143 lines
9.1 KiB
HTML
143 lines
9.1 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html lang="en-us" xml:lang="en-us">
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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="security" content="public" />
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<meta http-equiv="PICS-Label" content='(PICS-1.1 "http://www.icra.org/ratingsv02.html" l gen true r (cz 1 lz 1 nz 1 oz 1 vz 1) "http://www.rsac.org/ratingsv01.html" l gen true r (n 0 s 0 v 0 l 0) "http://www.classify.org/safesurf/" l gen true r (SS~~000 1))' />
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<meta name="DC.Type" content="concept" />
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<meta name="DC.Title" content="Signals" />
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<meta name="abstract" content="An application program can request to be notified asynchronously (request that the system send a signal) when a condition that the application is interested in occurs." />
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<meta name="description" content="An application program can request to be notified asynchronously (request that the system send a signal) when a condition that the application is interested in occurs." />
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<meta name="DC.Relation" scheme="URI" content="aconcepts.htm" />
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<meta name="DC.Relation" scheme="URI" content="coobd.htm" />
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<meta name="DC.Relation" scheme="URI" content="../apis/accept.htm" />
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<meta name="DC.Relation" scheme="URI" content="../apis/sendms.htm" />
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<meta name="DC.Relation" scheme="URI" content="../apis/writev.htm" />
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<meta name="DC.Relation" scheme="URI" content="../apis/read.htm" />
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<meta name="DC.Relation" scheme="URI" content="../apis/recv.htm" />
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<meta name="DC.Relation" scheme="URI" content="../apis/send.htm" />
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<meta name="DC.Relation" scheme="URI" content="../apis/sselect.htm" />
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<meta name="DC.Relation" scheme="URI" content="xsignals.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 2001, 2006" />
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<meta name="DC.Rights.Owner" content="(C) Copyright IBM Corporation 2001, 2006" />
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<meta name="DC.Format" content="XHTML" />
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<meta name="DC.Identifier" content="csignals" />
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<meta name="DC.Language" content="en-us" />
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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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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Signals</title>
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</head>
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<body id="csignals"><a name="csignals"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Signals</h1>
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<div><p>An application program can request to be notified asynchronously
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(request that the system send a <dfn class="term">signal</dfn>) when a condition that
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the application is interested in occurs.</p>
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<p>There are two asynchronous signals that sockets send to an application. </p>
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<ol><li><dfn class="term">SIGUR</dfn>G is a signal that is sent when out-of-band (OOB) data
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is received on a socket for which the concept of OOB data is supported. For
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example, a socket with an address family of AF_INET and a type of SOCK_STREAM
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can be conditioned to send a SIGURG signal.</li>
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<li><dfn class="term">SIGIO</dfn> is a signal that is sent when normal data, OOB data,
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error conditions, or just about anything happens on any type of socket.</li>
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</ol>
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<p>The application should ensure that it is able to handle receiving a signal
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before it requests the system to send signals. This is done by setting up <dfn class="term">signal
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handlers</dfn>. One way to set a signal handler is by issuing the <span class="apiname">sigaction()</span> call.</p>
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<p>An application requests the system to send the <dfn class="term">SIGURG</dfn> signal
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by one of the following methods: </p>
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<ul><li>Issuing a <span class="apiname">fcntl()</span> call and specifying a process ID
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or a process group ID with the F_SETOWN command.</li>
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<li>Issuing an <span class="apiname">ioctl()</span> call and specifying the FIOSETOWN
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or the SIOCSPGRP command (request).</li>
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</ul>
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<p>An application requests the system to send the <dfn class="term">SIGIO</dfn> signal
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in two steps. First it must set the process ID or the process group ID as
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previously described for the <dfn class="term">SIGURG</dfn> signal. This is to inform
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the system of where the application wants the signal to be delivered. Second,
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the application must do either of the following tasks: </p>
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<ul><li>Issue the <span class="apiname">fcntl()</span> call and specify the F_SETFL command
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with the FASYNC flag.</li>
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<li>Issue the <span class="apiname">ioctl()</span> call and specify the FIOASYNC command.</li>
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</ul>
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<p>This step requests the system to generate the SIGIO signal. Note that these
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steps can be done in any order. Also note that if an application issues these
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requests on a listening socket, the values set by the requests are inherited
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by all sockets that are returned to the application from the <span class="apiname">accept()</span> function.
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That is, newly accepted sockets also have the same process ID or process group
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ID as well as the same information with regard to sending the SIGIO signal.</p>
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<p>A socket can also generate synchronous signals on error conditions. Whenever
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an application receives [EPIPE] an <em>errno</em> on a socket function, a SIGPIPE
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signal is delivered to the process that issued the operation receiving the <em>errno</em> value.
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On a Berkeley Socket Distribution (BSD) implementation, by default the SIGPIPE
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signal ends the process that received the <em>errno</em> value. To remain compatible
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with previous releases of the i5/OS™ implementation, the i5/OS implementation
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uses a default behavior of ignoring for the SIGPIPE signal. This
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ensures that existing applications are not negatively affected by the addition
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of the signals function.</p>
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<div class="p">When a signal is delivered to a process that is blocked on a sockets function,
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the function returns from the wait with the <em>[EINTR] errno</em> value, allowing
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the application's signal handler to run. The functions for which this occur
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are: <ul><li><span class="apiname">accept()</span></li>
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<li><span class="apiname">connect()</span></li>
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<li><img src="./delta.gif" alt="Start of change" /><span class="apiname">poll()</span><img src="./deltaend.gif" alt="End of change" /></li>
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<li><span class="apiname">read()</span></li>
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<li><span class="apiname">readv()</span></li>
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<li><span class="apiname">recv()</span></li>
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<li><span class="apiname">recvfrom()</span></li>
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<li><span class="apiname">recvmsg()</span></li>
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<li><span class="apiname">select()</span></li>
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<li><span class="apiname">send()</span></li>
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<li><span class="apiname">sendto()</span></li>
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<li><span class="apiname">sendmsg()</span></li>
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<li><span class="apiname">write()</span></li>
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<li><span class="apiname">writev()</span></li>
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</ul>
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</div>
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<p>It is important to note that signals do not provide the
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application program with a socket descriptor that identifies where the condition
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being signalled actually exists. Thus, if the application program is using
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multiple socket descriptors, it must either poll the descriptors or use the <span class="apiname">select()</span> call
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to determine why the signal was received.</p>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="aconcepts.htm" title="Advanced socket concepts go beyond a general discussion of what sockets are and how they work. They provide ways to design socket applications for larger and more complex networks.">Advanced socket concepts</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="coobd.htm" title="Out-of-band (OOB) data is user-specific data that only has meaning for connection-oriented (stream) sockets.">Out-of-band data</a></div>
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</div>
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<div class="relref"><strong>Related reference</strong><br />
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<div><a href="xsignals.htm" title="Signals allow you to be notified when a process or application becomes blocked.">Example: Use signals with blocking socket APIs</a></div>
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</div>
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<div class="relinfo"><strong>Related information</strong><br />
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<div><a href="../apis/accept.htm">accept()</a></div>
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<div><a href="../apis/sendms.htm">sendmsg()</a></div>
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<div><a href="../apis/sendto.htm">sendto()</a></div>
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<div><a href="../apis/write.htm">write()</a></div>
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<div><a href="../apis/writev.htm">writev()</a></div>
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<div><a href="../apis/read.htm">read()</a></div>
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<div><a href="../apis/readv.htm">readv()</a></div>
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<div><a href="../apis/connec.htm">connect()</a></div>
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<div><a href="../apis/recvfr.htm">recvfrom()</a></div>
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<div><a href="../apis/recvms.htm">recvmsg()</a></div>
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<div><a href="../apis/recv.htm">recv()</a></div>
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<div><a href="../apis/send.htm">send()</a></div>
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<div><a href="../apis/sselect.htm">select()</a></div>
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</div>
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</div>
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</body>
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</html>
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