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

Comparing libev/ev.pod (file contents):
Revision 1.347 by sf-exg, Wed Nov 10 19:50:09 2010 UTC vs.
Revision 1.354 by root, Tue Jan 11 01:40:25 2011 UTC

    }
    ...
    ev_set_syserr_cb (fatal_error);
+=item ev_feed_signal (int signum)
+This function can be used to "simulate" a signal receive. It is completely
+safe to call this function at any time, from any context, including signal
+handlers or random threads.
+Its main use is to customise signal handling in your process, especially
+in the presence of threads. For example, you could block signals
+by default in all threads (and specifying C<EVFLAG_NOSIGMASK> when
+creating any loops), and in one thread, use C<sigwait> or any other
+mechanism to wait for signals, then "deliver" them to libev by calling
+C<ev_feed_signal>.
 =back
 =head1 FUNCTIONS CONTROLLING EVENT LOOPS
 An event loop is described by a C<struct ev_loop *> (the C<struct> is
 threads that are not interested in handling them.
 Signalfd will not be used by default as this changes your signal mask, and
 there are a lot of shoddy libraries and programs (glib's threadpool for
 example) that can't properly initialise their signal masks.
+=item C<EVFLAG_NOSIGMASK>
+When this flag is specified, then libev will avoid to modify the signal
+mask. Specifically, this means you ahve to make sure signals are unblocked
+when you want to receive them.
+This behaviour is useful when you want to do your own signal handling, or
+want to handle signals only in specific threads and want to avoid libev
+unblocking the signals.
+This flag's behaviour will become the default in future versions of libev.
 =item C<EVBACKEND_SELECT>  (value 1, portable select backend)
 This is your standard select(2) backend. Not I<completely> standard, as
 libev tries to roll its own fd_set with no limits on the number of fds,
 employing an additional generation counter and comparing that against the
 events to filter out spurious ones, recreating the set when required. Last
 not least, it also refuses to work with some file descriptors which work
 perfectly fine with C<select> (files, many character devices...).
-Epoll is truly the train wreck analog among event poll mechanisms.
+Epoll is truly the train wreck analog among event poll mechanisms,
+a frankenpoll, cobbled together in a hurry, no thought to design or
+interaction with others.
 While stopping, setting and starting an I/O watcher in the same iteration
 will result in some caching, there is still a system call per such
 incident (because the same I<file descriptor> could point to a different
 I<file description> now), so its best to avoid that. Also, C<dup ()>'ed
 =item C<EVBACKEND_PORT>    (value 32, Solaris 10)
 This uses the Solaris 10 event port mechanism. As with everything on Solaris,
 it's really slow, but it still scales very well (O(active_fds)).
-Please note that Solaris event ports can deliver a lot of spurious
-notifications, so you need to use non-blocking I/O or other means to avoid
-blocking when no data (or space) is available.
 While this backend scales well, it requires one system call per active
 file descriptor per loop iteration. For small and medium numbers of file
 descriptors a "slow" C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> backend
 might perform better.
-On the positive side, with the exception of the spurious readiness
+On the positive side, this backend actually performed fully to
-notifications, this backend actually performed fully to specification
-in all tests and is fully embeddable, which is a rare feat among the
+specification in all tests and is fully embeddable, which is a rare feat
-OS-specific backends (I vastly prefer correctness over speed hacks).
+among the OS-specific backends (I vastly prefer correctness over speed
+hacks).
+On the negative side, the interface is I<bizarre> - so bizarre that
+even sun itself gets it wrong in their code examples: The event polling
+function sometimes returning events to the caller even though an error
+occurred, but with no indication whether it has done so or not (yes, it's
+even documented that way) - deadly for edge-triggered interfaces where
+you absolutely have to know whether an event occurred or not because you
+have to re-arm the watcher.
+Fortunately libev seems to be able to work around these idiocies.
 This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
 C<EVBACKEND_POLL>.
 =item C<EVBACKEND_ALL>
 Try all backends (even potentially broken ones that wouldn't be tried
 with C<EVFLAG_AUTO>). Since this is a mask, you can do stuff such as
 C<EVBACKEND_ALL & ~EVBACKEND_KQUEUE>.
-It is definitely not recommended to use this flag.
+It is definitely not recommended to use this flag, use whatever
+C<ev_recommended_backends ()> returns, or simply do not specify a backend
+at all.
+=item C<EVBACKEND_MASK>
+Not a backend at all, but a mask to select all backend bits from a
+C<flags> value, in case you want to mask out any backends from a flags
+value (e.g. when modifying the C<LIBEV_FLAGS> environment variable).
 =back
 If one or more of the backend flags are or'ed into the flags value,
 then only these backends will be tried (in the reverse order as listed
 running when nothing else is active.
    ev_signal exitsig;
    ev_signal_init (&exitsig, sig_cb, SIGINT);
    ev_signal_start (loop, &exitsig);
-   evf_unref (loop);
+   ev_unref (loop);
 Example: For some weird reason, unregister the above signal handler again.
    ev_ref (loop);
    ev_signal_stop (loop, &exitsig);
 In general you can register as many read and/or write event watchers per
 fd as you want (as long as you don't confuse yourself). Setting all file
 descriptors to non-blocking mode is also usually a good idea (but not
 required if you know what you are doing).
-If you cannot use non-blocking mode, then force the use of a
-known-to-be-good backend (at the time of this writing, this includes only
-C<EVBACKEND_SELECT> and C<EVBACKEND_POLL>). The same applies to file
-descriptors for which non-blocking operation makes no sense (such as
-files) - libev doesn't guarantee any specific behaviour in that case.
 Another thing you have to watch out for is that it is quite easy to
-receive "spurious" readiness notifications, that is your callback might
+receive "spurious" readiness notifications, that is, your callback might
 be called with C<EV_READ> but a subsequent C<read>(2) will actually block
-because there is no data. Not only are some backends known to create a
+because there is no data. It is very easy to get into this situation even
-lot of those (for example Solaris ports), it is very easy to get into
+with a relatively standard program structure. Thus it is best to always
-this situation even with a relatively standard program structure. Thus
+use non-blocking I/O: An extra C<read>(2) returning C<EAGAIN> is far
-it is best to always use non-blocking I/O: An extra C<read>(2) returning
-C<EAGAIN> is far preferable to a program hanging until some data arrives.
+preferable to a program hanging until some data arrives.
 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 separately
 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
+interface such as poll (fortunately in the case of Xlib, it already does
-does this on its own, so its quite safe to use). Some people additionally
+this on its own, so its quite safe to use). Some people additionally
 use C<SIGALRM> and an interval timer, just to be sure you won't block
 indefinitely.
 But really, best use non-blocking mode.
 There is no workaround possible except not registering events
 for potentially C<dup ()>'ed file descriptors, or to resort to
 C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
+=head3 The special problem of files
+Many people try to use C<select> (or libev) on file descriptors
+representing files, and expect it to become ready when their program
+doesn't block on disk accesses (which can take a long time on their own).
+However, this cannot ever work in the "expected" way - you get a readiness
+notification as soon as the kernel knows whether and how much data is
+there, and in the case of open files, that's always the case, so you
+always get a readiness notification instantly, and your read (or possibly
+write) will still block on the disk I/O.
+Another way to view it is that in the case of sockets, pipes, character
+devices and so on, there is another party (the sender) that delivers data
+on it's own, but in the case of files, there is no such thing: the disk
+will not send data on it's own, simply because it doesn't know what you
+wish to read - you would first have to request some data.
+Since files are typically not-so-well supported by advanced notification
+mechanism, libev tries hard to emulate POSIX behaviour with respect
+to files, even though you should not use it. The reason for this is
+convenience: sometimes you want to watch STDIN or STDOUT, which is
+usually a tty, often a pipe, but also sometimes files or special devices
+(for example, C<epoll> on Linux works with F</dev/random> but not with
+F</dev/urandom>), and even though the file might better be served with
+asynchronous I/O instead of with non-blocking I/O, it is still useful when
+it "just works" instead of freezing.
+So avoid file descriptors pointing to files when you know it (e.g. use
+libeio), but use them when it is convenient, e.g. for STDIN/STDOUT, or
+when you rarely read from a file instead of from a socket, and want to
+reuse the same code path.
 =head3 The special problem of fork
 Some backends (epoll, kqueue) do not support C<fork ()> at all or exhibit
 useless behaviour. Libev fully supports fork, but needs to be told about
-it in the child.
+it in the child if you want to continue to use it in the child.
-To support fork in your programs, you either have to call
+To support fork in your child processes, you have to call C<ev_loop_fork
-C<ev_default_fork ()> or C<ev_loop_fork ()> after a fork in the child,
+()> after a fork in the child, enable C<EVFLAG_FORKCHECK>, or resort to
-enable C<EVFLAG_FORKCHECK>, or resort to C<EVBACKEND_SELECT> or
+C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
-C<EVBACKEND_POLL>.
 =head3 The special problem of SIGPIPE
 While not really specific to libev, it is easy to forget about C<SIGPIPE>:
 when writing to a pipe whose other end has been closed, your program gets
 I<has> to modify the signal mask, at least temporarily.
 So I can't stress this enough: I<If you do not reset your signal mask when
 you expect it to be empty, you have a race condition in your code>. This
 is not a libev-specific thing, this is true for most event libraries.
+=head3 The special problem of threads signal handling
+POSIX threads has problematic signal handling semantics, specifically,
+a lot of functionality (sigfd, sigwait etc.) only really works if all
+threads in a process block signals, which is hard to achieve.
+When you want to use sigwait (or mix libev signal handling with your own
+for the same signals), you can tackle this problem by globally blocking
+all signals before creating any threads (or creating them with a fully set
+sigprocmask) and also specifying the C<EVFLAG_NOSIGMASK> when creating
+loops. Then designate one thread as "signal receiver thread" which handles
+these signals. You can pass on any signals that libev might be interested
+in by calling C<ev_feed_signal>.
 =head3 Watcher-Specific Functions and Data Members
 =over 4
 it by calling C<ev_async_send>, which is thread- and signal safe.
 This functionality is very similar to C<ev_signal> watchers, as signals,
 too, are asynchronous in nature, and signals, too, will be compressed
 (i.e. the number of callback invocations may be less than the number of
-C<ev_async_sent> calls).
+C<ev_async_sent> calls). In fact, you could use signal watchers as a kind
+of "global async watchers" by using a watcher on an otherwise unused
+signal, and C<ev_feed_signal> to signal this watcher from another thread,
+even without knowing which loop owns the signal.
 Unlike C<ev_signal> watchers, C<ev_async> works with any event loop, not
 just the default loop.
 =head3 Queueing
 Feed an event on the given fd, as if a file descriptor backend detected
 the given events it.
 =item ev_feed_signal_event (loop, int signum)
-Feed an event as if the given signal occurred (C<loop> must be the default
+Feed an event as if the given signal occurred. See also C<ev_feed_signal>,
-loop!).
+which is async-safe.
 =back
 =head1 COMMON OR USEFUL IDIOMS (OR BOTH)
    // exit main program, after modal loop is finished
    exit_main_loop = 1;
    // exit both
    exit_main_loop = exit_nested_loop = 1;
+=item Thread locking example
+Here is a fictitious example of how to run an event loop in a different
+thread than where callbacks are being invoked and watchers are
+created/added/removed.
+For a real-world example, see the C<EV::Loop::Async> perl module,
+which uses exactly this technique (which is suited for many high-level
+languages).
+The example uses a pthread mutex to protect the loop data, a condition
+variable to wait for callback invocations, an async watcher to notify the
+event loop thread and an unspecified mechanism to wake up the main thread.
+First, you need to associate some data with the event loop:
+   typedef struct {
+     mutex_t lock; /* global loop lock */
+     ev_async async_w;
+     thread_t tid;
+     cond_t invoke_cv;
+   } userdata;
+   void prepare_loop (EV_P)
+   {
+      // for simplicity, we use a static userdata struct.
+      static userdata u;
+      ev_async_init (&u->async_w, async_cb);
+      ev_async_start (EV_A_ &u->async_w);
+      pthread_mutex_init (&u->lock, 0);
+      pthread_cond_init (&u->invoke_cv, 0);
+      // now associate this with the loop
+      ev_set_userdata (EV_A_ u);
+      ev_set_invoke_pending_cb (EV_A_ l_invoke);
+      ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
+      // then create the thread running ev_loop
+      pthread_create (&u->tid, 0, l_run, EV_A);
+   }
+The callback for the C<ev_async> watcher does nothing: the watcher is used
+solely to wake up the event loop so it takes notice of any new watchers
+that might have been added:
+   static void
+   async_cb (EV_P_ ev_async *w, int revents)
+   {
+      // just used for the side effects
+   }
+The C<l_release> and C<l_acquire> callbacks simply unlock/lock the mutex
+protecting the loop data, respectively.
+   static void
+   l_release (EV_P)
+   {
+     userdata *u = ev_userdata (EV_A);
+     pthread_mutex_unlock (&u->lock);
+   }
+   static void
+   l_acquire (EV_P)
+   {
+     userdata *u = ev_userdata (EV_A);
+     pthread_mutex_lock (&u->lock);
+   }
+The event loop thread first acquires the mutex, and then jumps straight
+into C<ev_run>:
+   void *
+   l_run (void *thr_arg)
+   {
+     struct ev_loop *loop = (struct ev_loop *)thr_arg;
+     l_acquire (EV_A);
+     pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
+     ev_run (EV_A_ 0);
+     l_release (EV_A);
+     return 0;
+   }
+Instead of invoking all pending watchers, the C<l_invoke> callback will
+signal the main thread via some unspecified mechanism (signals? pipe
+writes? C<Async::Interrupt>?) and then waits until all pending watchers
+have been called (in a while loop because a) spurious wakeups are possible
+and b) skipping inter-thread-communication when there are no pending
+watchers is very beneficial):
+   static void
+   l_invoke (EV_P)
+   {
+     userdata *u = ev_userdata (EV_A);
+     while (ev_pending_count (EV_A))
+       {
+         wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
+         pthread_cond_wait (&u->invoke_cv, &u->lock);
+       }
+   }
+Now, whenever the main thread gets told to invoke pending watchers, it
+will grab the lock, call C<ev_invoke_pending> and then signal the loop
+thread to continue:
+   static void
+   real_invoke_pending (EV_P)
+   {
+     userdata *u = ev_userdata (EV_A);
+     pthread_mutex_lock (&u->lock);
+     ev_invoke_pending (EV_A);
+     pthread_cond_signal (&u->invoke_cv);
+     pthread_mutex_unlock (&u->lock);
+   }
+Whenever you want to start/stop a watcher or do other modifications to an
+event loop, you will now have to lock:
+   ev_timer timeout_watcher;
+   userdata *u = ev_userdata (EV_A);
+   ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
+   pthread_mutex_lock (&u->lock);
+   ev_timer_start (EV_A_ &timeout_watcher);
+   ev_async_send (EV_A_ &u->async_w);
+   pthread_mutex_unlock (&u->lock);
+Note that sending the C<ev_async> watcher is required because otherwise
+an event loop currently blocking in the kernel will have no knowledge
+about the newly added timer. By waking up the loop it will pick up any new
+watchers in the next event loop iteration.
 =back
 =head1 LIBEVENT EMULATION
 default loop and triggering an C<ev_async> watcher from the default loop
 watcher callback into the event loop interested in the signal.
 =back
-=head4 THREAD LOCKING EXAMPLE
+See also L<Thread locking example>.
-Here is a fictitious example of how to run an event loop in a different
-thread than where callbacks are being invoked and watchers are
-created/added/removed.
-For a real-world example, see the C<EV::Loop::Async> perl module,
-which uses exactly this technique (which is suited for many high-level
-languages).
-The example uses a pthread mutex to protect the loop data, a condition
-variable to wait for callback invocations, an async watcher to notify the
-event loop thread and an unspecified mechanism to wake up the main thread.
-First, you need to associate some data with the event loop:
-   typedef struct {
-     mutex_t lock; /* global loop lock */
-     ev_async async_w;
-     thread_t tid;
-     cond_t invoke_cv;
-   } userdata;
-   void prepare_loop (EV_P)
-   {
-      // for simplicity, we use a static userdata struct.
-      static userdata u;
-      ev_async_init (&u->async_w, async_cb);
-      ev_async_start (EV_A_ &u->async_w);
-      pthread_mutex_init (&u->lock, 0);
-      pthread_cond_init (&u->invoke_cv, 0);
-      // now associate this with the loop
-      ev_set_userdata (EV_A_ u);
-      ev_set_invoke_pending_cb (EV_A_ l_invoke);
-      ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
-      // then create the thread running ev_loop
-      pthread_create (&u->tid, 0, l_run, EV_A);
-   }
-The callback for the C<ev_async> watcher does nothing: the watcher is used
-solely to wake up the event loop so it takes notice of any new watchers
-that might have been added:
-   static void
-   async_cb (EV_P_ ev_async *w, int revents)
-   {
-      // just used for the side effects
-   }
-The C<l_release> and C<l_acquire> callbacks simply unlock/lock the mutex
-protecting the loop data, respectively.
-   static void
-   l_release (EV_P)
-   {
-     userdata *u = ev_userdata (EV_A);
-     pthread_mutex_unlock (&u->lock);
-   }
-   static void
-   l_acquire (EV_P)
-   {
-     userdata *u = ev_userdata (EV_A);
-     pthread_mutex_lock (&u->lock);
-   }
-The event loop thread first acquires the mutex, and then jumps straight
-into C<ev_run>:
-   void *
-   l_run (void *thr_arg)
-   {
-     struct ev_loop *loop = (struct ev_loop *)thr_arg;
-     l_acquire (EV_A);
-     pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
-     ev_run (EV_A_ 0);
-     l_release (EV_A);
-     return 0;
-   }
-Instead of invoking all pending watchers, the C<l_invoke> callback will
-signal the main thread via some unspecified mechanism (signals? pipe
-writes? C<Async::Interrupt>?) and then waits until all pending watchers
-have been called (in a while loop because a) spurious wakeups are possible
-and b) skipping inter-thread-communication when there are no pending
-watchers is very beneficial):
-   static void
-   l_invoke (EV_P)
-   {
-     userdata *u = ev_userdata (EV_A);
-     while (ev_pending_count (EV_A))
-       {
-         wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
-         pthread_cond_wait (&u->invoke_cv, &u->lock);
-       }
-   }
-Now, whenever the main thread gets told to invoke pending watchers, it
-will grab the lock, call C<ev_invoke_pending> and then signal the loop
-thread to continue:
-   static void
-   real_invoke_pending (EV_P)
-   {
-     userdata *u = ev_userdata (EV_A);
-     pthread_mutex_lock (&u->lock);
-     ev_invoke_pending (EV_A);
-     pthread_cond_signal (&u->invoke_cv);
-     pthread_mutex_unlock (&u->lock);
-   }
-Whenever you want to start/stop a watcher or do other modifications to an
-event loop, you will now have to lock:
-   ev_timer timeout_watcher;
-   userdata *u = ev_userdata (EV_A);
-   ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
-   pthread_mutex_lock (&u->lock);
-   ev_timer_start (EV_A_ &timeout_watcher);
-   ev_async_send (EV_A_ &u->async_w);
-   pthread_mutex_unlock (&u->lock);
-Note that sending the C<ev_async> watcher is required because otherwise
-an event loop currently blocking in the kernel will have no knowledge
-about the newly added timer. By waking up the loop it will pick up any new
-watchers in the next event loop iteration.
 =head3 COROUTINES
 Libev is very accommodating to coroutines ("cooperative threads"):
 libev fully supports nesting calls to its functions from different

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Comparing libev/ev.pod (file contents): Revision 1.347 by sf-exg, Wed Nov 10 19:50:09 2010 UTC vs. Revision 1.354 by root, Tue Jan 11 01:40:25 2011 UTC

Diff Legend

Comparing libev/ev.pod (file contents):
Revision 1.347 by sf-exg, Wed Nov 10 19:50:09 2010 UTC vs.
Revision 1.354 by root, Tue Jan 11 01:40:25 2011 UTC