[ViewVC] Diff of: cvs/libev/ev.html

Comparing libev/ev.html (file contents):
Revision 1.70 by root, Sat Dec 8 14:27:39 2007 UTC vs.
Revision 1.77 by root, Wed Dec 12 04:53:58 2007 UTC

 <head>
 	<title>libev</title>
 	<meta name="description" content="Pod documentation for libev" />
 	<meta name="inputfile" content="&lt;standard input&gt;" />
 	<meta name="outputfile" content="&lt;standard output&gt;" />
-	<meta name="created" content="Sat Dec  8 15:27:35 2007" />
+	<meta name="created" content="Wed Dec 12 05:53:55 2007" />
 	<meta name="generator" content="Pod::Xhtml 1.57" />
 <link rel="stylesheet" href="http://res.tst.eu/pod.css"/></head>
 <body>
 <div class="pod">
 <!-- INDEX START -->
 <ul><li><a href="#GENERIC_WATCHER_FUNCTIONS">GENERIC WATCHER FUNCTIONS</a></li>
 <li><a href="#ASSOCIATING_CUSTOM_DATA_WITH_A_WATCH">ASSOCIATING CUSTOM DATA WITH A WATCHER</a></li>
 </ul>
 </li>
 <li><a href="#WATCHER_TYPES">WATCHER TYPES</a>
-<ul><li><a href="#code_ev_io_code_is_this_file_descrip"><code>ev_io</code> - is this file descriptor readable or writable?</a></li>
+<ul><li><a href="#code_ev_io_code_is_this_file_descrip"><code>ev_io</code> - is this file descriptor readable or writable?</a>
+<ul><li><a href="#The_special_problem_of_disappearing_">The special problem of disappearing file descriptors</a></li>
+</ul>
+</li>
 <li><a href="#code_ev_timer_code_relative_and_opti"><code>ev_timer</code> - relative and optionally repeating timeouts</a></li>
 <li><a href="#code_ev_periodic_code_to_cron_or_not"><code>ev_periodic</code> - to cron or not to cron?</a></li>
 <li><a href="#code_ev_signal_code_signal_me_when_a"><code>ev_signal</code> - signal me when a signal gets signalled!</a></li>
 <li><a href="#code_ev_child_code_watch_out_for_pro"><code>ev_child</code> - watch out for process status changes</a></li>
 <li><a href="#code_ev_stat_code_did_the_file_attri"><code>ev_stat</code> - did the file attributes just change?</a></li>
 you actually want to know.</p>
 	</dd>
 	<dt>int ev_version_major ()</dt>
 	<dt>int ev_version_minor ()</dt>
 	<dd>
-		<p>You can find out the major and minor version numbers of the library
+		<p>You can find out the major and minor ABI version numbers of the library
 you linked against by calling the functions <code>ev_version_major</code> and
 <code>ev_version_minor</code>. If you want, you can compare against the global
 symbols <code>EV_VERSION_MAJOR</code> and <code>EV_VERSION_MINOR</code>, which specify the
 version of the library your program was compiled against.</p>
+		<p>These version numbers refer to the ABI version of the library, not the
+release version.</p>
 		<p>Usually, it's a good idea to terminate if the major versions mismatch,
-as this indicates an incompatible change.  Minor versions are usually
+as this indicates an incompatible change. Minor versions are usually
 compatible to older versions, so a larger minor version alone is usually
 not a problem.</p>
 		<p>Example: Make sure we haven't accidentally been linked against the wrong
 version.</p>
 <pre>  assert ((&quot;libev version mismatch&quot;,
 one iteration of the loop. This is useful if you are waiting for some
 external event in conjunction with something not expressible using other
 libev watchers. However, a pair of <code>ev_prepare</code>/<code>ev_check</code> watchers is
 usually a better approach for this kind of thing.</p>
 		<p>Here are the gory details of what <code>ev_loop</code> does:</p>
+<pre>   - Before the first iteration, call any pending watchers.
-<pre>   * If there are no active watchers (reference count is zero), return.
+   * If there are no active watchers (reference count is zero), return.
-   - Queue prepare watchers and then call all outstanding watchers.
+   - Queue all prepare watchers and then call all outstanding watchers.
    - If we have been forked, recreate the kernel state.
    - Update the kernel state with all outstanding changes.
    - Update the &quot;event loop time&quot;.
    - Calculate for how long to block.
    - Block the process, waiting for any events.
 <p>If you cannot run the fd in non-blocking mode (for example you should not
 play around with an Xlib connection), then you have to seperately re-test
 whether a file descriptor is really ready with a known-to-be good interface
 such as poll (fortunately in our Xlib example, Xlib already does this on
 its own, so its quite safe to use).</p>
+</div>
+<h3 id="The_special_problem_of_disappearing_">The special problem of disappearing file descriptors</h3>
+<div id="The_special_problem_of_disappearing_-2">
+<p>Some backends (e.g kqueue, epoll) need to be told about closing a file
+descriptor (either by calling <code>close</code> explicitly or by any other means,
+such as <code>dup</code>). The reason is that you register interest in some file
+descriptor, but when it goes away, the operating system will silently drop
+this interest. If another file descriptor with the same number then is
+registered with libev, there is no efficient way to see that this is, in
+fact, a different file descriptor.</p>
+<p>To avoid having to explicitly tell libev about such cases, libev follows
+the following policy:  Each time <code>ev_io_set</code> is being called, libev
+will assume that this is potentially a new file descriptor, otherwise
+it is assumed that the file descriptor stays the same. That means that
+you <i>have</i> to call <code>ev_io_set</code> (or <code>ev_io_init</code>) when you change the
+descriptor even if the file descriptor number itself did not change.</p>
+<p>This is how one would do it normally anyway, the important point is that
+the libev application should not optimise around libev but should leave
+optimisations to libev.</p>
 <dl>
 	<dt>ev_io_init (ev_io *, callback, int fd, int events)</dt>
 	<dt>ev_io_set (ev_io *, int fd, int events)</dt>
 	<dd>
 		<p>Configures an <code>ev_io</code> watcher. The <code>fd</code> is the file descriptor to
 but on wallclock time (absolute time). You can tell a periodic watcher
 to trigger &quot;at&quot; some specific point in time. For example, if you tell a
 periodic watcher to trigger in 10 seconds (by specifiying e.g. <code>ev_now ()
 + 10.</code>) and then reset your system clock to the last year, then it will
 take a year to trigger the event (unlike an <code>ev_timer</code>, which would trigger
-roughly 10 seconds later and of course not if you reset your system time
+roughly 10 seconds later).</p>
-again).</p>
 <p>They can also be used to implement vastly more complex timers, such as
-triggering an event on eahc midnight, local time.</p>
+triggering an event on each midnight, local time or other, complicated,
+rules.</p>
 <p>As with timers, the callback is guarenteed to be invoked only when the
 time (<code>at</code>) has been passed, but if multiple periodic timers become ready
 during the same loop iteration then order of execution is undefined.</p>
 <dl>
 	<dt>ev_periodic_init (ev_periodic *, callback, ev_tstamp at, ev_tstamp interval, reschedule_cb)</dt>
 	<dd>
 		<p>Lots of arguments, lets sort it out... There are basically three modes of
 operation, and we will explain them from simplest to complex:</p>
 		<p>
 			<dl>
-				<dt>* absolute timer (interval = reschedule_cb = 0)</dt>
+				<dt>* absolute timer (at = time, interval = reschedule_cb = 0)</dt>
 				<dd>
 					<p>In this configuration the watcher triggers an event at the wallclock time
 <code>at</code> and doesn't repeat. It will not adjust when a time jump occurs,
 that is, if it is to be run at January 1st 2011 then it will run when the
 system time reaches or surpasses this time.</p>
 				</dd>
-				<dt>* non-repeating interval timer (interval &gt; 0, reschedule_cb = 0)</dt>
+				<dt>* non-repeating interval timer (at = offset, interval &gt; 0, reschedule_cb = 0)</dt>
 				<dd>
 					<p>In this mode the watcher will always be scheduled to time out at the next
-<code>at + N * interval</code> time (for some integer N) and then repeat, regardless
+<code>at + N * interval</code> time (for some integer N, which can also be negative)
-of any time jumps.</p>
+and then repeat, regardless of any time jumps.</p>
 					<p>This can be used to create timers that do not drift with respect to system
 time:</p>
 <pre>   ev_periodic_set (&amp;periodic, 0., 3600., 0);
 </pre>
 full hour (UTC), or more correctly, when the system time is evenly divisible
 by 3600.</p>
 					<p>Another way to think about it (for the mathematically inclined) is that
 <code>ev_periodic</code> will try to run the callback in this mode at the next possible
 time where <code>time = at (mod interval)</code>, regardless of any time jumps.</p>
+					<p>For numerical stability it is preferable that the <code>at</code> value is near
+<code>ev_now ()</code> (the current time), but there is no range requirement for
+this value.</p>
 				</dd>
-				<dt>* manual reschedule mode (reschedule_cb = callback)</dt>
+				<dt>* manual reschedule mode (at and interval ignored, reschedule_cb = callback)</dt>
 				<dd>
 					<p>In this mode the values for <code>interval</code> and <code>at</code> are both being
 ignored. Instead, each time the periodic watcher gets scheduled, the
 reschedule callback will be called with the watcher as first, and the
 current time as second argument.</p>
 					<p>NOTE: <i>This callback MUST NOT stop or destroy any periodic watcher,
 ever, or make any event loop modifications</i>. If you need to stop it,
 return <code>now + 1e30</code> (or so, fudge fudge) and stop it afterwards (e.g. by
-starting a prepare watcher).</p>
+starting an <code>ev_prepare</code> watcher, which is legal).</p>
 					<p>Its prototype is <code>ev_tstamp (*reschedule_cb)(struct ev_periodic *w,
 ev_tstamp now)</code>, e.g.:</p>
 <pre>   static ev_tstamp my_rescheduler (struct ev_periodic *w, ev_tstamp now)
    {
      return now + 60.;
 	<dd>
 		<p>Simply stops and restarts the periodic watcher again. This is only useful
 when you changed some parameters or the reschedule callback would return
 a different time than the last time it was called (e.g. in a crond like
 program when the crontabs have changed).</p>
+	</dd>
+	<dt>ev_tstamp offset [read-write]</dt>
+	<dd>
+		<p>When repeating, this contains the offset value, otherwise this is the
+absolute point in time (the <code>at</code> value passed to <code>ev_periodic_set</code>).</p>
+		<p>Can be modified any time, but changes only take effect when the periodic
+timer fires or <code>ev_periodic_again</code> is being called.</p>
 	</dd>
 	<dt>ev_tstamp interval [read-write]</dt>
 	<dd>
 		<p>The current interval value. Can be modified any time, but changes only
 take effect when the periodic timer fires or <code>ev_periodic_again</code> is being
 are ready to run (it's actually more complicated: it only runs coroutines
 with priority higher than or equal to the event loop and one coroutine
 of lower priority, but only once, using idle watchers to keep the event
 loop from blocking if lower-priority coroutines are active, thus mapping
 low-priority coroutines to idle/background tasks).</p>
+<p>It is recommended to give <code>ev_check</code> watchers highest (<code>EV_MAXPRI</code>)
+priority, to ensure that they are being run before any other watchers
+after the poll. Also, <code>ev_check</code> watchers (and <code>ev_prepare</code> watchers,
+too) should not activate (&quot;feed&quot;) events into libev. While libev fully
+supports this, they will be called before other <code>ev_check</code> watchers did
+their job. As <code>ev_check</code> watchers are often used to embed other event
+loops those other event loops might be in an unusable state until their
+<code>ev_check</code> watcher ran (always remind yourself to coexist peacefully with
+others).</p>
 <dl>
 	<dt>ev_prepare_init (ev_prepare *, callback)</dt>
 	<dt>ev_check_init (ev_check *, callback)</dt>
 	<dd>
 		<p>Initialises and configures the prepare or check watcher - they have no
 parameters of any kind. There are <code>ev_prepare_set</code> and <code>ev_check_set</code>
 macros, but using them is utterly, utterly and completely pointless.</p>
 	</dd>
 </dl>
-<p>Example: To include a library such as adns, you would add IO watchers
+<p>There are a number of principal ways to embed other event loops or modules
-and a timeout watcher in a prepare handler, as required by libadns, and
+into libev. Here are some ideas on how to include libadns into libev
+(there is a Perl module named <code>EV::ADNS</code> that does this, which you could
+use for an actually working example. Another Perl module named <code>EV::Glib</code>
+embeds a Glib main context into libev, and finally, <code>Glib::EV</code> embeds EV
+into the Glib event loop).</p>
+<p>Method 1: Add IO watchers and a timeout watcher in a prepare handler,
-in a check watcher, destroy them and call into libadns. What follows is
+and in a check watcher, destroy them and call into libadns. What follows
-pseudo-code only of course:</p>
+is pseudo-code only of course. This requires you to either use a low
+priority for the check watcher or use <code>ev_clear_pending</code> explicitly, as
+the callbacks for the IO/timeout watchers might not have been called yet.</p>
 <pre>  static ev_io iow [nfd];
   static ev_timer tw;
   static void
   io_cb (ev_loop *loop, ev_io *w, int revents)
   {
-    // set the relevant poll flags
-    // could also call adns_processreadable etc. here
-    struct pollfd *fd = (struct pollfd *)w-&gt;data;
-    if (revents &amp; EV_READ ) fd-&gt;revents |= fd-&gt;events &amp; POLLIN;
-    if (revents &amp; EV_WRITE) fd-&gt;revents |= fd-&gt;events &amp; POLLOUT;
   }
   // create io watchers for each fd and a timer before blocking
   static void
   adns_prepare_cb (ev_loop *loop, ev_prepare *w, int revents)
     /* the callback is illegal, but won't be called as we stop during check */
     ev_timer_init (&amp;tw, 0, timeout * 1e-3);
     ev_timer_start (loop, &amp;tw);
-    // create on ev_io per pollfd
+    // create one ev_io per pollfd
     for (int i = 0; i &lt; nfd; ++i)
       {
         ev_io_init (iow + i, io_cb, fds [i].fd,
           ((fds [i].events &amp; POLLIN ? EV_READ : 0)
            | (fds [i].events &amp; POLLOUT ? EV_WRITE : 0)));
         fds [i].revents = 0;
-        iow [i].data = fds + i;
         ev_io_start (loop, iow + i);
       }
   }
   // stop all watchers after blocking
   adns_check_cb (ev_loop *loop, ev_check *w, int revents)
   {
     ev_timer_stop (loop, &amp;tw);
     for (int i = 0; i &lt; nfd; ++i)
+      {
+        // set the relevant poll flags
+        // could also call adns_processreadable etc. here
+        struct pollfd *fd = fds + i;
+        int revents = ev_clear_pending (iow + i);
+        if (revents &amp; EV_READ ) fd-&gt;revents |= fd-&gt;events &amp; POLLIN;
+        if (revents &amp; EV_WRITE) fd-&gt;revents |= fd-&gt;events &amp; POLLOUT;
+        // now stop the watcher
-      ev_io_stop (loop, iow + i);
+        ev_io_stop (loop, iow + i);
+      }
     adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
+  }
+</pre>
+<p>Method 2: This would be just like method 1, but you run <code>adns_afterpoll</code>
+in the prepare watcher and would dispose of the check watcher.</p>
+<p>Method 3: If the module to be embedded supports explicit event
+notification (adns does), you can also make use of the actual watcher
+callbacks, and only destroy/create the watchers in the prepare watcher.</p>
+<pre>  static void
+  timer_cb (EV_P_ ev_timer *w, int revents)
+  {
+    adns_state ads = (adns_state)w-&gt;data;
+    update_now (EV_A);
+    adns_processtimeouts (ads, &amp;tv_now);
+  }
+  static void
+  io_cb (EV_P_ ev_io *w, int revents)
+  {
+    adns_state ads = (adns_state)w-&gt;data;
+    update_now (EV_A);
+    if (revents &amp; EV_READ ) adns_processreadable  (ads, w-&gt;fd, &amp;tv_now);
+    if (revents &amp; EV_WRITE) adns_processwriteable (ads, w-&gt;fd, &amp;tv_now);
+  }
+  // do not ever call adns_afterpoll
+</pre>
+<p>Method 4: Do not use a prepare or check watcher because the module you
+want to embed is too inflexible to support it. Instead, youc na override
+their poll function.  The drawback with this solution is that the main
+loop is now no longer controllable by EV. The <code>Glib::EV</code> module does
+this.</p>
+<pre>  static gint
+  event_poll_func (GPollFD *fds, guint nfds, gint timeout)
+  {
+    int got_events = 0;
+    for (n = 0; n &lt; nfds; ++n)
+      // create/start io watcher that sets the relevant bits in fds[n] and increment got_events
+    if (timeout &gt;= 0)
+      // create/start timer
+    // poll
+    ev_loop (EV_A_ 0);
+    // stop timer again
+    if (timeout &gt;= 0)
+      ev_timer_stop (EV_A_ &amp;to);
+    // stop io watchers again - their callbacks should have set
+    for (n = 0; n &lt; nfds; ++n)
+      ev_io_stop (EV_A_ iow [n]);
+    return got_events;
   }

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Comparing libev/ev.html (file contents): Revision 1.70 by root, Sat Dec 8 14:27:39 2007 UTC vs. Revision 1.77 by root, Wed Dec 12 04:53:58 2007 UTC

Diff Legend

Comparing libev/ev.html (file contents):
Revision 1.70 by root, Sat Dec 8 14:27:39 2007 UTC vs.
Revision 1.77 by root, Wed Dec 12 04:53:58 2007 UTC