
calling process.
.LP
If the value of \fImsgsz\fP is greater than {SSIZE_MAX}, the result
is implementation-defined.
.LP
The argument \fImsgtyp\fP specifies the type of message requested
as follows:
.IP " *" 3
If \fImsgtyp\fP is 0, the first message on the queue shall be received.
.LP
.IP " *" 3
If \fImsgtyp\fP is greater than 0, the first message of type \fImsgtyp\fP
shall be received.
.LP
.IP " *" 3
If \fImsgtyp\fP is less than 0, the first message of the lowest type
that is less than or equal to the absolute value of
\fImsgtyp\fP shall be received.
.LP
.LP
The argument \fImsgflg\fP specifies the action to be taken if a message
of the desired type is not on the queue. These are as
follows:
.IP " *" 3
If (\fImsgflg\fP & IPC_NOWAIT) is non-zero, the calling thread shall
return immediately with a return value of -1 and
\fIerrno\fP set to [ENOMSG].
.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
A message of the desired type is placed on the queue.
.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 a message is not received 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:
.IP " *" 3
\fBmsg_qnum\fP shall be decremented by 1.
.LP
.IP " *" 3
\fBmsg_lrpid\fP shall be set equal to the process ID of the calling
process.
.LP
.IP " *" 3
\fBmsg_rtime\fP shall be set equal to the current time.
.LP
.SH RETURN VALUE
.LP
Upon successful completion, \fImsgrcv\fP() shall return a value equal
to the number of bytes actually placed into the buffer
\fImtext\fP. Otherwise, no message shall be received, \fImsgrcv\fP()
shall return (\fBssize_t\fP)-1, and \fIerrno\fP shall be
set to indicate the error.
.SH ERRORS
.LP
The \fImsgrcv\fP() function shall fail if:
.TP 7
.B E2BIG
The value of \fImtext\fP is greater than \fImsgsz\fP and (\fImsgflg\fP
& MSG_NOERROR) is 0.
.TP 7
.B EACCES
Operation permission is denied to the calling process; see \fIXSI
Interprocess
Communication\fP .
.TP 7
.B EIDRM
The message queue identifier \fImsqid\fP is removed from the system.
.TP 7
.B EINTR
The \fImsgrcv\fP() function was interrupted by a signal.
.TP 7
.B EINVAL
\fImsqid\fP is not a valid message queue identifier.
.TP 7
.B ENOMSG
The queue does not contain a message of the desired type and (\fImsgflg\fP
& IPC_NOWAIT) is non-zero.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.SS Receiving a Message
.LP
The following example receives the first message on the queue (based
on the value of the \fImsgtyp\fP argument, 0). The queue
is identified by the \fImsqid\fP argument (assuming that the value
has previously been set). This call specifies that an error
should be reported if no message is available, but not if the message
is too large. 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;
long msgtyp = 0;
\&...
result = msgrcv(msqid, (void *) &msg, sizeof(msg.text),
         msgtyp, MSG_NOERROR | 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(),
\fImsgsnd\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 .