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424 lines
12 KiB
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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>poll()--Wait for Events on Multiple Descriptors</title>
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<!--End Header Records --><!-- Edited by Kersten Feb 02 -->
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<a name="Top_Of_Page"></a>
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<h2><img src="delta.gif" alt="Start of change">
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poll()--Wait for Events on Multiple Descriptors</h2>
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<div class="box" style="width: 60%;">
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<br>
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Syntax<br>
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<pre>
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#include <sys/poll.h>
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int poll(struct pollfd <em>fds[]</em>,
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nfds_t <em>nfds</em>,
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int <em>timeout</em>)
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</pre>
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<br>
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Service Program Name: QSOSRV1<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: Conditional; see <a href="#USAGE_NOTES">Usage Notes</a>.<br>
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<!-- iddvc RMBR -->
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<br>
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</div>
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<p>The <em>poll()</em> function is used to enable an application to multiplex
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I/O over a set of descriptors. For each member of the array pointed to
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by <em>fds,
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</em> <em>poll()</em> will examine the given descriptor for
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the event(s) specified. <em>nfds</em> is
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the number of <strong>pollfd</strong> structures in the <em>fds</em> array.
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The <em>poll()</em> function will determine which descriptors can
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read or write data, or whether certain events have occured on the
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descriptors within the <em>timeout</em> period.</p>
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<br>
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<h3>Parameters</h3>
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<dl>
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<dt><strong>fds</strong></dt>
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<dd>
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<br>
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(Input) Specifies an array of descriptors to be examined and the events of
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interest for each descriptor.
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<pre>
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struct pollfd {
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int fd; /* Descriptor */
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short events; /* Requested events */
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short revents; /* Returned events */
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};
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</pre>
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The array's members are <strong>pollfd</strong> structures within which
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<em>fd</em> specifies an open descriptor. The <em>events</em> and
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<em>revents</em> are bitmasks constructed by OR'ing a combination of the
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following event flags.<br>
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<br>
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The following events can be specified in the <em>events</em> field:
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<table cellpadding="5">
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<tr>
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<td align="left" valign="top"><em><samp>POLLIN</samp></em></td>
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<td align="left" valign="top">Data may be read without blocking.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em><samp>POLLPRI</samp></em></td>
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<td align="left" valign="top">High-priority(OOB) data may be read without blocking.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em><samp>POLLOUT</samp></em></td>
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<td align="left" valign="top">Data may be written without blocking.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em><samp>POLLWRNORM</samp></em></td>
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<td align="left" valign="top">Equivalent to POLLOUT.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em><samp>POLLRDNORM</samp></em></td>
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<td align="left" valign="top">Equivalent to POLLIN.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em><samp>POLLNORM</samp></em></td>
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<td align="left" valign="top">Equivalent to POLLIN.</td>
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</tr>
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</table>
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<br>
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Each of the <em>events</em> listed above may be returned in the <em>revents</em> field
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when the <em>poll()</em> API completes if that requested condition is valid for the
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specified descriptor. In addition to these events, the following event may also be
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returned in the <em>revents</em> field:
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<table cellpadding="5">
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<tr>
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<td align="left" valign="top"><em><samp>POLLNVAL</samp></em></td>
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<td align="left" valign="top">The specified <em>fd</em> value is invalid. This
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flag is ignored if it is specified in the <em>events</em> field.</td>
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</tr>
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</table>
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<br>
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The following events will never be returned in the <em>revents</em> field on the iSeries:
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<table cellpadding="5">
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<tr>
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<td align="left" valign="top"><em><samp>POLLRDBAND</samp></em></td>
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<td align="left" valign="top">Priority message ready to read.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em><samp>POLLWRBAND</samp></em></td>
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<td align="left" valign="top">Writable priority band exists.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em><samp>POLLERR</samp></em></td>
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<td align="left" valign="top">An error occurred.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em><samp>POLLHUP</samp></em></td>
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<td align="left" valign="top">Connection disconnected.</td>
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</tr>
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</table>
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<br>
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In each <strong>pollfd</strong> structure,
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<em>poll()</em> will clear the <em>revents</em> member, except that where
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the application requested a report on a condition by setting one of the
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bits of <em>events</em>, <em>poll()</em> will set the corresponding
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bit in <em>revents</em> if the requested condition is true. In addition,
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<em>poll()</em> shall set the POLLNVAL flag in <em>revents</em> if the
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condition is true, even if the application did not set the corresponding
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bit in events.<br>
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<br>
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</dd>
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<dt><strong>nfds</strong></dt>
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<dd>(Input) <em>nfds</em> is the number of <strong>pollfd</strong> structures
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in the <em>fds</em> array.<br>
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<br>
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</dd>
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<dt><strong>timeout</strong></dt>
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<dd>(Input) The <em>timeout</em> argument specifies how long <em>poll()</em>
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is to wait before returning. If none of the requested events have occurred on
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any of the descriptors, <em>poll()</em> will wait at least
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<em>timeout</em> milliseconds for an event to occur on any of the
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descriptors. If the value of <em>timeout</em> is 0, <em>poll()</em> will return
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immediately. If the value of <em>timeout</em> is -1, <em>poll()</em> will block
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until a requested event occurs or until the call is interrupted.
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<br>
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<br>
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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>No authorization is required.</p>
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<br>
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<h3>Return Value</h3>
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<p><em>poll()</em> returns an integer. Possible values are:</p>
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<ul>
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<li>-1 (unsuccessful)<br>
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<br>
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</li>
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<li>0 (the <em>timeout</em> expired before any of the requested <em>events</em> were
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satisfied)<br>
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<br>
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</li>
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<li>n (total number of descriptors that met poll criteria)</li>
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</ul>
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<br>
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<h3>Error Conditions</h3>
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<p>When <em>poll()</em> fails, <em>errno</em> can be set to one of the
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following:</p>
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<table cellpadding="5">
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<tr>
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<td align="left" valign="top"><em>[EINVAL]</em></td>
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<td align="left" valign="top">The <em>timeout</em> argument was less than -1.
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<br>
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</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em>[ENOTSAFE]</em></td>
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<td align="left" valign="top">Function not allowed.
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<br>
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</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em>[EINTR]</em></td>
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<td align="left" valign="top">A signal was caught during the <em>poll()</em>.
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<br>
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</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em>[EFAULT]</em></td>
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<td align="left" valign="top">The address used for an argument was not valid.
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<br>
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</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em>[EUNKNOWN]</em></td>
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<td align="left" valign="top">Unknown system state.
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<br>
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</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em>[ENOTSUP]</em></td>
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<td align="left" valign="top">Operation not supported.
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<br>
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</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 Messages</h3>
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<table cellpadding="5">
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<tr>
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<td align="left" valign="top">CPE3418 E</td>
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<td align="left" valign="top">Possible APAR condition or hardware failure.</td>
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</tr>
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<tr>
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<td align="left" valign="top">CPF9872 E</td>
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<td align="left" valign="top">Program or service program &1 in library
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&2 ended. Reason code &3.</td>
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</tr>
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<tr>
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<td align="left" valign="top">CPFA081 E</td>
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<td align="left" valign="top">Unable to set return value or error code.</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><a name="USAGE_NOTES">Usage Notes</a></h3>
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<ol>
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<li>When specifying the POLLIN flag for <em>events</em>,
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the following can be indicated:
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<ul>
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<li>Data is available to be read<br>
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<br>
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</li>
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<li>An error event exists on the descriptor.<br>
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<br>
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</li>
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<li>
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A socket descriptor that is listening for connections will indicate that
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it is ready for reading, once connections are available.<br>
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<br>
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</li>
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<li>No data can be read from the underlying instance represented by the
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descriptor. For example, a socket descriptor for which a <em>shutdown()</em>
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call has been done to disable the reception of data.<br>
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<br>
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</li>
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</ul>
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<br>
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<br>
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</li>
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<li>When specifying the POLLOUT flag for <em>events</em>,
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the following can be indicated:
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<ul>
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<li>When a <em>write()</em> can be successfully issued without blocking (or,
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for nonblocking, so it does not return <samp>[EWOULDBLOCK]</samp>).<br>
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<br>
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</li>
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<li>Completion of a non-blocking <em>connect()</em> call on a socket
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descriptor. This allows an application to set a socket descriptor to
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nonblocking (with <em>fcntl()</em> or <em>ioctl()</em>), issue a
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<em>connect()</em> and receive <samp>[EINPROGRESS]</samp>, and then use
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<em>poll()</em> to verify that the connection has completed.<br>
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<br>
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</li>
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<li>No data can be written to the underlying instance represented by the
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descriptor (for example, a socket descriptor for which a <em>shutdown()</em>
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has been done to disable the sending of data).<br>
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<br>
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</li>
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</ul>
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<br>
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<br>
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</li>
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<li>If the <em>revents</em> field is set to POLLPRI
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when the <em>poll()</em> API completes, this indicates that out-of-band data
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has arrived on the descriptor specified by the <em>fd</em> field.
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This is only supported for connection-oriented sockets with an address
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family of <samp>AF_INET</samp> or <samp>AF_INET6</samp>.
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<br>
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<br></li>
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<li>The <em>poll()</em> API will not be affected by the O_NONBLOCK flag.
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<br>
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<br></li>
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<li>The <em>poll()</em> API is more efficient than the <em>select()</em> API and
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therefore <em>poll()</em> is always recommended over <em>select()</em>.
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<br>
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<br></li>
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<li>The timeout mechanism is different between <em>poll()</em> and <em>select()</em>.
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The <em>poll()</em> API uses an integer with the unit of measure as milliseconds.
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The <em>select()</em> API uses a timeval structure.
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<br>
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<br></li>
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<li>
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Unpredictable results will appear if
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this function or any of its associated type and macro definitions are used
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in a thread executing one of the scan-related exit programs
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(or any of its' created threads). See
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<a href="ifsopenexit.htm">Integrated File System Scan on Open Exit Programs</a> and
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<a href="ifscloseexit.htm">Integrated File System Scan on Close Exit Programs</a>
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for more information.
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<br>
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<br>
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<br>
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</li>
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</ol>
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<h3>Related Information</h3>
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<ul>
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<li><a href="sselect.htm">select()</a>--Wait for events on Multiple
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Sockets</li>
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</ul>
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<img src="deltaend.gif" alt="End of change">
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<hr>
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<br>
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