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

Comparing libev/ev.pod (file contents):
Revision 1.264 by root, Wed Aug 19 23:44:51 2009 UTC vs.
Revision 1.284 by root, Sun Mar 14 21:05:52 2010 UTC

 Libev is very configurable. In this manual the default (and most common)
 configuration will be described, which supports multiple event loops. For
 more info about various configuration options please have a look at
 B<EMBED> section in this manual. If libev was configured without support
 for multiple event loops, then all functions taking an initial argument of
-name C<loop> (which is always of type C<ev_loop *>) will not have
+name C<loop> (which is always of type C<struct ev_loop *>) will not have
 this argument.
 =head2 TIME REPRESENTATION
 Libev represents time as a single floating point number, representing
 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
 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_NOSIGNALFD>
+=item C<EVFLAG_SIGNALFD>
-When this flag is specified, then libev will not attempt to use the
+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 is
+I<signalfd> API for it's C<ev_signal> (and C<ev_child>) watchers. This API
-probably only useful to work around any bugs in libev. Consequently, this
+delivers signals synchronously, which makes it both faster and might make
-flag might go away once the signalfd functionality is considered stable,
+it possible to get the queued signal data. It can also simplify signal
-so it's useful mostly in environment variables and not in program code.
+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<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,
 This backend maps C<EV_READ> to C<POLLIN | POLLERR | POLLHUP>, and
 C<EV_WRITE> to C<POLLOUT | POLLERR | POLLHUP>.
 =item C<EVBACKEND_EPOLL>   (value 4, Linux)
+Use the linux-specific epoll(7) interface (for both pre- and post-2.6.9
+kernels).
 For few fds, this backend is a bit little slower than poll and select,
 but it scales phenomenally better. While poll and select usually scale
 like O(total_fds) where n is the total number of fds (or the highest fd),
 epoll scales either O(1) or O(active_fds).
 as signal and child watchers) would need to be stopped manually.
 In general it is not advisable to call this function except in the
 rare occasion where you really need to free e.g. the signal handling
 pipe fds. If you need dynamically allocated loops it is better to use
-C<ev_loop_new> and C<ev_loop_destroy>).
+C<ev_loop_new> and C<ev_loop_destroy>.
 =item ev_loop_destroy (loop)
 Like C<ev_default_destroy>, but destroys an event loop created by an
 earlier call to C<ev_loop_new>.
 event loop time (see C<ev_now_update>).
 =item ev_loop (loop, int flags)
 Finally, this is it, the event handler. This function usually is called
-after you initialised all your watchers and you want to start handling
+after you have initialised all your watchers and you want to start
-events.
+handling events.
 If the flags argument is specified as C<0>, it will not return until
 either no event watchers are active anymore or C<ev_unloop> was called.
 Please note that an explicit C<ev_unloop> is usually better than
 Ref/unref can be used to add or remove a reference count on the event
 loop: Every watcher keeps one reference, and as long as the reference
 count is nonzero, C<ev_loop> will not return on its own.
-If you have a watcher you never unregister that should not keep C<ev_loop>
+This is useful when you have a watcher that you never intend to
-from returning, call ev_unref() after starting, and ev_ref() before
+unregister, but that nevertheless should not keep C<ev_loop> from
+returning. In such a case, call C<ev_unref> after starting, and C<ev_ref>
-stopping it.
+before stopping it.
 As an example, libev itself uses this for its internal signal pipe: It
 is not visible to the libev user and should not keep C<ev_loop> from
 exiting if no event watchers registered by it are active. It is also an
 excellent way to do this for generic recurring timers or from within
 While event loop modifications are allowed between invocations of
 C<release> and C<acquire> (that's their only purpose after all), no
 modifications done will affect the event loop, i.e. adding watchers will
 have no effect on the set of file descriptors being watched, or the time
-waited. USe an C<ev_async> watcher to wake up C<ev_loop> when you want it
+waited. Use an C<ev_async> watcher to wake up C<ev_loop> when you want it
 to take note of any changes you made.
 In theory, threads executing C<ev_loop> will be async-cancel safe between
 invocations of C<release> and C<acquire>.
    ev_io w;
    ev_init (&w, my_cb);
    ev_io_set (&w, STDIN_FILENO, EV_READ);
-=item C<ev_TYPE_set> (ev_TYPE *, [args])
+=item C<ev_TYPE_set> (ev_TYPE *watcher, [args])
 This macro initialises the type-specific parts of a watcher. You need to
 call C<ev_init> at least once before you call this macro, but you can
 call C<ev_TYPE_set> any number of times. You must not, however, call this
 macro on a watcher that is active (it can be pending, however, which is a
 Example: Initialise and set an C<ev_io> watcher in one step.
    ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
-=item C<ev_TYPE_start> (loop *, ev_TYPE *watcher)
+=item C<ev_TYPE_start> (loop, ev_TYPE *watcher)
 Starts (activates) the given watcher. Only active watchers will receive
 events. If the watcher is already active nothing will happen.
 Example: Start the C<ev_io> watcher that is being abused as example in this
 whole section.
    ev_io_start (EV_DEFAULT_UC, &w);
-=item C<ev_TYPE_stop> (loop *, ev_TYPE *watcher)
+=item C<ev_TYPE_stop> (loop, ev_TYPE *watcher)
 Stops the given watcher if active, and clears the pending status (whether
 the watcher was active or not).
 It is possible that stopped watchers are pending - for example,
 =item ev_cb_set (ev_TYPE *watcher, callback)
 Change the callback. You can change the callback at virtually any time
 (modulo threads).
-=item ev_set_priority (ev_TYPE *watcher, priority)
+=item ev_set_priority (ev_TYPE *watcher, int priority)
 =item int ev_priority (ev_TYPE *watcher)
 Set and query the priority of the watcher. The priority is a small
 integer between C<EV_MAXPRI> (default: C<2>) and C<EV_MINPRI>
 returns its C<revents> bitset (as if its callback was invoked). If the
 watcher isn't pending it does nothing and returns C<0>.
 Sometimes it can be useful to "poll" a watcher instead of waiting for its
 callback to be invoked, which can be accomplished with this function.
+=item ev_feed_event (loop, ev_TYPE *watcher, int revents)
+Feeds the given event set into the event loop, as if the specified event
+had happened for the specified watcher (which must be a pointer to an
+initialised but not necessarily started event watcher). Obviously you must
+not free the watcher as long as it has pending events.
+Stopping the watcher, letting libev invoke it, or calling
+C<ev_clear_pending> will clear the pending event, even if the watcher was
+not started in the first place.
+See also C<ev_feed_fd_event> and C<ev_feed_signal_event> for related
+functions that do not need a watcher.
 =back
 =head2 ASSOCIATING CUSTOM DATA WITH A WATCHER
 So when you encounter spurious, unexplained daemon exits, make sure you
 ignore SIGPIPE (and maybe make sure you log the exit status of your daemon
 somewhere, as that would have given you a big clue).
+=head3 The special problem of accept()ing when you can't
+Many implementations of the POSIX C<accept> function (for example,
+found in port-2004 Linux) have the peculiar behaviour of not removing a
+connection from the pending queue in all error cases.
+For example, larger servers often run out of file descriptors (because
+of resource limits), causing C<accept> to fail with C<ENFILE> but not
+rejecting the connection, leading to libev signalling readiness on
+the next iteration again (the connection still exists after all), and
+typically causing the program to loop at 100% CPU usage.
+Unfortunately, the set of errors that cause this issue differs between
+operating systems, there is usually little the app can do to remedy the
+situation, and no known thread-safe method of removing the connection to
+cope with overload is known (to me).
+One of the easiest ways to handle this situation is to just ignore it
+- when the program encounters an overload, it will just loop until the
+situation is over. While this is a form of busy waiting, no OS offers an
+event-based way to handle this situation, so it's the best one can do.
+A better way to handle the situation is to log any errors other than
+C<EAGAIN> and C<EWOULDBLOCK>, making sure not to flood the log with such
+messages, and continue as usual, which at least gives the user an idea of
+what could be wrong ("raise the ulimit!"). For extra points one could stop
+the C<ev_io> watcher on the listening fd "for a while", which reduces CPU
+usage.
+If your program is single-threaded, then you could also keep a dummy file
+descriptor for overload situations (e.g. by opening F</dev/null>), and
+when you run into C<ENFILE> or C<EMFILE>, close it, run C<accept>,
+close that fd, and create a new dummy fd. This will gracefully refuse
+clients under typical overload conditions.
+The last way to handle it is to simply log the error and C<exit>, as
+is often done with C<malloc> failures, but this results in an easy
+opportunity for a DoS attack.
 =head3 Watcher-Specific Functions
 =over 4
 C<repeat> value), or reset the running timer to the C<repeat> value.
 This sounds a bit complicated, see L<Be smart about timeouts>, above, for a
 usage example.
-=item ev_timer_remaining (loop, ev_timer *)
+=item ev_tstamp ev_timer_remaining (loop, ev_timer *)
 Returns the remaining time until a timer fires. If the timer is active,
 then this time is relative to the current event loop time, otherwise it's
 the timeout value currently configured.
 That is, after an C<ev_timer_set (w, 5, 7)>, C<ev_timer_remaining> returns
-C<5>. When the timer is started and one second passes, C<ev_timer_remain>
+C<5>. When the timer is started and one second passes, C<ev_timer_remaining>
 will return C<4>. When the timer expires and is restarted, it will return
 roughly C<7> (likely slightly less as callback invocation takes some time,
 too), and so on.
 =item ev_tstamp repeat [read-write]
 When the first watcher gets started will libev actually register something
 with the kernel (thus it coexists with your own signal handlers as long as
 you don't register any with libev for the same signal).
-Both the signal mask state (C<sigprocmask>) and the signal handler state
-(C<sigaction>) are unspecified after starting a signal watcher (and after
-sotpping it again), that is, libev might or might not block the signal,
-and might or might not set or restore the installed signal handler.
 If possible and supported, libev will install its handlers with
 C<SA_RESTART> (or equivalent) behaviour enabled, so system calls should
 not be unduly interrupted. If you have a problem with system calls getting
 interrupted by signals you can block all signals in an C<ev_check> watcher
 and unblock them in an C<ev_prepare> watcher.
+=head3 The special problem of inheritance over fork/execve/pthread_create
+Both the signal mask (C<sigprocmask>) and the signal disposition
+(C<sigaction>) are unspecified after starting a signal watcher (and after
+stopping it again), that is, libev might or might not block the signal,
+and might or might not set or restore the installed signal handler.
+While this does not matter for the signal disposition (libev never
+sets signals to C<SIG_IGN>, so handlers will be reset to C<SIG_DFL> on
+C<execve>), this matters for the signal mask: many programs do not expect
+certain signals to be blocked.
+This means that before calling C<exec> (from the child) you should reset
+the signal mask to whatever "default" you expect (all clear is a good
+choice usually).
+The simplest way to ensure that the signal mask is reset in the child is
+to install a fork handler with C<pthread_atfork> that resets it. That will
+catch fork calls done by libraries (such as the libc) as well.
+In current versions of libev, the signal will not be blocked indefinitely
+unless you use the C<signalfd> API (C<EV_SIGNALFD>). While this reduces
+the window of opportunity for problems, it will not go away, as libev
+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 Watcher-Specific Functions and Data Members
 =over 4
 =head3 Queueing
 C<ev_async> does not support queueing of data in any way. The reason
 is that the author does not know of a simple (or any) algorithm for a
 multiple-writer-single-reader queue that works in all cases and doesn't
-need elaborate support such as pthreads.
+need elaborate support such as pthreads or unportable memory access
+semantics.
 That means that if you want to queue data, you have to provide your own
 queue. But at least I can tell you how to implement locking around your
 queue:
        /* doh, nothing entered */;
    }
    ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
-=item ev_feed_event (struct ev_loop *, watcher *, int revents)
-Feeds the given event set into the event loop, as if the specified event
-had happened for the specified watcher (which must be a pointer to an
-initialised but not necessarily started event watcher).
-=item ev_feed_fd_event (struct ev_loop *, int fd, int revents)
+=item ev_feed_fd_event (loop, int fd, int revents)
 Feed an event on the given fd, as if a file descriptor backend detected
 the given events it.
-=item ev_feed_signal_event (struct ev_loop *loop, int signum)
+=item ev_feed_signal_event (loop, int signum)
 Feed an event as if the given signal occurred (C<loop> must be the default
 loop!).
 =back
 =over 4
 =item ev::TYPE::TYPE ()
-=item ev::TYPE::TYPE (struct ev_loop *)
+=item ev::TYPE::TYPE (loop)
 =item ev::TYPE::~TYPE
 The constructor (optionally) takes an event loop to associate the watcher
 with. If it is omitted, it will use C<EV_DEFAULT>.
 Example: Use a plain function as callback.
    static void io_cb (ev::io &w, int revents) { }
    iow.set <io_cb> ();
-=item w->set (struct ev_loop *)
+=item w->set (loop)
 Associates a different C<struct ev_loop> with this watcher. You can only
 do this when the watcher is inactive (and not pending either).
 =item w->set ([arguments])
 Erkki Seppala has written Ocaml bindings for libev, to be found at
 L<http://modeemi.cs.tut.fi/~flux/software/ocaml-ev/>.
 =item Lua
-Brian Maher has written a partial interface to libev
+Brian Maher has written a partial interface to libev for lua (at the
-for lua (only C<ev_io> and C<ev_timer>), to be found at
+time of this writing, only C<ev_io> and C<ev_timer>), to be found at
 L<http://github.com/brimworks/lua-ev>.
 =back
    libev.m4
 =head2 PREPROCESSOR SYMBOLS/MACROS
 Libev can be configured via a variety of preprocessor symbols you have to
-define before including any of its files. The default in the absence of
+define before including (or compiling) any of its files. The default in
-autoconf is documented for every option.
+the absence of autoconf is documented for every option.
+Symbols marked with "(h)" do not change the ABI, and can have different
+values when compiling libev vs. including F<ev.h>, so it is permissible
+to redefine them before including F<ev.h> without breakign compatibility
+to a compiled library. All other symbols change the ABI, which means all
+users of libev and the libev code itself must be compiled with compatible
+settings.
 =over 4
-=item EV_STANDALONE
+=item EV_STANDALONE (h)
 Must always be C<1> if you do not use autoconf configuration, which
 keeps libev from including F<config.h>, and it also defines dummy
 implementations for some libevent functions (such as logging, which is not
 supported). It will also not define any of the structs usually found in
 as well as for signal and thread safety in C<ev_async> watchers.
 In the absence of this define, libev will use C<sig_atomic_t volatile>
 (from F<signal.h>), which is usually good enough on most platforms.
-=item EV_H
+=item EV_H (h)
 The name of the F<ev.h> header file used to include it. The default if
 undefined is C<"ev.h"> in F<event.h>, F<ev.c> and F<ev++.h>. This can be
 used to virtually rename the F<ev.h> header file in case of conflicts.
-=item EV_CONFIG_H
+=item EV_CONFIG_H (h)
 If C<EV_STANDALONE> isn't C<1>, this variable can be used to override
 F<ev.c>'s idea of where to find the F<config.h> file, similarly to
 C<EV_H>, above.
-=item EV_EVENT_H
+=item EV_EVENT_H (h)
 Similarly to C<EV_H>, this macro can be used to override F<event.c>'s idea
 of how the F<event.h> header can be found, the default is C<"event.h">.
-=item EV_PROTOTYPES
+=item EV_PROTOTYPES (h)
 If defined to be C<0>, then F<ev.h> will not define any function
 prototypes, but still define all the structs and other symbols. This is
 occasionally useful if you want to provide your own wrapper functions
 around libev functions.
 fine.
 If your embedding application does not need any priorities, defining these
 both to C<0> will save some memory and CPU.
-=item EV_PERIODIC_ENABLE
+=item EV_PERIODIC_ENABLE, EV_IDLE_ENABLE, EV_EMBED_ENABLE, EV_STAT_ENABLE,
+EV_PREPARE_ENABLE, EV_CHECK_ENABLE, EV_FORK_ENABLE, EV_SIGNAL_ENABLE,
+EV_ASYNC_ENABLE, EV_CHILD_ENABLE.
-If undefined or defined to be C<1>, then periodic timers are supported. If
+If undefined or defined to be C<1> (and the platform supports it), then
-defined to be C<0>, then they are not. Disabling them saves a few kB of
+the respective watcher type is supported. If defined to be C<0>, then it
-code.
+is not. Disabling watcher types mainly saves codesize.
-=item EV_IDLE_ENABLE
-If undefined or defined to be C<1>, then idle watchers are supported. If
-defined to be C<0>, then they are not. Disabling them saves a few kB of
-code.
-=item EV_EMBED_ENABLE
-If undefined or defined to be C<1>, then embed watchers are supported. If
-defined to be C<0>, then they are not. Embed watchers rely on most other
-watcher types, which therefore must not be disabled.
-=item EV_STAT_ENABLE
-If undefined or defined to be C<1>, then stat watchers are supported. If
-defined to be C<0>, then they are not.
-=item EV_FORK_ENABLE
-If undefined or defined to be C<1>, then fork watchers are supported. If
-defined to be C<0>, then they are not.
-=item EV_ASYNC_ENABLE
-If undefined or defined to be C<1>, then async watchers are supported. If
-defined to be C<0>, then they are not.
 =item EV_MINIMAL
 If you need to shave off some kilobytes of code at the expense of some
 speed (but with the full API), define this symbol to C<1>. Currently this
 on amd64. It also selects a much smaller 2-heap for timer management over
 the default 4-heap.
 You can save even more by disabling watcher types you do not need
 and setting C<EV_MAXPRI> == C<EV_MINPRI>. Also, disabling C<assert>
-(C<-DNDEBUG>) will usually reduce code size a lot.
+(C<-DNDEBUG>) will usually reduce code size a lot. Disabling inotify,
+eventfd and signalfd will further help, and disabling backends one doesn't
+need (e.g. poll, epoll, kqueue, ports) will help further.
 Defining C<EV_MINIMAL> to C<2> will additionally reduce the core API to
 provide a bare-bones event library. See C<ev.h> for details on what parts
 of the API are still available, and do not complain if this subset changes
 over time.
+This example set of settings reduces the compiled size of libev from
+23.9Kb to 7.7Kb on my GNU/Linux amd64 system (and leaves little
+in - there is also an effect on the amount of memory used). With
+an intelligent-enough linker (gcc+binutils do this when you use
+C<-Wl,--gc-sections -ffunction-sections>) further unused functions might
+be left out as well automatically - a binary starting a timer and an I/O
+watcher then might come out at only 5Kb.
+   // tuning and API changes
+   #define EV_MINIMAL 2
+   #define EV_MULTIPLICITY 0
+   #define EV_MINPRI 0
+   #define EV_MAXPRI 0
+   // OS-specific backends
+   #define EV_USE_INOTIFY 0
+   #define EV_USE_EVENTFD 0
+   #define EV_USE_SIGNALFD 0
+   #define EV_USE_REALTIME 0
+   #define EV_USE_MONOTONIC 0
+   #define EV_USE_CLOCK_SYSCALL 0
+   // disable all backends except select
+   #define EV_USE_POLL 0
+   #define EV_USE_PORT 0
+   #define EV_USE_KQUEUE 0
+   #define EV_USE_EPOLL 0
+   // disable all watcher types that cna be disabled
+   #define EV_STAT_ENABLE 0
+   #define EV_PERIODIC_ENABLE 0
+   #define EV_IDLE_ENABLE 0
+   #define EV_CHECK_ENABLE 0
+   #define EV_PREPARE_ENABLE 0
+   #define EV_FORK_ENABLE 0
+   #define EV_SIGNAL_ENABLE 0
+   #define EV_CHILD_ENABLE 0
+   #define EV_ASYNC_ENABLE 0
+   #define EV_EMBED_ENABLE 0
+=item EV_AVOID_STDIO
+If this is set to C<1> at compiletime, then libev will avoid using stdio
+functions (printf, scanf, perror etc.). This will increase the codesize
+somewhat, but if your program doesn't otherwise depend on stdio and your
+libc allows it, this avoids linking in the stdio library which is quite
+big.
+Note that error messages might become less precise when this option is
+enabled.
 =item EV_NSIG
 The highest supported signal number, +1 (or, the number of
 signals): Normally, libev tries to deduce the maximum number of signals

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Comparing libev/ev.pod (file contents): Revision 1.264 by root, Wed Aug 19 23:44:51 2009 UTC vs. Revision 1.284 by root, Sun Mar 14 21:05:52 2010 UTC

Diff Legend

Comparing libev/ev.pod (file contents):
Revision 1.264 by root, Wed Aug 19 23:44:51 2009 UTC vs.
Revision 1.284 by root, Sun Mar 14 21:05:52 2010 UTC