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

Comparing libev/ev.pod (file contents):
Revision 1.337 by root, Sun Oct 31 20:20:20 2010 UTC vs.
Revision 1.350 by sf-exg, Mon Jan 10 08:36:41 2011 UTC

 the current system, you would need to look at C<ev_embeddable_backends ()
 & ev_supported_backends ()>, likewise for recommended ones.
 See the description of C<ev_embed> watchers for more info.
-=item ev_set_allocator (void *(*cb)(void *ptr, long size)) [NOT REENTRANT]
+=item ev_set_allocator (void *(*cb)(void *ptr, long size))
 Sets the allocation function to use (the prototype is similar - the
 semantics are identical to the C<realloc> C89/SuS/POSIX function). It is
 used to allocate and free memory (no surprises here). If it returns zero
 when memory needs to be allocated (C<size != 0>), the library might abort
    }
    ...
    ev_set_allocator (persistent_realloc);
-=item ev_set_syserr_cb (void (*cb)(const char *msg)); [NOT REENTRANT]
+=item ev_set_syserr_cb (void (*cb)(const char *msg))
 Set the callback function to call on a retryable system call error (such
 as failed select, poll, epoll_wait). The message is a printable string
 indicating the system call or subsystem causing the problem. If this
 callback is set, then libev will expect it to remedy the situation, no
    }
    ...
    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
 =item struct ev_loop *ev_loop_new (unsigned int flags)
 This will create and initialise a new event loop object. If the loop
 could not be initialised, returns false.
-Note that this function I<is> thread-safe, and one common way to use
+This function is thread-safe, and one common way to use libev with
-libev with threads is indeed to create one loop per thread, and using the
+threads is indeed to create one loop per thread, and using the default
-default loop in the "main" or "initial" thread.
+loop in the "main" or "initial" thread.
 The flags argument can be used to specify special behaviour or specific
 backends to use, and is usually specified as C<0> (or C<EVFLAG_AUTO>).
 The following flags are supported:
 environment variable.
 =item C<EVFLAG_NOINOTIFY>
 When this flag is specified, then libev will not attempt to use the
-I<inotify> API for it's C<ev_stat> watchers. Apart from debugging and
+I<inotify> API for its C<ev_stat> watchers. Apart from debugging and
 testing, this flag can be useful to conserve inotify file descriptors, as
 otherwise each loop using C<ev_stat> watchers consumes one inotify handle.
 =item C<EVFLAG_SIGNALFD>
 When this flag is specified, then libev will attempt to use the
-I<signalfd> API for it's C<ev_signal> (and C<ev_child>) watchers. This API
+I<signalfd> API for its C<ev_signal> (and C<ev_child>) watchers. This API
 delivers signals synchronously, which makes it both faster and might make
 it possible to get the queued signal data. It can also simplify signal
 handling with threads, as long as you properly block signals in your
 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,
 The epoll mechanism deserves honorable mention as the most misdesigned
 of the more advanced event mechanisms: mere annoyances include silently
 dropping file descriptors, requiring a system call per change per file
 descriptor (and unnecessary guessing of parameters), problems with dup,
-returning before the timeout value requiring additional iterations and so
+returning before the timeout value, resulting in additional iterations
+(and only giving 5ms accuracy while select on the same platform gives
-on. The biggest issue is fork races, however - if a program forks then
+0.1ms) and so on. The biggest issue is fork races, however - if a program
-I<both> parent and child process have to recreate the epoll set, which can
+forks then I<both> parent and child process have to recreate the epoll
-take considerable time (one syscall per file descriptor) and is of course
+set, which can take considerable time (one syscall per file descriptor)
-hard to detect.
+and is of course hard to detect.
 Epoll is also notoriously buggy - embedding epoll fds I<should> work, but
 of course I<doesn't>, and epoll just loves to report events for totally
 I<different> file descriptors (even already closed ones, so one cannot
 even remove them from the set) than registered in the set (especially
 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.
 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
 file descriptors might not work very well if you register events for both
 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
 This function is normally used on loop objects allocated by
 C<ev_loop_new>, but it can also be used on the default loop returned by
 C<ev_default_loop>, in which case it is not thread-safe.
 Note that it is not advisable to call this function on the default loop
-except in the rare occasion where you really need to free it's resources.
+except in the rare occasion where you really need to free its resources.
 If you need dynamically allocated loops it is better to use C<ev_loop_new>
 and C<ev_loop_destroy>.
 =item ev_loop_fork (loop)
 prepare and check phases.
 =item unsigned int ev_depth (loop)
 Returns the number of times C<ev_run> was entered minus the number of
-times C<ev_run> was exited, in other words, the recursion depth.
+times C<ev_run> was exited normally, in other words, the recursion depth.
 Outside C<ev_run>, this number is zero. In a callback, this number is
 C<1>, unless C<ev_run> was invoked recursively (or from another thread),
 in which case it is higher.
-Leaving C<ev_run> abnormally (setjmp/longjmp, cancelling the thread
+Leaving C<ev_run> abnormally (setjmp/longjmp, cancelling the thread,
-etc.), doesn't count as "exit" - consider this as a hint to avoid such
+throwing an exception etc.), doesn't count as "exit" - consider this
-ungentleman-like behaviour unless it's really convenient.
+as a hint to avoid such ungentleman-like behaviour unless it's really
+convenient, in which case it is fully supported.
 =item unsigned int ev_backend (loop)
 Returns one of the C<EVBACKEND_*> flags indicating the event backend in
 use.
 relying on all watchers to be stopped when deciding when a program has
 finished (especially in interactive programs), but having a program
 that automatically loops as long as it has to and no longer by virtue
 of relying on its watchers stopping correctly, that is truly a thing of
 beauty.
+This function is also I<mostly> exception-safe - you can break out of
+a C<ev_run> call by calling C<longjmp> in a callback, throwing a C++
+exception and so on. This does not decrement the C<ev_depth> value, nor
+will it clear any outstanding C<EVBREAK_ONE> breaks.
 A flags value of C<EVRUN_NOWAIT> will look for new events, will handle
 those events and any already outstanding ones, but will not wait and
 block your process in case there are no events and will return after one
 iteration of the loop. This is sometimes useful to poll and handle new
 Can be used to make a call to C<ev_run> return early (but only after it
 has processed all outstanding events). The C<how> argument must be either
 C<EVBREAK_ONE>, which will make the innermost C<ev_run> call return, or
 C<EVBREAK_ALL>, which will make all nested C<ev_run> calls return.
-This "break state" will be cleared when entering C<ev_run> again.
+This "break state" will be cleared on the next call to C<ev_run>.
-It is safe to call C<ev_break> from outside any C<ev_run> calls, too.
+It is safe to call C<ev_break> from outside any C<ev_run> calls, too, in
+which case it will have no effect.
 =item ev_ref (loop)
 =item ev_unref (loop)
 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);
 See also the locking example in the C<THREADS> section later in this
 document.
 =item ev_set_userdata (loop, void *data)
-=item ev_userdata (loop)
+=item void *ev_userdata (loop)
 Set and retrieve a single C<void *> associated with a loop. When
 C<ev_set_userdata> has never been called, then C<ev_userdata> returns
-C<0.>
+C<0>.
 These two functions can be used to associate arbitrary data with a loop,
 and are intended solely for the C<invoke_pending_cb>, C<release> and
 C<acquire> callbacks described above, but of course can be (ab-)used for
 any other purpose as well.
 =head2 C<ev_signal> - signal me when a signal gets signalled!
 Signal watchers will trigger an event when the process receives a specific
 signal one or more times. Even though signals are very asynchronous, libev
-will try it's best to deliver signals synchronously, i.e. as part of the
+will try its best to deliver signals synchronously, i.e. as part of the
 normal event processing, like any other event.
 If you want signals to be delivered truly asynchronously, just use
 C<sigaction> as you would do without libev and forget about sharing
 the signal. You can even use C<ev_async> from a signal handler to
 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)
+This section explains some common idioms that are not immediately
+obvious. Note that examples are sprinkled over the whole manual, and this
+section only contains stuff that wouldn't fit anywhere else.
+=over 4
+=item Model/nested event loop invocations and exit conditions.
+Often (especially in GUI toolkits) there are places where you have
+I<modal> interaction, which is most easily implemented by recursively
+invoking C<ev_run>.
+This brings the problem of exiting - a callback might want to finish the
+main C<ev_run> call, but not the nested one (e.g. user clicked "Quit", but
+a modal "Are you sure?" dialog is still waiting), or just the nested one
+and not the main one (e.g. user clocked "Ok" in a modal dialog), or some
+other combination: In these cases, C<ev_break> will not work alone.
+The solution is to maintain "break this loop" variable for each C<ev_run>
+invocation, and use a loop around C<ev_run> until the condition is
+triggered, using C<EVRUN_ONCE>:
+   // main loop
+   int exit_main_loop = 0;
+   while (!exit_main_loop)
+     ev_run (EV_DEFAULT_ EVRUN_ONCE);
+   // in a model watcher
+   int exit_nested_loop = 0;
+   while (!exit_nested_loop)
+     ev_run (EV_A_ EVRUN_ONCE);
+To exit from any of these loops, just set the corresponding exit variable:
+   // exit modal loop
+   exit_nested_loop = 1;
+   // exit main program, after modal loop is finished
+   exit_main_loop = 1;
+   // exit both
+   exit_main_loop = exit_nested_loop = 1;
 =back
 =head1 LIBEVENT EMULATION
 Libev offers a compatibility emulation layer for libevent. It cannot
 emulate the internals of libevent, so here are some usage hints:
 =over 4
+=item * Only the libevent-1.4.1-beta API is being emulated.
+This was the newest libevent version available when libev was implemented,
+and is still mostly unchanged in 2010.
 =item * Use it by including <event.h>, as usual.
 =item * The following members are fully supported: ev_base, ev_callback,
 ev_arg, ev_fd, ev_res, ev_events.
 =item * Priorities are not currently supported. Initialising priorities
 will fail and all watchers will have the same priority, even though there
 is an ev_pri field.
 =item * In libevent, the last base created gets the signals, in libev, the
-first base created (== the default loop) gets the signals.
+base that registered the signal gets the signals.
 =item * Other members are not supported.
 =item * The libev emulation is I<not> ABI compatible to libevent, you need
 to use the libev header file and library.
 Care has been taken to keep the overhead low. The only data member the C++
 classes add (compared to plain C-style watchers) is the event loop pointer
 that the watcher is associated with (or no additional members at all if
 you disable C<EV_MULTIPLICITY> when embedding libev).
-Currently, functions, and static and non-static member functions can be
+Currently, functions, static and non-static member functions and classes
-used as callbacks. Other types should be easy to add as long as they only
+with C<operator ()> can be used as callbacks. Other types should be easy
-need one additional pointer for context. If you need support for other
+to add as long as they only need one additional pointer for context. If
-types of functors please contact the author (preferably after implementing
+you need support for other types of functors please contact the author
-it).
+(preferably after implementing it).
 Here is a list of things available in the C<ev> namespace:
 =over 4

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Comparing libev/ev.pod (file contents): Revision 1.337 by root, Sun Oct 31 20:20:20 2010 UTC vs. Revision 1.350 by sf-exg, Mon Jan 10 08:36:41 2011 UTC

Diff Legend

Comparing libev/ev.pod (file contents):
Revision 1.337 by root, Sun Oct 31 20:20:20 2010 UTC vs.
Revision 1.350 by sf-exg, Mon Jan 10 08:36:41 2011 UTC