453 lines
13 KiB
HTML
453 lines
13 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation">
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<title>msgrcv()--Receive Message Operation</title>
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<!-- Begin Header Records ========================================== -->
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- Use, duplication or disclosure restricted by -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<!-- Change History: -->
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<!-- YYMMDD USERID Change description -->
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<!-- Direct1 SCRIPT J converted by B2H R4.1 (346) (CMS) by V2KEA304 -->
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<!-- at RCHVMW2 on 17 Feb 1999 at 11:05:09 -->
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<!-- Edited by Kersten Feb 02 -->
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<!-- This file has undergone error condition cleanup on 05/01/02 by JET -->
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<link rel="stylesheet" type="text/css" href="../rzahg/ic.css">
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</head>
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<body>
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<!--End Header Records -->
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<!-- Java sync-link -->
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<script type="text/javascript" language="Javascript" src="../rzahg/synch.js">
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</script>
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<a name="Top_Of_Page"></a>
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<h2>msgrcv()--Receive Message Operation</h2>
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<div class="box" style="width: 70%;">
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<br>
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Syntax<br>
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<pre>
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#include <sys/msg.h>
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int msgrcv(int <em>msqid</em>, void <em>*msgp</em>, size_t <em>msgsz</em>,
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long int <em>msgtyp</em>, int <em>msgflg</em>);
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</pre>
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<br>
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Service Program Name: QP0ZUSHR<br>
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<!-- iddvc RMBR -->
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<br>
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Default Public Authority: *USE<br>
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<!-- iddvc RMBR -->
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<br>
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Threadsafe: Yes<br>
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<!-- iddvc RMBR -->
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<br>
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</div>
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<p>The <strong>msgrcv()</strong> function reads a message from the message queue
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specified by the <em>msqid</em> parameter and
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places it in the user-defined buffer pointed to by the <em>*msgp</em> parameter.</p>
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<p>The <em>*msgp</em> parameter points to a user-defined buffer that must
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contain the following:</p>
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<ol>
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<li>A field of type <samp>long int</samp> that specifies the type of the
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message.</li>
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<li>A data part that contains the data bytes of the message.</li>
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</ol>
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<p>The following structure is an example of what this user-defined buffer might
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look like for a message that has 5 bytes of data:</p>
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<pre>
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struct mymsg {
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long int mtype; /* message type */
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char mtext[5]; /* message text */
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}
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</pre>
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<p>The value of <samp>mtype</samp> is the type of the received message,
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as specified by the sender of the message.</p>
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<p>The <em>msgtyp</em> parameter specifies the type of message to receive from
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the message queue as follows:</p>
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<ul>
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<li>If <em>msgtyp</em> is equal to zero, the first message is
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received.</li>
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<li>If <em>msgtyp</em> is greater than zero, the first message of type <em>
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msgtyp</em> is received.</li>
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<li>If <em>msgtyp</em> is less than zero, the first message of the lowest type
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that is less than or equal to the absolute value of <em>msgtyp</em> is
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received.</li>
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</ul>
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<p>The <em>msgsz</em> parameter specifies the size in bytes of the data part of the
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message. The received message is truncated to <em>msgsz</em> bytes if it
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is larger than <em>msgsz</em> and the MSG_NOERROR flag is set in the <em>msgflg</em>
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parameter. The truncated part of the message is lost and no indication of the
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truncation is given to the calling thread. Otherwise, <strong>msgrcv()</strong>
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returns with return value -1 and <em>errno</em> set to E2BIG.</p>
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<p>If a message of the desired type is not available on the message queue, the
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<em>msgflg</em> parameter specifies the action to be taken. The actions are as follows:</p>
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<br>
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<ul>
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<li>If the IPC_NOWAIT flag is set in the <em>msgflg</em> parameter, <strong>msgrcv()</strong>
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returns immediately with a return value of -1 and <em>errno</em> set to
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ENOMSG.<br>
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<br>
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</li>
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<li>If the IPC_NOWAIT flag is not set in the <em>msgflg</em> parameter, the calling thread suspends
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processing until one of the following occurs:<br><br>
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<ul>
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<li>A message of the desired type is sent to the message queue.</li>
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<li>The message queue identifier <em>msqid</em> is removed from the system.
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When this occurs, the <strong>msgrcv()</strong> function returns with a return value of -1 and
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<em>errno</em> set to EIDRM.</li>
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<li>A signal is delivered to the calling thread.
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When this occurs, the <strong>msgrcv()</strong> function returns with a return value of -1 and
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<em>errno</em> set to EINTR.</li>
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</ul>
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</ul>
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<p>The system maintains status information about a message queue which can be
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retrieved with the <a href="ipcmsgct.htm">msgctl()</a> function. When a message
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is successfully received from the message queue, the system sets the members of the
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<samp>msqid_ds</samp> structure as follows:</p>
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<br>
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<ul>
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<li><samp>msg_qnum</samp> is decremented by 1.<br>
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<br>
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</li>
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<li><samp>msg_lrpid</samp> is set to the process ID of the calling thread.<br>
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<br>
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</li>
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<li><samp>msg_rtime</samp> is set to the current time.</li>
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</ul>
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<h3>Parameters</h3>
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<dl>
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<dt><strong>msqid</strong></dt>
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<dd>(Input) Message queue identifier, a positive integer. It is returned by the
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<a href="ipcmsggt.htm">msgget()</a> function and used to identify the message queue
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to receive the message from.<br>
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<br>
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</dd>
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<dt><strong>msgp</strong></dt>
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<dd>(Output) Pointer to a buffer in which the received message will be stored.
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See above for the details on the format of the buffer.<br>
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<br>
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</dd>
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<dt><strong>msgsz</strong></dt>
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<dd>(Input) Length of the data part of the buffer.<br>
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<br>
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</dd>
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<dt><strong>msgtyp</strong></dt>
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<dd>(Input) Type of message to be received.<br>
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<br>
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</dd>
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<dt><strong>msgflg</strong></dt>
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<dd>(Input) Operations flags. The value of <em>msgflg</em> is either zero or is
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obtained by performing an OR operation on one or more of the following constants:
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<dl>
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<dt><strong>IPC_NOWAIT (0x00000800)</strong></dt>
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<dd>If a message is not available, do not wait for the message and return immediately.</dd>
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<dt><strong>MSG_NOERROR (0x00001000)</strong></dt>
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<dd>If the data part of the message is larger than <em>msgsz</em>, do not return an error.<p></dd>
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</dl>
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</dd>
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</dl>
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<br>
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<h3>Authorities</h3>
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<p><strong><a name="TBLAMRCV1">Authorization Required for
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msgrcv()</a></strong></p>
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<table border cellpadding="5">
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<!-- cols="60 20 20" -->
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<tr>
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<th align="left" valign="bottom">Object Referred to</th>
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<th align="left" valign="bottom">Authority Required</th>
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<th align="left" valign="bottom">errno</th>
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</tr>
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<tr>
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<td align="left" valign="top">Message queue from which message is received</td>
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<td align="left" valign="top">Read</td>
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<td align="left" valign="top">EACCES</td>
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</tr>
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</table>
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<br>
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<br>
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<h3>Return Value</h3>
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<table cellpadding="5">
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<!-- cols="15 85" -->
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<tr>
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<td align="left" valign="top"><em>value</em></td>
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<td align="left" valign="top"><strong>msgrcv()</strong> was successful. The
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value returned is the number of bytes of data placed in the data part of the buffer
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pointed to by the <em>msgp</em> parameter.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em>-1</em></td>
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<td align="left" valign="top"><strong>msgrcv()</strong> was not successful. The
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<em>errno</em> variable is set to indicate the error.</td>
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</tr>
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</table>
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<br>
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<br>
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<h3>Error Conditions</h3>
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<p>If <strong>msgrcv()</strong> is not successful, <em>errno</em> usually
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indicates one of the following errors. Under some conditions, <em>errno</em>
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could indicate an error other than those listed here.</p>
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<dl compact>
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<dt><em>[E2BIG]</em></dt>
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<dd>
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<p>Argument list too long.</p>
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<p>The size in bytes of the message is greater than <em>msgsz</em> and
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the MSG_NOERROR flag is not set in the <em>msgflg</em> parameter.</p>
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</dd>
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<dt><em>[EACCES]</em></dt>
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<dd>
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<p>Permission denied.</p>
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<p>An attempt was made to access an object in a way forbidden by its object
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access permissions.</p>
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<p>The thread does not have access to the specified file, directory, component,
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or path.</p>
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<p>The calling thread does not have read permission to the message queue.</p>
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</dd>
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<dt><em>[EDAMAGE]</em></dt>
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<dd>
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<p>A damaged object was encountered.</p>
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<p>A referenced object is damaged. The object cannot be used.</p>
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<p>The message queue has been damaged by a previous message queue
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operation.</p>
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</dd>
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<dt><em>[EFAULT]</em></dt>
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<dd>
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<p>The address used for an argument is not correct.</p>
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<p>In attempting to use an argument in a call, the system detected an address
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that is not valid.</p>
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<p>While attempting to access a parameter passed to this function, the system
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detected an address that is not valid.</p>
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</dd>
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<dt><em>[EIDRM]</em></dt>
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<dd>
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<p>ID has been removed.</p>
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<p>The message queue identifier <em>msqid</em> was removed from the system.</p>
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</dd>
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<dt><em>[EINTR]</em></dt>
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<dd>
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<p>Interrupted function call.</p>
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<p>The function <strong>msgrcv()</strong> was interrupted by a signal.</p>
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</dd>
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<dt><em>[EINVAL]</em></dt>
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<dd>
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<p>The value specified for the argument is not correct.</p>
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<p>A function was passed incorrect argument values, or an operation was
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attempted on an object and the operation specified is not supported for that
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type of object.</p>
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<p>An argument value is not valid, out of range, or NULL.</p>
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<p>One of the following has occurred:</p>
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<ul>
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<li>The <em>*msgp</em> parameter is NULL.</li>
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<li>The <em>msqid</em> parameter is not a valid message queue identifier.</li>
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</ul>
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</dd>
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<dt><em>[ENOMSG]</em></dt>
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<dd>
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<p>Message does not exist.</p>
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<p>The message queue does not contain a message of the desired type and the
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IPC_NOWAIT flag is set in the <em>msgflg</em> parameter.</p>
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</dd>
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<dt><em>[EUNKNOWN]</em></dt>
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<dd>
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<p>Unknown system state.</p>
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<p>The operation failed because of an unknown system state. See any messages in
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the job log and correct any errors that are indicated, then retry the
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operation.</p>
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</dd>
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</dl>
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<br>
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||
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<h3>Error Messages</h3>
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<p>None.</p>
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<br>
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<h3><a name="USAGE_NOTES">Usage Notes</a></h3>
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<ol>
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<li>The parameter <em>msgsz</em> should include any bytes inserted by the
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compiler for padding or alignment purposes. These bytes are part of the message
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data and affect the total number of bytes in the message queue.</li>
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<li>The following example shows pad data and how it affects the size of a
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message using datamodel *P128:</li>
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<li><pre>
|
||
|
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struct mymsg {
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long int mtype; /* 12 bytes padding inserted after */
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char *pointer; /* the mtype field by the compiler.*/
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char c; /* 15 bytes padding inserted after */
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char *pointer2; /* the c field by the compiler. */
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} msg; /* After the mtype field, there are*/
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/* 33 bytes of user data, but 60 */
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/* bytes of data including padding.*/
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msgsz = sizeof(msg) - sizeof(long int); /* 60 bytes. */
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|
|
</pre>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>The <strong>msgrcv()</strong> function does not tag message data with a
|
||
|
|
CCSID (coded character set identifier) value. If a CCSID value is required to
|
||
|
|
correctly interpret the message data, it is the responsibility of the caller to
|
||
|
|
include the CCSID value as part of the data.<br>
|
||
|
|
<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li>If the <strong>msgrcv()</strong> function does not complete successfully,
|
||
|
|
the requested message is not removed from the message queue.</li>
|
||
|
|
</ol>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
<h3>Related Information</h3>
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>The <<strong>sys/msg.h</strong>> file (see <a href="unix13.htm">
|
||
|
|
Header Files for UNIX-Type Functions</a>)<br>
|
||
|
|
<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="unix3a.htm#messagequeue">Message Queues</a> for the current system limits.
|
||
|
|
<br><br></li>
|
||
|
|
|
||
|
|
<li><a href="ipcmsgct.htm">msgctl()</a>--Perform Message Control Operations<br>
|
||
|
|
<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="ipcmsggt.htm">msgget()</a>--Get Message Queue<br>
|
||
|
|
<br>
|
||
|
|
</li>
|
||
|
|
|
||
|
|
<li><a href="ipcmsgsn.htm">msgsnd()</a>--Send Message Operation</li>
|
||
|
|
|
||
|
|
<li><a href="sigactn.htm">sigaction()</a>--Examine and change signal action<p></li>
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
<h3>Example</h3>
|
||
|
|
|
||
|
|
<p>See <a href="../apiref/aboutapis.htm#codedisclaimer">Code disclaimer information</a>
|
||
|
|
for information pertaining to code examples.</p>
|
||
|
|
|
||
|
|
<p>The following example receives a message from a message queue:</p>
|
||
|
|
|
||
|
|
<pre>
|
||
|
|
#include <sys/msg.h>
|
||
|
|
|
||
|
|
main() {
|
||
|
|
int msqid = 2;
|
||
|
|
int rc;
|
||
|
|
size_t msgsz;
|
||
|
|
long int msgtyp;
|
||
|
|
struct mymsg {
|
||
|
|
long int mtype;
|
||
|
|
char mtext[256];
|
||
|
|
};
|
||
|
|
|
||
|
|
msgsz = sizeof(struct mymsg) - sizeof(long int);
|
||
|
|
msgtyp = 1;
|
||
|
|
rc = msgrcv(msqid, &mymsg, msgsz, msgtyp, IPC_NOWAIT);
|
||
|
|
}
|
||
|
|
</pre>
|
||
|
|
|
||
|
|
<br>
|
||
|
|
<hr>
|
||
|
|
API introduced: V3R6
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<center>
|
||
|
|
<table cellpadding="2" cellspacing="2">
|
||
|
|
<tr align="center">
|
||
|
|
<td valign="middle" align="center"><a href="#Top_Of_Page">Top</a> | <a href=
|
||
|
|
"unix.htm">UNIX-Type APIs</a> | <a href="aplist.htm">APIs by category</a></td>
|
||
|
|
</tr>
|
||
|
|
</table>
|
||
|
|
</center>
|
||
|
|
</body>
|
||
|
|
</html>
|
||
|
|
|