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[software/libev.git] / ev.pod

diff --git a/ev.pod b/ev.pod
index 62118f167549cea2539d0b591c0b01b7fbc5758b..ab57ffbd559a2819c7d245ec80c9900ebb82d558 100644 (file)
--- a/ev.pod
+++ b/ev.pod
@@ -119,14 +119,17 @@ you actually want to know.
 
 =item int ev_version_minor ()
 
 
 =item int ev_version_minor ()
 

-You can find out the major and minor version numbers of the library
+You can find out the major and minor ABI version numbers of the library

 you linked against by calling the functions C<ev_version_major> and
 C<ev_version_minor>. If you want, you can compare against the global
 symbols C<EV_VERSION_MAJOR> and C<EV_VERSION_MINOR>, which specify the
 version of the library your program was compiled against.
 
 you linked against by calling the functions C<ev_version_major> and
 C<ev_version_minor>. If you want, you can compare against the global
 symbols C<EV_VERSION_MAJOR> and C<EV_VERSION_MINOR>, which specify the
 version of the library your program was compiled against.
 

+These version numbers refer to the ABI version of the library, not the
+release version.
+

 Usually, it's a good idea to terminate if the major versions mismatch,
 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.
 
 compatible to older versions, so a larger minor version alone is usually
 not a problem.
 


@@ -488,8 +491,9 @@ usually a better approach for this kind of thing.
 
 Here are the gory details of what C<ev_loop> does:
 
 
 Here are the gory details of what C<ev_loop> does:
 

+   - Before the first iteration, call any pending watchers.

    * 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 "event loop time".
    - If we have been forked, recreate the kernel state.
    - Update the kernel state with all outstanding changes.
    - Update the "event loop time".


@@ -909,6 +913,28 @@ 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).
 
 such as poll (fortunately in our Xlib example, Xlib already does this on
 its own, so its quite safe to use).
 

+=head3 The special problem of disappearing file descriptors
+
+Some backends (e.g kqueue, epoll) need to be told about closing a file
+descriptor (either by calling C<close> explicitly or by any other means,
+such as C<dup>). 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.
+
+To avoid having to explicitly tell libev about such cases, libev follows
+the following policy:  Each time C<ev_io_set> 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> to call C<ev_io_set> (or C<ev_io_init>) when you change the
+descriptor even if the file descriptor number itself did not change.
+
+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.
+
+

 =over 4
 
 =item ev_io_init (ev_io *, callback, int fd, int events)
 =over 4
 
 =item ev_io_init (ev_io *, callback, int fd, int events)


@@ -1075,11 +1101,11 @@ to trigger "at" some specific point in time. For example, if you tell a
 periodic watcher to trigger in 10 seconds (by specifiying e.g. C<ev_now ()
 + 10.>) and then reset your system clock to the last year, then it will
 take a year to trigger the event (unlike an C<ev_timer>, which would trigger
 periodic watcher to trigger in 10 seconds (by specifiying e.g. C<ev_now ()
 + 10.>) and then reset your system clock to the last year, then it will
 take a year to trigger the event (unlike an C<ev_timer>, which would trigger

-roughly 10 seconds later and of course not if you reset your system time
-again).
+roughly 10 seconds later).

 
 They can also be used to implement vastly more complex timers, such as
 
 They can also be used to implement vastly more complex timers, such as

-triggering an event on eahc midnight, local time.
+triggering an event on each midnight, local time or other, complicated,
+rules.

 
 As with timers, the callback is guarenteed to be invoked only when the
 time (C<at>) has been passed, but if multiple periodic timers become ready
 
 As with timers, the callback is guarenteed to be invoked only when the
 time (C<at>) has been passed, but if multiple periodic timers become ready


@@ -1096,18 +1122,18 @@ operation, and we will explain them from simplest to complex:
 
 =over 4
 
 
 =over 4
 

-=item * absolute timer (interval = reschedule_cb = 0)
+=item * absolute timer (at = time, interval = reschedule_cb = 0)

 
 In this configuration the watcher triggers an event at the wallclock time
 C<at> 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.
 
 
 In this configuration the watcher triggers an event at the wallclock time
 C<at> 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.
 

-=item * non-repeating interval timer (interval > 0, reschedule_cb = 0)
+=item * non-repeating interval timer (at = offset, interval > 0, reschedule_cb = 0)

 
 In this mode the watcher will always be scheduled to time out at the next
 
 In this mode the watcher will always be scheduled to time out at the next

-C<at + N * interval> time (for some integer N) and then repeat, regardless
-of any time jumps.
+C<at + N * interval> time (for some integer N, which can also be negative)
+and then repeat, regardless of any time jumps.

 
 This can be used to create timers that do not drift with respect to system
 time:
 
 This can be used to create timers that do not drift with respect to system
 time:


@@ -1123,7 +1149,11 @@ Another way to think about it (for the mathematically inclined) is that
 C<ev_periodic> will try to run the callback in this mode at the next possible
 time where C<time = at (mod interval)>, regardless of any time jumps.
 
 C<ev_periodic> will try to run the callback in this mode at the next possible
 time where C<time = at (mod interval)>, regardless of any time jumps.
 

-=item * manual reschedule mode (reschedule_cb = callback)
+For numerical stability it is preferable that the C<at> value is near
+C<ev_now ()> (the current time), but there is no range requirement for
+this value.
+
+=item * manual reschedule mode (at and interval ignored, reschedule_cb = callback)

 
 In this mode the values for C<interval> and C<at> are both being
 ignored. Instead, each time the periodic watcher gets scheduled, the
 
 In this mode the values for C<interval> and C<at> are both being
 ignored. Instead, each time the periodic watcher gets scheduled, the


@@ -1133,7 +1163,7 @@ current time as second argument.
 NOTE: I<This callback MUST NOT stop or destroy any periodic watcher,
 ever, or make any event loop modifications>. If you need to stop it,
 return C<now + 1e30> (or so, fudge fudge) and stop it afterwards (e.g. by
 NOTE: I<This callback MUST NOT stop or destroy any periodic watcher,
 ever, or make any event loop modifications>. If you need to stop it,
 return C<now + 1e30> (or so, fudge fudge) and stop it afterwards (e.g. by

-starting a prepare watcher).
+starting an C<ev_prepare> watcher, which is legal).

 
 Its prototype is C<ev_tstamp (*reschedule_cb)(struct ev_periodic *w,
 ev_tstamp now)>, e.g.:
 
 Its prototype is C<ev_tstamp (*reschedule_cb)(struct ev_periodic *w,
 ev_tstamp now)>, e.g.:


@@ -1166,6 +1196,14 @@ 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).
 
 a different time than the last time it was called (e.g. in a crond like
 program when the crontabs have changed).
 

+=item ev_tstamp offset [read-write]
+
+When repeating, this contains the offset value, otherwise this is the
+absolute point in time (the C<at> value passed to C<ev_periodic_set>).
+
+Can be modified any time, but changes only take effect when the periodic
+timer fires or C<ev_periodic_again> is being called.
+

 =item ev_tstamp interval [read-write]
 
 The current interval value. Can be modified any time, but changes only
 =item ev_tstamp interval [read-write]
 
 The current interval value. Can be modified any time, but changes only


@@ -1483,6 +1521,16 @@ 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).
 
 loop from blocking if lower-priority coroutines are active, thus mapping
 low-priority coroutines to idle/background tasks).
 

+It is recommended to give C<ev_check> watchers highest (C<EV_MAXPRI>)
+priority, to ensure that they are being run before any other watchers
+after the poll. Also, C<ev_check> watchers (and C<ev_prepare> watchers,
+too) should not activate ("feed") events into libev. While libev fully
+supports this, they will be called before other C<ev_check> watchers did
+their job. As C<ev_check> watchers are often used to embed other event
+loops those other event loops might be in an unusable state until their
+C<ev_check> watcher ran (always remind yourself to coexist peacefully with
+others).
+

 =over 4
 
 =item ev_prepare_init (ev_prepare *, callback)
 =over 4
 
 =item ev_prepare_init (ev_prepare *, callback)


@@ -1495,10 +1543,18 @@ macros, but using them is utterly, utterly and completely pointless.
 
 =back
 
 
 =back
 

-Example: To include a library such as adns, you would add IO watchers
-and a timeout watcher in a prepare handler, as required by libadns, and
-in a check watcher, destroy them and call into libadns. What follows is
-pseudo-code only of course:
+There are a number of principal ways to embed other event loops or modules
+into libev. Here are some ideas on how to include libadns into libev
+(there is a Perl module named C<EV::ADNS> that does this, which you could
+use for an actually working example. Another Perl module named C<EV::Glib>
+embeds a Glib main context into libev, and finally, C<Glib::EV> embeds EV
+into the Glib event loop).
+
+Method 1: Add IO watchers and a timeout watcher in a prepare handler,
+and in a check watcher, destroy them and call into libadns. What follows
+is pseudo-code only of course. This requires you to either use a low
+priority for the check watcher or use C<ev_clear_pending> explicitly, as
+the callbacks for the IO/timeout watchers might not have been called yet.

 
   static ev_io iow [nfd];
   static ev_timer tw;
 
   static ev_io iow [nfd];
   static ev_timer tw;


@@ -1506,11 +1562,6 @@ pseudo-code only of course:
   static void
   io_cb (ev_loop *loop, ev_io *w, int revents)
   {
   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->data;
-    if (revents & EV_READ ) fd->revents |= fd->events & POLLIN;
-    if (revents & EV_WRITE) fd->revents |= fd->events & POLLOUT;

   }
 
   // create io watchers for each fd and a timer before blocking
   }
 
   // create io watchers for each fd and a timer before blocking


@@ -1526,7 +1577,7 @@ pseudo-code only of course:
     ev_timer_init (&tw, 0, timeout * 1e-3);
     ev_timer_start (loop, &tw);
 
     ev_timer_init (&tw, 0, timeout * 1e-3);
     ev_timer_start (loop, &tw);
 

-    // create on ev_io per pollfd
+    // create one ev_io per pollfd

     for (int i = 0; i < nfd; ++i)
       {
         ev_io_init (iow + i, io_cb, fds [i].fd,
     for (int i = 0; i < nfd; ++i)
       {
         ev_io_init (iow + i, io_cb, fds [i].fd,


@@ -1534,7 +1585,6 @@ pseudo-code only of course:
            | (fds [i].events & POLLOUT ? EV_WRITE : 0)));
 
         fds [i].revents = 0;
            | (fds [i].events & POLLOUT ? EV_WRITE : 0)));
 
         fds [i].revents = 0;

-        iow [i].data = fds + i;

         ev_io_start (loop, iow + i);
       }
   }
         ev_io_start (loop, iow + i);
       }
   }


@@ -1546,11 +1596,80 @@ pseudo-code only of course:
     ev_timer_stop (loop, &tw);
 
     for (int i = 0; i < nfd; ++i)
     ev_timer_stop (loop, &tw);
 
     for (int i = 0; i < nfd; ++i)

-      ev_io_stop (loop, iow + 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 & EV_READ ) fd->revents |= fd->events & POLLIN;
+        if (revents & EV_WRITE) fd->revents |= fd->events & POLLOUT;
+
+        // now stop the watcher
+        ev_io_stop (loop, iow + i);
+      }

 
     adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
   }
 
 
     adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
   }
 

+Method 2: This would be just like method 1, but you run C<adns_afterpoll>
+in the prepare watcher and would dispose of the check watcher.
+
+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.
+
+  static void
+  timer_cb (EV_P_ ev_timer *w, int revents)
+  {
+    adns_state ads = (adns_state)w->data;
+    update_now (EV_A);
+
+    adns_processtimeouts (ads, &tv_now);
+  }
+
+  static void
+  io_cb (EV_P_ ev_io *w, int revents)
+  {
+    adns_state ads = (adns_state)w->data;
+    update_now (EV_A);
+
+    if (revents & EV_READ ) adns_processreadable  (ads, w->fd, &tv_now);
+    if (revents & EV_WRITE) adns_processwriteable (ads, w->fd, &tv_now);
+  }
+
+  // do not ever call adns_afterpoll
+
+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 C<Glib::EV> module does
+this.
+
+  static gint
+  event_poll_func (GPollFD *fds, guint nfds, gint timeout)
+  {
+    int got_events = 0;
+
+    for (n = 0; n < nfds; ++n)
+      // create/start io watcher that sets the relevant bits in fds[n] and increment got_events
+
+    if (timeout >= 0)
+      // create/start timer
+
+    // poll
+    ev_loop (EV_A_ 0);
+
+    // stop timer again
+    if (timeout >= 0)
+      ev_timer_stop (EV_A_ &to);
+
+    // stop io watchers again - their callbacks should have set
+    for (n = 0; n < nfds; ++n)
+      ev_io_stop (EV_A_ iow [n]);
+
+    return got_events;
+  }
+

 
 =head2 C<ev_embed> - when one backend isn't enough...
 
 
 =head2 C<ev_embed> - when one backend isn't enough...