1AnyEvent::FAQ(3) User Contributed Perl Documentation AnyEvent::FAQ(3)
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6 AnyEvent::FAQ - frequently asked questions
7
9 The newest version of this document can be found at
10 <http://pod.tst.eu/http://cvs.schmorp.de/AnyEvent/lib/AnyEvent/FAQ.pod>.
11 My program exits before doing anything, what's going on?
13 Programmers new to event-based programming often forget that you can
14 actually do other stuff while "waiting" for an event to occur and
15 therefore forget to actually wait when they do not, in fact, have
16 anything else to do.
17 Here is an example:
19 use AnyEvent;
21 my $timer = AnyEvent->timer (after => 5, cb => sub { say "hi" });
23 The expectation might be for the program to print "hi" after 5 seconds
25 and then probably to exit. However, if you run this, your program will
26 exit almost instantly: Creating the timer does not wait for it, instead
27 the "timer" method returns immediately and perl executes the rest of
28 the program. But there is nothing left to execute, so perl exits.
29 To force AnyEvent to wait for something, use a condvar:
31 use AnyEvent;
33 my $quit_program = AnyEvent->condvar;
35 my $timer = AnyEvent->timer (after => 5, cb => sub { $quit_program->send });
36 $quit_program->recv;
38 Here the program doesn't immediately exit, because it first waits for
40 the "quit_program" condition.
41 In most cases, your main program should call the event library "loop"
43 function directly:
44 use EV;
46 use AnyEvent;
47 ...
49 EV::loop;
51 Why is my "tcp_connect" callback never called?
53 Tricky: "tcp_connect" (and a few other functions in AnyEvent::Socket)
54 is critically sensitive to the caller context.
55 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
58 "guard" object - when it is destroyed (e.g. when you don't store it but
59 throw it away), tcp_connect will no longer try to connect or call any
60 callbacks.
61 Often this happens when the "tcp_connect" call is at the end of a
63 function:
64 sub do_connect {
66 tcp_connect "www.example.com", 80, sub {
67 ... lengthy code
68 };
69 }
70 Then the caller decides whether there is a void context or not. One can
72 avoid these cases by explicitly returning nothing:
73 sub do_connect {
75 tcp_connect "www.example.com", 80, sub {
76 ... lengthy code
77 };
78 () # return nothing
80 }
81 Why do some backends use a lot of CPU in "AE::cv->recv"?
83 Many people try out this simple program, or its equivalent:
84 use AnyEvent;
86 AnyEvent->condvar->recv;
87 They are then shocked to see that this basically idles with the Perl
89 backend, but uses 100% CPU with the EV backend, which is supposed to be
90 sooo efficient.
91 The key to understand this is to understand that the above program is
93 actually buggy: Nothing calls "->send" on the condvar, ever. Worse,
94 there are no event watchers whatsoever. Basically, it creates a
95 deadlock: there is no way to make progress, this program doesn't do
96 anything useful, and this will not change in the future: it is already
97 an ex-parrot.
98 Some backends react to this by freezing, some by idling, and some do a
100 100% CPU loop.
101 Since this program is not useful (and behaves as documented with all
103 backends, as AnyEvent makes no CPU time guarantees), this shouldn't be
104 a big deal: as soon as your program actually implements something, the
105 CPU usage will be normal.
106 Why does this FAQ not deal with AnyEvent::Handle questions?
108 Because AnyEvent::Handle has a NONFAQ on its own that already deals
109 with common issues.
110 How can I combine Win32::GUI applications with AnyEvent?
112 Well, not in the same OS thread, that's for sure :) What you can do is
113 create another ithread (or fork) and run AnyEvent inside that thread,
114 or better yet, run all your GUI code in a second ithread.
115 For example, you could load Win32::GUI and AnyEvent::Util, then create
117 a portable socketpair for GUI->AnyEvent communication.
118 Then fork/create a new ithread, in there, create a Window and send the
120 "$WINDOW->{-Handle}" to the AnyEvent ithread so it can "PostMessage".
121 GUI to AnyEvent communication could work by pushing some data into a
123 Thread::Queue and writing a byte into the socket. The AnyEvent watcher
124 on the other side will then look at the queue.
125 AnyEvent to GUI communications can also use a Thread::Queue, but to
127 wake up the GUI thread, it would instead use "Win32::GUI::PostMessage
128 $WINDOW, 1030, 0, """, and the GUI thread would listen for these
129 messages by using "$WINDOW->Hook (1030 (), sub { ... })".
130 My callback dies and...
132 It must not - part of the contract betwene AnyEvent and user code is
133 that callbacks do not throw exceptions (and don't do even more evil
134 things, such as using "last" outside a loop :). If your callback might
135 die sometimes, you need to use "eval".
136 If you want to track down such a case and you can reproduce it, you can
138 enable wrapping (by calling "AnyEvent::Debug::wrap" or by setting
139 "PERL_ANYEVENT_DEBUG_WRAP=1" before starting your program). This will
140 wrap every callback into an eval and will report any exception complete
141 with a backtrace and some information about which watcher died, where
142 it was created and so on.
143
145 Marc Lehmann <schmorp@schmorp.de>.
146perl v5.32.0 2020-07-28 AnyEvent::FAQ(3)