📄 poll.html

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html><head><!-- Copyright 1997 The Open Group, All Rights Reserved --><title>poll</title></head><body bgcolor=white><center><font size=2>The Single UNIX &reg; Specification, Version 2<br>Copyright &copy; 1997 The Open Group</font></center><hr size=2 noshade><h4><a name = "tag_000_007_988">&nbsp;</a>NAME</h4><blockquote>poll - input/output multiplexing</blockquote><h4><a name = "tag_000_007_989">&nbsp;</a>SYNOPSIS</h4><blockquote><pre><code>#include &lt;<a href="poll.h.html">poll.h</a>&gt;int poll(struct pollfd <i>fds</i>[], nfds_t <i>nfds</i>, int <i>timeout</i>);</code></pre></blockquote><h4><a name = "tag_000_007_990">&nbsp;</a>DESCRIPTION</h4><blockquote>The<i>poll()</i>function provides applications with a mechanism for multiplexing input/outputover a set of file descriptors.  For each member of the array pointed to by<i>fds</i>,<i>poll()</i>examines the given file descriptor for theevent(s) specified in <i>events</i>.The number of<b>pollfd</b>structures in the<i>fds</i>array is specified by <i>nfds</i>.  The<i>poll()</i>function identifies those file descriptors on which an application can read orwrite data, or on which certain events have occurred.<p>The <i>fds</i> argument specifies the file descriptors to be examined and theevents of interest for each file descriptor.  It is a pointer to an array withone member for each open file descriptor of interest.  The array's members are<b>pollfd</b> structures within which <b>fd</b> specifies an open filedescriptor and <b>events</b> and <b>revents</b> are bitmasks constructed byOR-ing a combination of the following event flags:<dl compact><dt>POLLIN<dd>Data other than high-priority data may be read without blocking.  For STREAMS,this flag is set in <b>revents</b> even if the message is of zero length.<dt>POLLRDNORM<dd>Normal data (priority band equals 0) may be read without blocking.  ForSTREAMS, this flag is set in <b>revents</b> even if the message is of zerolength.<dt>POLLRDBAND<dd>Data from a non-zero priority band may be read without blocking.  For STREAMS,this flag is set in <b>revents</b> even if the message is of zero length.<dt>POLLPRI<dd>High-priority data may be received without blocking.  For STREAMS, this flagis set in <b>revents</b> even if the message is of zero length.<dt>POLLOUT<dd>Normal data (priority band equals 0) may be written without blocking.<dt>POLLWRNORM<dd>Same as POLLOUT.<dt>POLLWRBAND<dd>Priority data (priority band &gt; 0) may be written.  This event onlyexamines bands that have been written to at least once.<dt>POLLERR<dd>An error has occurred on the device or stream.This flag is only valid in the<b>revents</b> bitmask; it is ignored in the <b>events</b> member.<dt>POLLHUP<dd>The device has been disconnected.  This event and POLLOUT are mutuallyexclusive; a stream can never be writable if a hangup has occurred.  However,this event and POLLIN, POLLRDNORM, POLLRDBAND or POLLPRI are not mutuallyexclusive.This flag is only valid in the <b>revents</b> bitmask; it isignored in the <b>events</b> member.<dt>POLLNVAL<dd>The specified <b>fd</b> value is invalid.This flag is only valid in the<b>revents</b> member; it is ignored in the <b>events</b> member.</dl><p>If the value of <b>fd</b> is less than 0, <b>events</b> is ignored and<b>revents</b> is set to 0 in that entry on return from<i>poll()</i>.<p>In each <b>pollfd</b> structure,<i>poll()</i>clears the <b>revents</b> member except that where the applicationrequested a report on a condition by setting one of the bits of<b>events</b> listed above,<i>poll()</i>sets the corresponding bit in <b>revents</b> if the requested condition is true.In addition,<i>poll()</i>sets the POLLHUP, POLLERR and POLLNVAL flag in <b>revents</b> if thecondition is true, even if the application did not set thecorresponding bit in <b>events</b>.<p>If none of the defined events have occurred on any selected file descriptor,<i>poll()</i>waits at least <i>timeout</i> milliseconds for an event to occur on any of theselected file descriptors.  If the value of <i>timeout</i> is 0,<i>poll()</i>returns immediately.  If the value of <i>timeout</i> is -1,<i>poll()</i>blocks until a requested event occurs or until the call is interrupted.<p>Implementations may place limitations on the granularity of timeoutintervals.  If the requested timeout interval requires a finer granularitythan the implementation supports, the actual timeout interval will be roundedup to the next supported value.<p>The<i>poll()</i>function is not affected by the O_NONBLOCK flag.<p>The<i>poll()</i>function supports regular files, terminal and pseudo-terminal devices,STREAMS-based files, FIFOs and pipes.  The behaviour of<i>poll()</i>on elements of<i>fds</i>that refer to other types of file is unspecified.<p>Regular files always poll TRUE for reading and writing.</blockquote><h4><a name = "tag_000_007_991">&nbsp;</a>RETURN VALUE</h4><blockquote>Upon successful completion,<i>poll()</i>returns a non-negative value.  A positive value indicates the total number offile descriptors that have been selected (that is, file descriptors for whichthe <b>revents</b> member is non-zero).  A value of 0 indicates that the calltimed out and no file descriptors have been selected.  Upon failure,<i>poll()</i>returns -1 and sets <i>errno</i> to indicate the error.</blockquote><h4><a name = "tag_000_007_992">&nbsp;</a>ERRORS</h4><blockquote>The<i>poll()</i>function will fail if:<dl compact><dt>[EAGAIN]<dd>The allocation of internal data structures failed but a subsequent requestmay succeed.<dt>[EINTR]<dd>A signal was caught during<i>poll()</i>.<dt>[EINVAL]<dd>The <i>nfds</i> argument is greater than {OPEN_MAX},or one of the <b>fd</b> members refers to a STREAM or multiplexer that islinked (directly or indirectly) downstream from a multiplexer.</dl></blockquote><h4><a name = "tag_000_007_993">&nbsp;</a>EXAMPLES</h4><blockquote>None.</blockquote><h4><a name = "tag_000_007_994">&nbsp;</a>APPLICATION USAGE</h4><blockquote>None.</blockquote><h4><a name = "tag_000_007_995">&nbsp;</a>FUTURE DIRECTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_000_007_996">&nbsp;</a>SEE ALSO</h4><blockquote><i><a href="getmsg.html">getmsg()</a></i>,<i><a href="putmsg.html">putmsg()</a></i>,<i><a href="read.html">read()</a></i>,<i><a href="select.html">select()</a></i>,<i><a href="write.html">write()</a></i>,<i><a href="poll.h.html">&lt;poll.h&gt;</a></i>,<i><a href="stropts.h.html">&lt;stropts.h&gt;</a></i>,<a href="STREAMS.html">STREAMS overview</a>.</blockquote><hr size=2 noshade><center><font size=2>UNIX &reg; is a registered Trademark of The Open Group.<br>Copyright &copy; 1997 The Open Group<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]</font></center><hr size=2 noshade></body></html>
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