lib/AnyEvent/FAQ.pod

=head1 NAME

AnyEvent::FAQ - frequently asked questions

=head1 FAQs

The newest version of this document can be found at
L<http://pod.tst.eu/http://cvs.schmorp.de/AnyEvent/lib/AnyEvent/FAQ.pod>.

=head2 My program exits before doing anything, what's going on?

Programmers new to event-based programming often forget that you can
actually do other stuff while "waiting" for an event to occur and
therefore forget to actually wait when they do not, in fact, have anything
else to do.

Here is an example:

   use AnyEvent;

   my $timer = AnyEvent->timer (after => 5, cb => sub { say "hi" });

The expectation might be for the program to print "hi" after 5 seconds
and then probably to exit. However, if you run this, your program will
exit almost instantly: Creating the timer does not wait for it, instead
the C<timer> method returns immediately and perl executes the rest of the
program. But there is nothing left to execute, so perl exits.

To force AnyEvent to wait for something, use a condvar:

   use AnyEvent;

   my $quit_program = AnyEvent->condvar;
   my $timer = AnyEvent->timer (after => 5, cb => sub { $quit_program->send });

   $quit_program->recv;

Here the program doesn't immediately exit, because it first waits for
the "quit_program" condition.

In most cases, your main program should call the event library "loop"
function directly:

   use EV;
   use AnyEvent;

   ...

   EV::loop;

=head2 Why is my C<tcp_connect> callback never called?

Tricky: C<tcp_connect> (and a few other functions in L<AnyEvent::Socket>)
is critically sensitive to the caller context.

In void context, it will just do its thing and eventually call the
callback. In any other context, however, it will return a special "guard"
object - when it is destroyed (e.g. when you don't store it but throw it
away), tcp_connect will no longer try to connect or call any callbacks.

Often this happens when the C<tcp_connect> call is at the end of a function:

   sub do_connect {
      tcp_connect "www.example.com", 80, sub {
         ... lengthy code
      };
   }

Then the caller decides whether there is a void context or not. One can
avoid these cases by explicitly returning nothing:

   sub do_connect {
      tcp_connect "www.example.com", 80, sub {
         ... lengthy code
      };

      () # return nothing
   }

=head2 Why do some backends use a lot of CPU in C<< AE::cv->recv >>?

Many people try out this simple program, or its equivalent:

   use AnyEvent;
   AnyEvent->condvar->recv;

They are then shocked to see that this basically idles with the Perl
backend, but uses 100% CPU with the EV backend, which is supposed to be
sooo efficient.

The key to understand this is to understand that the above program
is actually I<buggy>: Nothing calls C<< ->send >> on the condvar,
ever. Worse, there are no event watchers whatsoever. Basically, it creates
a deadlock: there is no way to make progress, this program doesn't do
anything useful, and this will not change in the future: it is already an
ex-parrot.

Some backends react to this by freezing, some by idling, and some do a
100% CPU loop.

Since this program is not useful (and behaves as documented with all
backends, as AnyEvent makes no CPU time guarantees), this shouldn't be a
big deal: as soon as your program actually implements I<something>, the
CPU usage will be normal.

=head2 Why does this FAQ not deal with L<AnyEvent::Handle> questions?

Because L<AnyEvent::Handle> has a NONFAQ on its own that already deals
with common issues.

=head2 How can I combine L<Win32::GUI> applications with AnyEvent?

Well, not in the same OS thread, that's for sure :) What you can do is
create another ithread (or fork) and run AnyEvent inside that thread, or
better yet, run all your GUI code in a second ithread.

For example, you could load L<Win32::GUI> and L<AnyEvent::Util>, then
create a portable socketpair for GUI->AnyEvent communication.

Then fork/create a new ithread, in there, create a Window and send the C<<
$WINDOW->{-Handle} >> to the AnyEvent ithread so it can C<PostMessage>.

GUI to AnyEvent communication could work by pushing some data into a
L<Thread::Queue> and writing a byte into the socket. The AnyEvent watcher
on the other side will then look at the queue.

AnyEvent to GUI communications can also use a L<Thread::Queue>, but to
wake up the GUI thread, it would instead use C<< Win32::GUI::PostMessage
$WINDOW, 1030, 0, "" >>, and the GUI thread would listen for these
messages by using C<< $WINDOW->Hook (1030 (), sub { ... }) >>.

=head2 My callback dies and...

It must not - part of the contract betwene AnyEvent and user code is that
callbacks do not throw exceptions (and don't do even more evil things,
such as using C<last> outside a loop :). If your callback might die
sometimes, you need to use C<eval>.

If you want to track down such a case and you can reproduce it, you can
enable wrapping (by calling C<< L<AnyEvent::Debug>::wrap >> or by setting
C<PERL_ANYEVENT_DEBUG_WRAP=1> before starting your program). This will
wrap every callback into an eval and will report any exception complete
with a backtrace and some information about which watcher died, where it
was created and so on.

=head1 Author

Marc Lehmann <schmorp@schmorp.de>.

Revision:	1.10
Committed:	Sat Jul 7 14:03:05 2018 UTC (6 years, 5 months ago) by root
Branch:	MAIN
CVS Tags:	rel-7_16, rel-7_15, HEAD
Changes since 1.9:	+2 -0 lines
Log Message:	* empty log message *
#	Content
1	=head1 NAME
2
3	AnyEvent::FAQ - frequently asked questions
4
5	=head1 FAQs
6
7	The newest version of this document can be found at
8	L<http://pod.tst.eu/http://cvs.schmorp.de/AnyEvent/lib/AnyEvent/FAQ.pod>.
9
10	=head2 My program exits before doing anything, what's going on?
11
12	Programmers new to event-based programming often forget that you can
13	actually do other stuff while "waiting" for an event to occur and
14	therefore forget to actually wait when they do not, in fact, have anything
15	else to do.
16
17	Here is an example:
18
19	use AnyEvent;
20
21	my $timer = AnyEvent->timer (after => 5, cb => sub { say "hi" });
22
23	The expectation might be for the program to print "hi" after 5 seconds
24	and then probably to exit. However, if you run this, your program will
25	exit almost instantly: Creating the timer does not wait for it, instead
26	the C<timer> method returns immediately and perl executes the rest of the
27	program. But there is nothing left to execute, so perl exits.
28
29	To force AnyEvent to wait for something, use a condvar:
30
31	use AnyEvent;
32
33	my $quit_program = AnyEvent->condvar;
34	my $timer = AnyEvent->timer (after => 5, cb => sub { $quit_program->send });
35
36	$quit_program->recv;
37
38	Here the program doesn't immediately exit, because it first waits for
39	the "quit_program" condition.
40
41	In most cases, your main program should call the event library "loop"
42	function directly:
43
44	use EV;
45	use AnyEvent;
46
47	...
48
49	EV::loop;
50
51	=head2 Why is my C<tcp_connect> callback never called?
52
53	Tricky: C<tcp_connect> (and a few other functions in L<AnyEvent::Socket>)
54	is critically sensitive to the caller context.
55
56	In void context, it will just do its thing and eventually call the
57	callback. In any other context, however, it will return a special "guard"
58	object - when it is destroyed (e.g. when you don't store it but throw it
59	away), tcp_connect will no longer try to connect or call any callbacks.
60
61	Often this happens when the C<tcp_connect> call is at the end of a function:
62
63	sub do_connect {
64	tcp_connect "www.example.com", 80, sub {
65	... lengthy code
66	};
67	}
68
69	Then the caller decides whether there is a void context or not. One can
70	avoid these cases by explicitly returning nothing:
71
72	sub do_connect {
73	tcp_connect "www.example.com", 80, sub {
74	... lengthy code
75	};
76
77	() # return nothing
78	}
79
80	=head2 Why do some backends use a lot of CPU in C<< AE::cv->recv >>?
81
82	Many people try out this simple program, or its equivalent:
83
84	use AnyEvent;
85	AnyEvent->condvar->recv;
86
87	They are then shocked to see that this basically idles with the Perl
88	backend, but uses 100% CPU with the EV backend, which is supposed to be
89	sooo efficient.
90
91	The key to understand this is to understand that the above program
92	is actually I<buggy>: Nothing calls C<< ->send >> on the condvar,
93	ever. Worse, there are no event watchers whatsoever. Basically, it creates
94	a deadlock: there is no way to make progress, this program doesn't do
95	anything useful, and this will not change in the future: it is already an
96	ex-parrot.
97
98	Some backends react to this by freezing, some by idling, and some do a
99	100% CPU loop.
100
101	Since this program is not useful (and behaves as documented with all
102	backends, as AnyEvent makes no CPU time guarantees), this shouldn't be a
103	big deal: as soon as your program actually implements I<something>, the
104	CPU usage will be normal.
105
106	=head2 Why does this FAQ not deal with L<AnyEvent::Handle> questions?
107
108	Because L<AnyEvent::Handle> has a NONFAQ on its own that already deals
109	with common issues.
110
111	=head2 How can I combine L<Win32::GUI> applications with AnyEvent?
112
113	Well, not in the same OS thread, that's for sure :) What you can do is
114	create another ithread (or fork) and run AnyEvent inside that thread, or
115	better yet, run all your GUI code in a second ithread.
116
117	For example, you could load L<Win32::GUI> and L<AnyEvent::Util>, then
118	create a portable socketpair for GUI->AnyEvent communication.
119
120	Then fork/create a new ithread, in there, create a Window and send the C<<
121	$WINDOW->{-Handle} >> to the AnyEvent ithread so it can C<PostMessage>.
122
123	GUI to AnyEvent communication could work by pushing some data into a
124	L<Thread::Queue> and writing a byte into the socket. The AnyEvent watcher
125	on the other side will then look at the queue.
126
127	AnyEvent to GUI communications can also use a L<Thread::Queue>, but to
128	wake up the GUI thread, it would instead use C<< Win32::GUI::PostMessage
129	$WINDOW, 1030, 0, "" >>, and the GUI thread would listen for these
130	messages by using C<< $WINDOW->Hook (1030 (), sub { ... }) >>.
131
132	=head2 My callback dies and...
133
134	It must not - part of the contract betwene AnyEvent and user code is that
135	callbacks do not throw exceptions (and don't do even more evil things,
136	such as using C<last> outside a loop :). If your callback might die
137	sometimes, you need to use C<eval>.
138
139	If you want to track down such a case and you can reproduce it, you can
140	enable wrapping (by calling C<< L<AnyEvent::Debug>::wrap >> or by setting
141	C<PERL_ANYEVENT_DEBUG_WRAP=1> before starting your program). This will
142	wrap every callback into an eval and will report any exception complete
143	with a backtrace and some information about which watcher died, where it
144	was created and so on.
145
146	=head1 Author
147
148	Marc Lehmann <schmorp@schmorp.de>.
149