156 lines
6.0 KiB
HTML
156 lines
6.0 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>Message Queue Functions</title>
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<h2><a name="messagequeue">Message Queue Functions</a></h2>
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<p><strong>Message queues</strong> provide a form of message passing in which
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any process (given that it has the necessary permissions) can read a message
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from or write a message to any IPC message queue on the system. There are no
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requirements that a process be waiting to receive a message from a queue before
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another process sends one, or that a message exist on the queue before a
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process requests to receive one.</p>
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<p>Every message on a queue has the following attributes:</p>
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<ul>
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<li>Message type</li>
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<li>Message length (length of data part of message)</li>
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<li>Message data (if length is greater than 0)</li>
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</ul>
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<p>A thread gets a message queue identifier by calling the <strong>
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msgget()</strong> function. Depending on the <em>key</em> and <em>msgflg</em>
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parameters passed in, either a new message queue is created or an existing
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message queue is accessed. When a new message queue is created, a data
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structure is also created to contain information about the message queue. This
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structure is defined in the <strong><sys/msg.h></strong> header file as
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follows:</p>
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<pre>
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typedef struct msqid_ds {
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struct ipc_perm msg_perm; /* Operation permission struct */
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msgqnum_t msg_qnum; /* # msgs currently on queue */
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msglen_t msg_qbytes; /* Max # bytes allowed on queue*/
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pid_t msg_lspid; /* Process ID of last msgsnd() */
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pid_t msg_lrpid; /* Process ID of last msgrcv() */
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time_t msg_stime; /* Time of last msgsnd() */
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time_t msg_rtime; /* Time of last msgrcv() */
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time_t msg_ctime; /* Time of last change */
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} msqid_ds_t;
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</pre>
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<p>A thread puts a message on a message queue by calling the <strong>
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msgsnd()</strong> function. The following parameters are passed in:</p>
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<ul>
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<li>Message queue ID</li>
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<li>Pointer to a buffer containing the message type and message data</li>
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<li>Length of the message</li>
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<li>Flag that specifies whether or not the thread is willing to wait to send
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the message.</li>
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</ul>
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<p>A thread gets a message from a message queue by calling the <strong>
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msgrcv()</strong> function. The following parameters are passed in:</p>
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<ul>
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<li>Message queue ID</li>
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<li>Pointer to a buffer in which to receive the message</li>
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<li>Length of the buffer</li>
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<li>Type of message</li>
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<li>Flag that specifies whether or not the thread is willing to wait and
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whether or not the thread is willing to truncate a message to receive it</li>
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</ul>
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<p>A thread removes a message queue ID by calling the <strong>msgctl()</strong>
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function. The thread also can use the <strong>msgctl()</strong> function to
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change the data structure values associated with the message queue ID or to
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retrieve the data structure values associated with the message queue ID. The
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following parameters are passed in:</p>
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<ul>
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<li>Message queue ID</li>
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<li>Command the thread wants to perform (remove ID, set data structure values,
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receive data structure values)</li>
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<li>Pointer to a buffer from which to set data structure values or in which to
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receive data structure values</li>
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</ul>
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<br>
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<h4>Message Queue Differences and Restrictions</h4>
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<p>i5/OS message queues differ from the message queue definition in the
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Single UNIX Specification in the following ways:</p>
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<ul>
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<li>The maximum message size is 65535 bytes.</li>
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<li>The maximum number of bytes on a message queue is
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16 777 216.</li>
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<li>The maximum number of message queues that can be created (system-wide) is
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2 147 483 646.</li>
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</ul>
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<br>
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<p>The message queue functions are:</p>
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<!-- ***** NOTE ***** Do not manually update text or links in this section. -->
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<!-- Updates made in this section *will* be overlaid by automated tools -->
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<!-- Notify User Technologies of needed updates to be made in XML for API finder.-->
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<!--***************API BEGIN PASTE***************-->
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<ul>
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<li><A HREF="ipcmsgct.htm">msgctl()</A> (Perform Message Control Operations) provides message control operations as specified by cmd on the message queue specified by msqid.</li>
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<li><A HREF="ipcmsggt.htm">msgget()</A> (Get Message Queue) returns the message queue identifier associated with the parameter key.</li>
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<li><A HREF="ipcmsgrc.htm">msgrcv()</A> (Receive Message Operation) reads a message from the queue associated with the message queue identifier specified by msqid and places it in the user-defined buffer pointed to by msgp.</li>
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<li><A HREF="ipcmsgsn.htm">msgsnd()</A> (Send Message Operation) is used to send a message to the queue associated with the message queue identifier specified by msqid.</li>
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</ul>
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<!--***************API END PASTE***************-->
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<p>See also <a href="unix3a4.htm">IPC Key Generation Functions</a> for additional message queue functions.</p>
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<br>
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<hr>
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<center>
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<table cellpadding="2" cellspacing="2">
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<tr align="center">
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<td valign="middle" align="center"><a href="#Top_Of_Page">Top</a> | <a href=
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"unix.htm">UNIX-Type APIs</a> | <a href="aplist.htm">APIs by category</a></td>
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</tr>
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</table>
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</center>
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</body>
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</html>
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