*** empty log message ***

1 files changed, 186 insertions, 157 deletions

diff --git a/ev.pod b/ev.pod
index 4d470a2..63c37a5 100644
--- a/ev.pod
+++ b/ev.pod
@@ -508,7 +508,9 @@ 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
@@ -1616,26 +1618,19 @@ 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
-lot of those (for example Solaris ports), it is very easy to get into
-this situation even with a relatively standard program structure. Thus
-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.
+because there is no data. It is very easy to get into this situation even
+with a relatively standard program structure. Thus 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.
 
 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
-does this on its own, so its quite safe to use). Some people additionally
+interface such as poll (fortunately in the case of Xlib, it already does
+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.
 
@@ -1673,16 +1668,48 @@ 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
-C<ev_default_fork ()> or C<ev_loop_fork ()> after a fork in the child,
-enable C<EVFLAG_FORKCHECK>, or resort to C<EVBACKEND_SELECT> or
-C<EVBACKEND_POLL>.
+To support fork in your child processes, you have to call C<ev_loop_fork
+()> after a fork in the child, enable C<EVFLAG_FORKCHECK>, or resort to
+C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
 
 =head3 The special problem of SIGPIPE
 
@@ -3472,6 +3499,144 @@ To exit from any of these loops, just set the corresponding exit variable:
    // 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
 
 
@@ -4473,143 +4638,7 @@ watcher callback into the event loop interested in the signal.
 
 =back
 
-=head4 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.
+See also L<Thread locking example>.
 
 =head3 COROUTINES