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Comparing libev/ev.pod (file contents):
Revision 1.123 by root, Thu Jan 31 13:25:10 2008 UTC vs.
Revision 1.128 by root, Fri Feb 1 13:53:56 2008 UTC

… …
2070C<ev_async_sent> calls). 2070C<ev_async_sent> calls).
2071 2071
2072Unlike C<ev_signal> watchers, C<ev_async> works with any event loop, not 2072Unlike C<ev_signal> watchers, C<ev_async> works with any event loop, not
2073just the default loop. 2073just the default loop.
2074 2074
2075=head3 Queueing
2076
2077C<ev_async> does not support queueing of data in any way. The reason
2078is that the author does not know of a simple (or any) algorithm for a
2079multiple-writer-single-reader queue that works in all cases and doesn't
2080need elaborate support such as pthreads.
2081
2082That means that if you want to queue data, you have to provide your own
2083queue. And here is how you would implement locking:
2084
2085=over 4
2086
2087=item queueing from a signal handler context
2088
2089To implement race-free queueing, you simply add to the queue in the signal
2090handler but you block the signal handler in the watcher callback. Here is an example that does that for
2091some fictitiuous SIGUSR1 handler:
2092
2093 static ev_async mysig;
2094
2095 static void
2096 sigusr1_handler (void)
2097 {
2098 sometype data;
2099
2100 // no locking etc.
2101 queue_put (data);
2102 ev_async_send (DEFAULT_ &mysig);
2103 }
2104
2105 static void
2106 mysig_cb (EV_P_ ev_async *w, int revents)
2107 {
2108 sometype data;
2109 sigset_t block, prev;
2110
2111 sigemptyset (&block);
2112 sigaddset (&block, SIGUSR1);
2113 sigprocmask (SIG_BLOCK, &block, &prev);
2114
2115 while (queue_get (&data))
2116 process (data);
2117
2118 if (sigismember (&prev, SIGUSR1)
2119 sigprocmask (SIG_UNBLOCK, &block, 0);
2120 }
2121
2122(Note: pthreads in theory requires you to use C<pthread_setmask>
2123instead of C<sigprocmask> when you use threads, but libev doesn't do it
2124either...).
2125
2126=item queueing from a thread context
2127
2128The strategy for threads is different, as you cannot (easily) block
2129threads but you can easily preempt them, so to queue safely you need to
2130emply a traditional mutex lock, such as in this pthread example:
2131
2132 static ev_async mysig;
2133 static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
2134
2135 static void
2136 otherthread (void)
2137 {
2138 // only need to lock the actual queueing operation
2139 pthread_mutex_lock (&mymutex);
2140 queue_put (data);
2141 pthread_mutex_unlock (&mymutex);
2142
2143 ev_async_send (DEFAULT_ &mysig);
2144 }
2145
2146 static void
2147 mysig_cb (EV_P_ ev_async *w, int revents)
2148 {
2149 pthread_mutex_lock (&mymutex);
2150
2151 while (queue_get (&data))
2152 process (data);
2153
2154 pthread_mutex_unlock (&mymutex);
2155 }
2156
2157=back
2158
2159
2075=head3 Watcher-Specific Functions and Data Members 2160=head3 Watcher-Specific Functions and Data Members
2076 2161
2077=over 4 2162=over 4
2078 2163
2079=item ev_async_init (ev_async *, callback) 2164=item ev_async_init (ev_async *, callback)
… …
2610be detected at runtime. 2695be detected at runtime.
2611 2696
2612=item EV_ATOMIC_T 2697=item EV_ATOMIC_T
2613 2698
2614Libev requires an integer type (suitable for storing C<0> or C<1>) whose 2699Libev requires an integer type (suitable for storing C<0> or C<1>) whose
2615access is atomic with respect to other threads or signal contexts. No such type 2700access is atomic with respect to other threads or signal contexts. No such
2616is easily found using, so you cna provide your own type that you know is safe. 2701type is easily found in the C language, so you can provide your own type
2702that you know is safe for your purposes. It is used both for signal handler "locking"
2703as well as for signal and thread safety in C<ev_async> watchers.
2617 2704
2618In the absense of this define, libev will use C<sig_atomic_t volatile> 2705In the absense of this define, libev will use C<sig_atomic_t volatile>
2619from F<signal.h>, which is usually good enough on most platforms. 2706(from F<signal.h>), which is usually good enough on most platforms.
2620 2707
2621=item EV_H 2708=item EV_H
2622 2709
2623The name of the F<ev.h> header file used to include it. The default if 2710The name of the F<ev.h> header file used to include it. The default if
2624undefined is C<"ev.h"> in F<event.h>, F<ev.c> and F<ev++.h>. This can be 2711undefined is C<"ev.h"> in F<event.h>, F<ev.c> and F<ev++.h>. This can be
… …
2825=item Changing timer/periodic watchers (by autorepeat or calling again): O(log skipped_other_timers) 2912=item Changing timer/periodic watchers (by autorepeat or calling again): O(log skipped_other_timers)
2826 2913
2827That means that changing a timer costs less than removing/adding them 2914That means that changing a timer costs less than removing/adding them
2828as only the relative motion in the event queue has to be paid for. 2915as only the relative motion in the event queue has to be paid for.
2829 2916
2830=item Starting io/check/prepare/idle/signal/child watchers: O(1) 2917=item Starting io/check/prepare/idle/signal/child/fork/async watchers: O(1)
2831 2918
2832These just add the watcher into an array or at the head of a list. 2919These just add the watcher into an array or at the head of a list.
2833 2920
2834=item Stopping check/prepare/idle watchers: O(1) 2921=item Stopping check/prepare/idle/fork/async watchers: O(1)
2835 2922
2836=item Stopping an io/signal/child watcher: O(number_of_watchers_for_this_(fd/signal/pid % EV_PID_HASHSIZE)) 2923=item Stopping an io/signal/child watcher: O(number_of_watchers_for_this_(fd/signal/pid % EV_PID_HASHSIZE))
2837 2924
2838These watchers are stored in lists then need to be walked to find the 2925These watchers are stored in lists then need to be walked to find the
2839correct watcher to remove. The lists are usually short (you don't usually 2926correct watcher to remove. The lists are usually short (you don't usually
… …
2856 2943
2857Priorities are implemented by allocating some space for each 2944Priorities are implemented by allocating some space for each
2858priority. When doing priority-based operations, libev usually has to 2945priority. When doing priority-based operations, libev usually has to
2859linearly search all the priorities, but starting/stopping and activating 2946linearly search all the priorities, but starting/stopping and activating
2860watchers becomes O(1) w.r.t. prioritiy handling. 2947watchers becomes O(1) w.r.t. prioritiy handling.
2948
2949=item Sending an ev_async: O(1)
2950
2951=item Processing ev_async_send: O(number_of_async_watchers)
2952
2953=item Processing signals: O(max_signal_number)
2954
2955Sending involves a syscall I<iff> there were no other C<ev_async_send>
2956calls in the current loop iteration. Checking for async and signal events
2957involves iterating over all running async watchers or all signal numbers.
2861 2958
2862=back 2959=back
2863 2960
2864 2961
2865=head1 Win32 platform limitations and workarounds 2962=head1 Win32 platform limitations and workarounds

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