NAME¶

SYNOPSIS¶

   #include <ev.h>

EXAMPLE PROGRAM¶

   // a single header file is required
   #include <ev.h>
   #include <stdio.h> // for puts
   // every watcher type has its own typedef'd struct
   // with the name ev_TYPE
   ev_io stdin_watcher;
   ev_timer timeout_watcher;
   // all watcher callbacks have a similar signature
   // this callback is called when data is readable on stdin
   static void
   stdin_cb (EV_P_ ev_io *w, int revents)
   {
     puts ("stdin ready");
     // for one-shot events, one must manually stop the watcher
     // with its corresponding stop function.
     ev_io_stop (EV_A_ w);
     // this causes all nested ev_run's to stop iterating
     ev_break (EV_A_ EVBREAK_ALL);
   }
   // another callback, this time for a time-out
   static void
   timeout_cb (EV_P_ ev_timer *w, int revents)
   {
     puts ("timeout");
     // this causes the innermost ev_run to stop iterating
     ev_break (EV_A_ EVBREAK_ONE);
   }
   int
   main (void)
   {
     // use the default event loop unless you have special needs
     struct ev_loop *loop = EV_DEFAULT;
     // initialise an io watcher, then start it
     // this one will watch for stdin to become readable
     ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
     ev_io_start (loop, &stdin_watcher);
     // initialise a timer watcher, then start it
     // simple non-repeating 5.5 second timeout
     ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
     ev_timer_start (loop, &timeout_watcher);
     // now wait for events to arrive
     ev_run (loop, 0);
     // break was called, so exit
     return 0;
   }

ABOUT THIS DOCUMENT¶

WHAT TO READ WHEN IN A HURRY¶

ABOUT LIBEV¶

FEATURES¶

CONVENTIONS¶

TIME REPRESENTATION¶

ERROR HANDLING¶

GLOBAL FUNCTIONS¶

ev_tstamp ev_time ()

Returns the current time as libev would use it. Please note that the "ev_now" function is usually faster and also often returns the timestamp you actually want to know. Also interesting is the combination of "ev_now_update" and "ev_now".

ev_sleep (ev_tstamp interval)

Sleep for the given interval: The current thread will be blocked until either it is interrupted or the given time interval has passed (approximately - it might return a bit earlier even if not interrupted). Returns immediately if "interval <= 0".

 

Basically this is a sub-second-resolution "sleep ()".

 

The range of the "interval" is limited - libev only guarantees to work with sleep times of up to one day ("interval <= 86400").

int ev_version_major ()

int ev_version_minor ()

You can find out the major and minor ABI version numbers of the library you linked against by calling the functions "ev_version_major" and "ev_version_minor". If you want, you can compare against the global symbols "EV_VERSION_MAJOR" and "EV_VERSION_MINOR", which specify the version of the library your program was compiled against.

 

These version numbers refer to the ABI version of the library, not the release version.

 

Usually, it's a good idea to terminate if the major versions mismatch, as this indicates an incompatible change. Minor versions are usually compatible to older versions, so a larger minor version alone is usually not a problem.

 

Example: Make sure we haven't accidentally been linked against the wrong version (note, however, that this will not detect other ABI mismatches, such as LFS or reentrancy).

 

   assert (("libev version mismatch",
            ev_version_major () == EV_VERSION_MAJOR
            && ev_version_minor () >= EV_VERSION_MINOR));
    

unsigned int ev_supported_backends ()

Return the set of all backends (i.e. their corresponding "EV_BACKEND_*" value) compiled into this binary of libev (independent of their availability on the system you are running on). See "ev_default_loop" for a description of the set values.

 

Example: make sure we have the epoll method, because yeah this is cool and a must have and can we have a torrent of it please!!!11

 

   assert (("sorry, no epoll, no sex",
            ev_supported_backends () & EVBACKEND_EPOLL));
    

unsigned int ev_recommended_backends ()

Return the set of all backends compiled into this binary of libev and also recommended for this platform, meaning it will work for most file descriptor types. This set is often smaller than the one returned by "ev_supported_backends", as for example kqueue is broken on most BSDs and will not be auto-detected unless you explicitly request it (assuming you know what you are doing). This is the set of backends that libev will probe for if you specify no backends explicitly.

unsigned int ev_embeddable_backends ()

Returns the set of backends that are embeddable in other event loops. This value is platform-specific but can include backends not available on the current system. To find which embeddable backends might be supported on the current system, you would need to look at "ev_embeddable_backends () & ev_supported_backends ()", likewise for recommended ones.

 

See the description of "ev_embed" watchers for more info.

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 "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 ("size != 0"), the library might abort or take some potentially destructive action.

 

Since some systems (at least OpenBSD and Darwin) fail to implement correct "realloc" semantics, libev will use a wrapper around the system "realloc" and "free" functions by default.

 

You could override this function in high-availability programs to, say, free some memory if it cannot allocate memory, to use a special allocator, or even to sleep a while and retry until some memory is available.

 

Example: Replace the libev allocator with one that waits a bit and then retries (example requires a standards-compliant "realloc").

 

   static void *
   persistent_realloc (void *ptr, size_t size)
   {
     for (;;)
       {
         void *newptr = realloc (ptr, size);
         if (newptr)
           return newptr;
         sleep (60);
       }
   }
   ...
   ev_set_allocator (persistent_realloc);
    

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 matter what, when it returns. That is, libev will generally retry the requested operation, or, if the condition doesn't go away, do bad stuff (such as abort).

 

Example: This is basically the same thing that libev does internally, too.

 

   static void
   fatal_error (const char *msg)
   {
     perror (msg);
     abort ();
   }
   ...
   ev_set_syserr_cb (fatal_error);
    

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 "EVFLAG_NOSIGMASK" when creating any loops), and in one thread, use "sigwait" or any other mechanism to wait for signals, then "deliver" them to libev by calling "ev_feed_signal".

FUNCTIONS CONTROLLING EVENT LOOPS¶

struct ev_loop *ev_default_loop (unsigned int flags)

This returns the "default" event loop object, which is what you should normally use when you just need "the event loop". Event loop objects and the "flags" parameter are described in more detail in the entry for "ev_loop_new".

 

If the default loop is already initialised then this function simply returns it (and ignores the flags. If that is troubling you, check "ev_backend ()" afterwards). Otherwise it will create it with the given flags, which should almost always be 0, unless the caller is also the one calling "ev_run" or otherwise qualifies as "the main program".

 

If you don't know what event loop to use, use the one returned from this function (or via the "EV_DEFAULT" macro).

 

Note that this function is not thread-safe, so if you want to use it from multiple threads, you have to employ some kind of mutex (note also that this case is unlikely, as loops cannot be shared easily between threads anyway).

 

The default loop is the only loop that can handle "ev_child" watchers, and to do this, it always registers a handler for "SIGCHLD". If this is a problem for your application you can either create a dynamic loop with "ev_loop_new" which doesn't do that, or you can simply overwrite the "SIGCHLD" signal handler after calling "ev_default_init".

 

Example: This is the most typical usage.

 

   if (!ev_default_loop (0))
     fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");
    

 

Example: Restrict libev to the select and poll backends, and do not allow environment settings to be taken into account:

 

   ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);
    

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.

 

This function is thread-safe, and one common way to use libev with threads is indeed to create one loop per thread, and using the default 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 0 (or "EVFLAG_AUTO").

 

The following flags are supported:

   struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
   if (!epoller)
     fatal ("no epoll found here, maybe it hides under your chair");

   struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);

ev_loop_destroy (loop)

Destroys an event loop object (frees all memory and kernel state etc.). None of the active event watchers will be stopped in the normal sense, so e.g. "ev_is_active" might still return true. It is your responsibility to either stop all watchers cleanly yourself before calling this function, or cope with the fact afterwards (which is usually the easiest thing, you can just ignore the watchers and/or "free ()" them for example).

 

Note that certain global state, such as signal state (and installed signal handlers), will not be freed by this function, and related watchers (such as signal and child watchers) would need to be stopped manually.

 

This function is normally used on loop objects allocated by "ev_loop_new", but it can also be used on the default loop returned by "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 its resources. If you need dynamically allocated loops it is better to use "ev_loop_new" and "ev_loop_destroy".

ev_loop_fork (loop)

This function sets a flag that causes subsequent "ev_run" iterations to reinitialise the kernel state for backends that have one. Despite the name, you can call it anytime, but it makes most sense after forking, in the child process. You must call it (or use "EVFLAG_FORKCHECK") in the child before resuming or calling "ev_run".

 

Again, you have to call it on any loop that you want to re-use after a fork, even if you do not plan to use the loop in the parent. This is because some kernel interfaces *cough* kqueue *cough* do funny things during fork.

 

On the other hand, you only need to call this function in the child process if and only if you want to use the event loop in the child. If you just fork+exec or create a new loop in the child, you don't have to call it at all (in fact, "epoll" is so badly broken that it makes a difference, but libev will usually detect this case on its own and do a costly reset of the backend).

 

The function itself is quite fast and it's usually not a problem to call it just in case after a fork.

 

Example: Automate calling "ev_loop_fork" on the default loop when using pthreads.

 

   static void
   post_fork_child (void)
   {
     ev_loop_fork (EV_DEFAULT);
   }
   ...
   pthread_atfork (0, 0, post_fork_child);
    

int ev_is_default_loop (loop)

Returns true when the given loop is, in fact, the default loop, and false otherwise.

unsigned int ev_iteration (loop)

Returns the current iteration count for the event loop, which is identical to the number of times libev did poll for new events. It starts at 0 and happily wraps around with enough iterations.

 

This value can sometimes be useful as a generation counter of sorts (it "ticks" the number of loop iterations), as it roughly corresponds with "ev_prepare" and "ev_check" calls - and is incremented between the prepare and check phases.

unsigned int ev_depth (loop)

Returns the number of times "ev_run" was entered minus the number of times "ev_run" was exited normally, in other words, the recursion depth.

 

Outside "ev_run", this number is zero. In a callback, this number is 1, unless "ev_run" was invoked recursively (or from another thread), in which case it is higher.

 

Leaving "ev_run" abnormally (setjmp/longjmp, cancelling the thread, throwing an exception etc.), doesn't count as "exit" - consider this as a hint to avoid such ungentleman-like behaviour unless it's really convenient, in which case it is fully supported.

unsigned int ev_backend (loop)

Returns one of the "EVBACKEND_*" flags indicating the event backend in use.

ev_tstamp ev_now (loop)

Returns the current "event loop time", which is the time the event loop received events and started processing them. This timestamp does not change as long as callbacks are being processed, and this is also the base time used for relative timers. You can treat it as the timestamp of the event occurring (or more correctly, libev finding out about it).

ev_now_update (loop)

Establishes the current time by querying the kernel, updating the time returned by "ev_now ()" in the progress. This is a costly operation and is usually done automatically within "ev_run ()".

 

This function is rarely useful, but when some event callback runs for a very long time without entering the event loop, updating libev's idea of the current time is a good idea.

 

See also "The special problem of time updates" in the "ev_timer" section.

ev_suspend (loop)

ev_resume (loop)

These two functions suspend and resume an event loop, for use when the loop is not used for a while and timeouts should not be processed.

 

A typical use case would be an interactive program such as a game: When the user presses "^Z" to suspend the game and resumes it an hour later it would be best to handle timeouts as if no time had actually passed while the program was suspended. This can be achieved by calling "ev_suspend" in your "SIGTSTP" handler, sending yourself a "SIGSTOP" and calling "ev_resume" directly afterwards to resume timer processing.

 

Effectively, all "ev_timer" watchers will be delayed by the time spend between "ev_suspend" and "ev_resume", and all "ev_periodic" watchers will be rescheduled (that is, they will lose any events that would have occurred while suspended).

 

After calling "ev_suspend" you must not call any function on the given loop other than "ev_resume", and you must not call "ev_resume" without a previous call to "ev_suspend".

 

Calling "ev_suspend"/"ev_resume" has the side effect of updating the event loop time (see "ev_now_update").

ev_run (loop, int flags)

Finally, this is it, the event handler. This function usually is called after you have initialised all your watchers and you want to start handling events. It will ask the operating system for any new events, call the watcher callbacks, an then repeat the whole process indefinitely: This is why event loops are called loops.

 

If the flags argument is specified as 0, it will keep handling events until either no event watchers are active anymore or "ev_break" was called.

 

Please note that an explicit "ev_break" is usually better than 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 mostly exception-safe - you can break out of a "ev_run" call by calling "longjmp" in a callback, throwing a C++ exception and so on. This does not decrement the "ev_depth" value, nor will it clear any outstanding "EVBREAK_ONE" breaks.

 

A flags value of "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 events while doing lengthy calculations, to keep the program responsive.

 

A flags value of "EVRUN_ONCE" will look for new events (waiting if necessary) and will handle those and any already outstanding ones. It will block your process until at least one new event arrives (which could be an event internal to libev itself, so there is no guarantee that a user-registered callback will be called), and will return after one iteration of the loop.

 

This is useful if you are waiting for some external event in conjunction with something not expressible using other libev watchers (i.e. "roll your own "ev_run""). However, a pair of "ev_prepare"/"ev_check" watchers is usually a better approach for this kind of thing.

 

Here are the gory details of what "ev_run" does (this is for your understanding, not a guarantee that things will work exactly like this in future versions):

 

   - Increment loop depth.
   - Reset the ev_break status.
   - Before the first iteration, call any pending watchers.
   LOOP:
   - If EVFLAG_FORKCHECK was used, check for a fork.
   - If a fork was detected (by any means), queue and call all fork watchers.
   - Queue and call all prepare watchers.
   - If ev_break was called, goto FINISH.
   - If we have been forked, detach and recreate the kernel state
     as to not disturb the other process.
   - Update the kernel state with all outstanding changes.
   - Update the "event loop time" (ev_now ()).
   - Calculate for how long to sleep or block, if at all
     (active idle watchers, EVRUN_NOWAIT or not having
     any active watchers at all will result in not sleeping).
   - Sleep if the I/O and timer collect interval say so.
   - Increment loop iteration counter.
   - Block the process, waiting for any events.
   - Queue all outstanding I/O (fd) events.
   - Update the "event loop time" (ev_now ()), and do time jump adjustments.
   - Queue all expired timers.
   - Queue all expired periodics.
   - Queue all idle watchers with priority higher than that of pending events.
   - Queue all check watchers.
   - Call all queued watchers in reverse order (i.e. check watchers first).
     Signals and child watchers are implemented as I/O watchers, and will
     be handled here by queueing them when their watcher gets executed.
   - If ev_break has been called, or EVRUN_ONCE or EVRUN_NOWAIT
     were used, or there are no active watchers, goto FINISH, otherwise
     continue with step LOOP.
   FINISH:
   - Reset the ev_break status iff it was EVBREAK_ONE.
   - Decrement the loop depth.
   - Return.
    

 

Example: Queue some jobs and then loop until no events are outstanding anymore.

 

   ... queue jobs here, make sure they register event watchers as long
   ... as they still have work to do (even an idle watcher will do..)
   ev_run (my_loop, 0);
   ... jobs done or somebody called break. yeah!
    

ev_break (loop, how)

Can be used to make a call to "ev_run" return early (but only after it has processed all outstanding events). The "how" argument must be either "EVBREAK_ONE", which will make the innermost "ev_run" call return, or "EVBREAK_ALL", which will make all nested "ev_run" calls return.

 

This "break state" will be cleared on the next call to "ev_run".

 

It is safe to call "ev_break" from outside any "ev_run" calls, too, in which case it will have no effect.

ev_ref (loop)

ev_unref (loop)

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, "ev_run" will not return on its own.

 

This is useful when you have a watcher that you never intend to unregister, but that nevertheless should not keep "ev_run" from returning. In such a case, call "ev_unref" after starting, and "ev_ref" 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 "ev_run" 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 third-party libraries. Just remember to unref after start and ref before stop (but only if the watcher wasn't active before, or was active before, respectively. Note also that libev might stop watchers itself (e.g. non-repeating timers) in which case you have to "ev_ref" in the callback).

 

Example: Create a signal watcher, but keep it from keeping "ev_run" running when nothing else is active.

 

   ev_signal exitsig;
   ev_signal_init (&exitsig, sig_cb, SIGINT);
   ev_signal_start (loop, &exitsig);
   ev_unref (loop);
    

 

Example: For some weird reason, unregister the above signal handler again.

 

   ev_ref (loop);
   ev_signal_stop (loop, &exitsig);
    

ev_set_io_collect_interval (loop, ev_tstamp interval)

ev_set_timeout_collect_interval (loop, ev_tstamp interval)

These advanced functions influence the time that libev will spend waiting for events. Both time intervals are by default 0, meaning that libev will try to invoke timer/periodic callbacks and I/O callbacks with minimum latency.

 

Setting these to a higher value (the "interval" must be >= 0) allows libev to delay invocation of I/O and timer/periodic callbacks to increase efficiency of loop iterations (or to increase power-saving opportunities).

 

The idea is that sometimes your program runs just fast enough to handle one (or very few) event(s) per loop iteration. While this makes the program responsive, it also wastes a lot of CPU time to poll for new events, especially with backends like "select ()" which have a high overhead for the actual polling but can deliver many events at once.

 

By setting a higher io collect interval you allow libev to spend more time collecting I/O events, so you can handle more events per iteration, at the cost of increasing latency. Timeouts (both "ev_periodic" and "ev_timer") will not be affected. Setting this to a non-null value will introduce an additional "ev_sleep ()" call into most loop iterations. The sleep time ensures that libev will not poll for I/O events more often then once per this interval, on average (as long as the host time resolution is good enough).

 

Likewise, by setting a higher timeout collect interval you allow libev to spend more time collecting timeouts, at the expense of increased latency/jitter/inexactness (the watcher callback will be called later). "ev_io" watchers will not be affected. Setting this to a non-null value will not introduce any overhead in libev.

 

Many (busy) programs can usually benefit by setting the I/O collect interval to a value near 0.1 or so, which is often enough for interactive servers (of course not for games), likewise for timeouts. It usually doesn't make much sense to set it to a lower value than 0.01, as this approaches the timing granularity of most systems. Note that if you do transactions with the outside world and you can't increase the parallelity, then this setting will limit your transaction rate (if you need to poll once per transaction and the I/O collect interval is 0.01, then you can't do more than 100 transactions per second).

 

Setting the timeout collect interval can improve the opportunity for saving power, as the program will "bundle" timer callback invocations that are "near" in time together, by delaying some, thus reducing the number of times the process sleeps and wakes up again. Another useful technique to reduce iterations/wake-ups is to use "ev_periodic" watchers and make sure they fire on, say, one-second boundaries only.

 

Example: we only need 0.1s timeout granularity, and we wish not to poll more often than 100 times per second:

 

   ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1);
   ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01);
    

ev_invoke_pending (loop)

This call will simply invoke all pending watchers while resetting their pending state. Normally, "ev_run" does this automatically when required, but when overriding the invoke callback this call comes handy. This function can be invoked from a watcher - this can be useful for example when you want to do some lengthy calculation and want to pass further event handling to another thread (you still have to make sure only one thread executes within "ev_invoke_pending" or "ev_run" of course).

int ev_pending_count (loop)

Returns the number of pending watchers - zero indicates that no watchers are pending.

ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(EV_P))

This overrides the invoke pending functionality of the loop: Instead of invoking all pending watchers when there are any, "ev_run" will call this callback instead. This is useful, for example, when you want to invoke the actual watchers inside another context (another thread etc.).

 

If you want to reset the callback, use "ev_invoke_pending" as new callback.

ev_set_loop_release_cb (loop, void (*release)(EV_P), void (*acquire)(EV_P))

Sometimes you want to share the same loop between multiple threads. This can be done relatively simply by putting mutex_lock/unlock calls around each call to a libev function.

 

However, "ev_run" can run an indefinite time, so it is not feasible to wait for it to return. One way around this is to wake up the event loop via "ev_break" and "ev_async_send", another way is to set these release and acquire callbacks on the loop.

 

When set, then "release" will be called just before the thread is suspended waiting for new events, and "acquire" is called just afterwards.

 

Ideally, "release" will just call your mutex_unlock function, and "acquire" will just call the mutex_lock function again.

 

While event loop modifications are allowed between invocations of "release" and "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 "ev_async" watcher to wake up "ev_run" when you want it to take note of any changes you made.

 

In theory, threads executing "ev_run" will be async-cancel safe between invocations of "release" and "acquire".

 

See also the locking example in the "THREADS" section later in this document.

ev_set_userdata (loop, void *data)

void *ev_userdata (loop)

Set and retrieve a single "void *" associated with a loop. When "ev_set_userdata" has never been called, then "ev_userdata" returns 0.

 

These two functions can be used to associate arbitrary data with a loop, and are intended solely for the "invoke_pending_cb", "release" and "acquire" callbacks described above, but of course can be (ab-)used for any other purpose as well.

ev_verify (loop)

This function only does something when "EV_VERIFY" support has been compiled in, which is the default for non-minimal builds. It tries to go through all internal structures and checks them for validity. If anything is found to be inconsistent, it will print an error message to standard error and call "abort ()".

 

This can be used to catch bugs inside libev itself: under normal circumstances, this function will never abort as of course libev keeps its data structures consistent.

ANATOMY OF A WATCHER¶

   static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
   {
     ev_io_stop (w);
     ev_break (loop, EVBREAK_ALL);
   }
   struct ev_loop *loop = ev_default_loop (0);
   ev_io stdin_watcher;
   ev_init (&stdin_watcher, my_cb);
   ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
   ev_io_start (loop, &stdin_watcher);
   ev_run (loop, 0);

"EV_READ"

"EV_WRITE"

The file descriptor in the "ev_io" watcher has become readable and/or writable.

"EV_TIMER"

The "ev_timer" watcher has timed out.

"EV_PERIODIC"

The "ev_periodic" watcher has timed out.

"EV_SIGNAL"

The signal specified in the "ev_signal" watcher has been received by a thread.

"EV_CHILD"

The pid specified in the "ev_child" watcher has received a status change.

"EV_STAT"

The path specified in the "ev_stat" watcher changed its attributes somehow.

"EV_IDLE"

The "ev_idle" watcher has determined that you have nothing better to do.

"EV_PREPARE"

"EV_CHECK"

All "ev_prepare" watchers are invoked just before "ev_run" starts to gather new events, and all "ev_check" watchers are invoked just after "ev_run" has gathered them, but before it invokes any callbacks for any received events. Callbacks of both watcher types can start and stop as many watchers as they want, and all of them will be taken into account (for example, a "ev_prepare" watcher might start an idle watcher to keep "ev_run" from blocking).

"EV_EMBED"

The embedded event loop specified in the "ev_embed" watcher needs attention.

"EV_FORK"

The event loop has been resumed in the child process after fork (see "ev_fork").

"EV_CLEANUP"

The event loop is about to be destroyed (see "ev_cleanup").

"EV_ASYNC"

The given async watcher has been asynchronously notified (see "ev_async").

"EV_CUSTOM"

Not ever sent (or otherwise used) by libev itself, but can be freely used by libev users to signal watchers (e.g. via "ev_feed_event").

"EV_ERROR"

An unspecified error has occurred, the watcher has been stopped. This might happen because the watcher could not be properly started because libev ran out of memory, a file descriptor was found to be closed or any other problem. Libev considers these application bugs.

 

You best act on it by reporting the problem and somehow coping with the watcher being stopped. Note that well-written programs should not receive an error ever, so when your watcher receives it, this usually indicates a bug in your program.

 

Libev will usually signal a few "dummy" events together with an error, for example it might indicate that a fd is readable or writable, and if your callbacks is well-written it can just attempt the operation and cope with the error from read() or write(). This will not work in multi-threaded programs, though, as the fd could already be closed and reused for another thing, so beware.

GENERIC WATCHER FUNCTIONS¶

"ev_init" (ev_TYPE *watcher, callback)

This macro initialises the generic portion of a watcher. The contents of the watcher object can be arbitrary (so "malloc" will do). Only the generic parts of the watcher are initialised, you need to call the type-specific "ev_TYPE_set" macro afterwards to initialise the type-specific parts. For each type there is also a "ev_TYPE_init" macro which rolls both calls into one.

 

You can reinitialise a watcher at any time as long as it has been stopped (or never started) and there are no pending events outstanding.

 

The callback is always of type "void (*)(struct ev_loop *loop, ev_TYPE *watcher, int revents)".

 

Example: Initialise an "ev_io" watcher in two steps.

 

   ev_io w;
   ev_init (&w, my_cb);
   ev_io_set (&w, STDIN_FILENO, EV_READ);
    

"ev_TYPE_set" (ev_TYPE *watcher, [args])

This macro initialises the type-specific parts of a watcher. You need to call "ev_init" at least once before you call this macro, but you can call "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 difference to the "ev_init" macro).

 

Although some watcher types do not have type-specific arguments (e.g. "ev_prepare") you still need to call its "set" macro.

 

See "ev_init", above, for an example.

"ev_TYPE_init" (ev_TYPE *watcher, callback, [args])

This convenience macro rolls both "ev_init" and "ev_TYPE_set" macro calls into a single call. This is the most convenient method to initialise a watcher. The same limitations apply, of course.

 

Example: Initialise and set an "ev_io" watcher in one step.

 

   ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
    

"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 "ev_io" watcher that is being abused as example in this whole section.

 

   ev_io_start (EV_DEFAULT_UC, &w);
    

"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, non-repeating timers are being stopped when they become pending - but calling "ev_TYPE_stop" ensures that the watcher is neither active nor pending. If you want to free or reuse the memory used by the watcher it is therefore a good idea to always call its "ev_TYPE_stop" function.

bool ev_is_active (ev_TYPE *watcher)

Returns a true value iff the watcher is active (i.e. it has been started and not yet been stopped). As long as a watcher is active you must not modify it.

bool ev_is_pending (ev_TYPE *watcher)

Returns a true value iff the watcher is pending, (i.e. it has outstanding events but its callback has not yet been invoked). As long as a watcher is pending (but not active) you must not call an init function on it (but "ev_TYPE_set" is safe), you must not change its priority, and you must make sure the watcher is available to libev (e.g. you cannot "free ()" it).

callback ev_cb (ev_TYPE *watcher)

Returns the callback currently set on the watcher.

ev_cb_set (ev_TYPE *watcher, callback)

Change the callback. You can change the callback at virtually any time (modulo threads).

ev_set_priority (ev_TYPE *watcher, int priority)

int ev_priority (ev_TYPE *watcher)

Set and query the priority of the watcher. The priority is a small integer between "EV_MAXPRI" (default: 2) and "EV_MINPRI" (default: "-2"). Pending watchers with higher priority will be invoked before watchers with lower priority, but priority will not keep watchers from being executed (except for "ev_idle" watchers).

 

If you need to suppress invocation when higher priority events are pending you need to look at "ev_idle" watchers, which provide this functionality.

 

You must not change the priority of a watcher as long as it is active or pending.

 

Setting a priority outside the range of "EV_MINPRI" to "EV_MAXPRI" is fine, as long as you do not mind that the priority value you query might or might not have been clamped to the valid range.

 

The default priority used by watchers when no priority has been set is always 0, which is supposed to not be too high and not be too low :).

 

See "WATCHER PRIORITY MODELS", below, for a more thorough treatment of priorities.

ev_invoke (loop, ev_TYPE *watcher, int revents)

Invoke the "watcher" with the given "loop" and "revents". Neither "loop" nor "revents" need to be valid as long as the watcher callback can deal with that fact, as both are simply passed through to the callback.

int ev_clear_pending (loop, ev_TYPE *watcher)

If the watcher is pending, this function clears its pending status and returns its "revents" bitset (as if its callback was invoked). If the watcher isn't pending it does nothing and returns 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.

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 "ev_clear_pending" will clear the pending event, even if the watcher was not started in the first place.

 

See also "ev_feed_fd_event" and "ev_feed_signal_event" for related functions that do not need a watcher.

WATCHER STATES¶

initialiased

Before a watcher can be registered with the event loop it has to be initialised. This can be done with a call to "ev_TYPE_init", or calls to "ev_init" followed by the watcher-specific "ev_TYPE_set" function.

 

In this state it is simply some block of memory that is suitable for use in an event loop. It can be moved around, freed, reused etc. at will - as long as you either keep the memory contents intact, or call "ev_TYPE_init" again.

started/running/active

Once a watcher has been started with a call to "ev_TYPE_start" it becomes property of the event loop, and is actively waiting for events. While in this state it cannot be accessed (except in a few documented ways), moved, freed or anything else - the only legal thing is to keep a pointer to it, and call libev functions on it that are documented to work on active watchers.

pending

If a watcher is active and libev determines that an event it is interested in has occurred (such as a timer expiring), it will become pending. It will stay in this pending state until either it is stopped or its callback is about to be invoked, so it is not normally pending inside the watcher callback.

 

The watcher might or might not be active while it is pending (for example, an expired non-repeating timer can be pending but no longer active). If it is stopped, it can be freely accessed (e.g. by calling "ev_TYPE_set"), but it is still property of the event loop at this time, so cannot be moved, freed or reused. And if it is active the rules described in the previous item still apply.

 

It is also possible to feed an event on a watcher that is not active (e.g. via "ev_feed_event"), in which case it becomes pending without being active.

stopped

A watcher can be stopped implicitly by libev (in which case it might still be pending), or explicitly by calling its "ev_TYPE_stop" function. The latter will clear any pending state the watcher might be in, regardless of whether it was active or not, so stopping a watcher explicitly before freeing it is often a good idea.

 

While stopped (and not pending) the watcher is essentially in the initialised state, that is, it can be reused, moved, modified in any way you wish (but when you trash the memory block, you need to "ev_TYPE_init" it again).

WATCHER PRIORITY MODELS¶

   ev_idle idle; // actual processing watcher
   ev_io io;     // actual event watcher
   static void
   io_cb (EV_P_ ev_io *w, int revents)
   {
     // stop the I/O watcher, we received the event, but
     // are not yet ready to handle it.
     ev_io_stop (EV_A_ w);
     // start the idle watcher to handle the actual event.
     // it will not be executed as long as other watchers
     // with the default priority are receiving events.
     ev_idle_start (EV_A_ &idle);
   }
   static void
   idle_cb (EV_P_ ev_idle *w, int revents)
   {
     // actual processing
     read (STDIN_FILENO, ...);
     // have to start the I/O watcher again, as
     // we have handled the event
     ev_io_start (EV_P_ &io);
   }
   // initialisation
   ev_idle_init (&idle, idle_cb);
   ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ);
   ev_io_start (EV_DEFAULT_ &io);

WATCHER TYPES¶

"ev_io" - is this file descriptor readable or writable?¶

ev_io_init (ev_io *, callback, int fd, int events)

ev_io_set (ev_io *, int fd, int events)

Configures an "ev_io" watcher. The "fd" is the file descriptor to receive events for and "events" is either "EV_READ", "EV_WRITE" or "EV_READ | EV_WRITE", to express the desire to receive the given events.

int fd [read-only]

The file descriptor being watched.

int events [read-only]

The events being watched.

   static void
   stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
   {
      ev_io_stop (loop, w);
     .. read from stdin here (or from w->fd) and handle any I/O errors
   }
   ...
   struct ev_loop *loop = ev_default_init (0);
   ev_io stdin_readable;
   ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
   ev_io_start (loop, &stdin_readable);
   ev_run (loop, 0);

"ev_timer" - relative and optionally repeating timeouts¶

1. Use a timer and stop, reinitialise and start it on activity.

This is the most obvious, but not the most simple way: In the beginning, start the watcher:

 

   ev_timer_init (timer, callback, 60., 0.);
   ev_timer_start (loop, timer);
    

 

Then, each time there is some activity, "ev_timer_stop" it, initialise it and start it again:

 

   ev_timer_stop (loop, timer);
   ev_timer_set (timer, 60., 0.);
   ev_timer_start (loop, timer);
    

 

This is relatively simple to implement, but means that each time there is some activity, libev will first have to remove the timer from its internal data structure and then add it again. Libev tries to be fast, but it's still not a constant-time operation.

2. Use a timer and re-start it with "ev_timer_again" inactivity.

This is the easiest way, and involves using "ev_timer_again" instead of "ev_timer_start".

 

To implement this, configure an "ev_timer" with a "repeat" value of 60 and then call "ev_timer_again" at start and each time you successfully read or write some data. If you go into an idle state where you do not expect data to travel on the socket, you can "ev_timer_stop" the timer, and "ev_timer_again" will automatically restart it if need be.

 

That means you can ignore both the "ev_timer_start" function and the "after" argument to "ev_timer_set", and only ever use the "repeat" member and "ev_timer_again".

 

At start:

 

   ev_init (timer, callback);
   timer->repeat = 60.;
   ev_timer_again (loop, timer);
    

 

Each time there is some activity:

 

   ev_timer_again (loop, timer);
    

 

It is even possible to change the time-out on the fly, regardless of whether the watcher is active or not:

 

   timer->repeat = 30.;
   ev_timer_again (loop, timer);
    

 

This is slightly more efficient then stopping/starting the timer each time you want to modify its timeout value, as libev does not have to completely remove and re-insert the timer from/into its internal data structure.

 

It is, however, even simpler than the "obvious" way to do it.

3. Let the timer time out, but then re-arm it as required.

This method is more tricky, but usually most efficient: Most timeouts are relatively long compared to the intervals between other activity - in our example, within 60 seconds, there are usually many I/O events with associated activity resets.

 

In this case, it would be more efficient to leave the "ev_timer" alone, but remember the time of last activity, and check for a real timeout only within the callback:

 

   ev_tstamp timeout = 60.;
   ev_tstamp last_activity; // time of last activity
   ev_timer timer;
   static void
   callback (EV_P_ ev_timer *w, int revents)
   {
     // calculate when the timeout would happen
     ev_tstamp after = last_activity - ev_now (EV_A) + timeout;
     // if negative, it means we the timeout already occured
     if (after < 0.)
       {
         // timeout occurred, take action
       }
     else
       {
         // callback was invoked, but there was some recent 
         // activity. simply restart the timer to time out
         // after "after" seconds, which is the earliest time
         // the timeout can occur.
         ev_timer_set (w, after, 0.);
         ev_timer_start (EV_A_ w);
       }
   }
    

 

To summarise the callback: first calculate in how many seconds the timeout will occur (by calculating the absolute time when it would occur, "last_activity + timeout", and subtracting the current time, "ev_now (EV_A)" from that).

 

If this value is negative, then we are already past the timeout, i.e. we timed out, and need to do whatever is needed in this case.

 

Otherwise, we now the earliest time at which the timeout would trigger, and simply start the timer with this timeout value.

 

In other words, each time the callback is invoked it will check whether the timeout cocured. If not, it will simply reschedule itself to check again at the earliest time it could time out. Rinse. Repeat.

 

This scheme causes more callback invocations (about one every 60 seconds minus half the average time between activity), but virtually no calls to libev to change the timeout.

 

To start the machinery, simply initialise the watcher and set "last_activity" to the current time (meaning there was some activity just now), then call the callback, which will "do the right thing" and start the timer:

 

   last_activity = ev_now (EV_A);
   ev_init (&timer, callback);
   callback (EV_A_ &timer, 0);
    

 

When there is some activity, simply store the current time in "last_activity", no libev calls at all:

 

   if (activity detected)
     last_activity = ev_now (EV_A);
    

 

When your timeout value changes, then the timeout can be changed by simply providing a new value, stopping the timer and calling the callback, which will agaion do the right thing (for example, time out immediately :).

 

   timeout = new_value;
   ev_timer_stop (EV_A_ &timer);
   callback (EV_A_ &timer, 0);
    

 

This technique is slightly more complex, but in most cases where the time-out is unlikely to be triggered, much more efficient.

4. Wee, just use a double-linked list for your timeouts.

If there is not one request, but many thousands (millions...), all employing some kind of timeout with the same timeout value, then one can do even better:

 

When starting the timeout, calculate the timeout value and put the timeout at the end of the list.

 

Then use an "ev_timer" to fire when the timeout at the beginning of the list is expected to fire (for example, using the technique #3).

 

When there is some activity, remove the timer from the list, recalculate the timeout, append it to the end of the list again, and make sure to update the "ev_timer" if it was taken from the beginning of the list.

 

This way, one can manage an unlimited number of timeouts in O(1) time for starting, stopping and updating the timers, at the expense of a major complication, and having to use a constant timeout. The constant timeout ensures that the list stays sorted.

   ev_timer_set (&timer, after + ev_now () - ev_time (), 0.);

ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)

ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)

Configure the timer to trigger after "after" seconds. If "repeat" is 0., then it will automatically be stopped once the timeout is reached. If it is positive, then the timer will automatically be configured to trigger again "repeat" seconds later, again, and again, until stopped manually.

 

The timer itself will do a best-effort at avoiding drift, that is, if you configure a timer to trigger every 10 seconds, then it will normally trigger at exactly 10 second intervals. If, however, your program cannot keep up with the timer (because it takes longer than those 10 seconds to do stuff) the timer will not fire more than once per event loop iteration.

ev_timer_again (loop, ev_timer *)

This will act as if the timer timed out, and restarts it again if it is repeating. It basically works like calling "ev_timer_stop", updating the timeout to the "repeat" value and calling "ev_timer_start".

 

The exact semantics are as in the following rules, all of which will be applied to the watcher:

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 "ev_timer_set (w, 5, 7)", "ev_timer_remaining" returns 5. When the timer is started and one second passes, "ev_timer_remaining" will return 4. When the timer expires and is restarted, it will return roughly 7 (likely slightly less as callback invocation takes some time, too), and so on.

ev_tstamp repeat [read-write]

The current "repeat" value. Will be used each time the watcher times out or "ev_timer_again" is called, and determines the next timeout (if any), which is also when any modifications are taken into account.

   static void
   one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
   {
     .. one minute over, w is actually stopped right here
   }
   ev_timer mytimer;
   ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
   ev_timer_start (loop, &mytimer);

   static void
   timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
   {
     .. ten seconds without any activity
   }
   ev_timer mytimer;
   ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
   ev_timer_again (&mytimer); /* start timer */
   ev_run (loop, 0);
   // and in some piece of code that gets executed on any "activity":
   // reset the timeout to start ticking again at 10 seconds
   ev_timer_again (&mytimer);

"ev_periodic" - to cron or not to cron?¶

ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)

ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)

Lots of arguments, let's sort it out... There are basically three modes of operation, and we will explain them from simplest to most complex:

ev_periodic_again (loop, ev_periodic *)

Simply stops and restarts the periodic watcher again. This is only useful when you changed some parameters or the reschedule callback would return a different time than the last time it was called (e.g. in a crond like program when the crontabs have changed).

ev_tstamp ev_periodic_at (ev_periodic *)

When active, returns the absolute time that the watcher is supposed to trigger next. This is not the same as the "offset" argument to "ev_periodic_set", but indeed works even in interval and manual rescheduling modes.

ev_tstamp offset [read-write]

When repeating, this contains the offset value, otherwise this is the absolute point in time (the "offset" value passed to "ev_periodic_set", although libev might modify this value for better numerical stability).

 

Can be modified any time, but changes only take effect when the periodic timer fires or "ev_periodic_again" is being called.

ev_tstamp interval [read-write]

The current interval value. Can be modified any time, but changes only take effect when the periodic timer fires or "ev_periodic_again" is being called.

ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]

The current reschedule callback, or 0, if this functionality is switched off. Can be changed any time, but changes only take effect when the periodic timer fires or "ev_periodic_again" is being called.

   static void
   clock_cb (struct ev_loop *loop, ev_periodic *w, int revents)
   {
     ... its now a full hour (UTC, or TAI or whatever your clock follows)
   }
   ev_periodic hourly_tick;
   ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
   ev_periodic_start (loop, &hourly_tick);

   #include <math.h>
   static ev_tstamp
   my_scheduler_cb (ev_periodic *w, ev_tstamp now)
   {
     return now + (3600. - fmod (now, 3600.));
   }
   ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);

   ev_periodic hourly_tick;
   ev_periodic_init (&hourly_tick, clock_cb,
                     fmod (ev_now (loop), 3600.), 3600., 0);
   ev_periodic_start (loop, &hourly_tick);

"ev_signal" - signal me when a signal gets signalled!¶

ev_signal_init (ev_signal *, callback, int signum)

ev_signal_set (ev_signal *, int signum)

Configures the watcher to trigger on the given signal number (usually one of the "SIGxxx" constants).

int signum [read-only]

The signal the watcher watches out for.

   static void
   sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
   {
     ev_break (loop, EVBREAK_ALL);
   }
   ev_signal signal_watcher;
   ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
   ev_signal_start (loop, &signal_watcher);

"ev_child" - watch out for process status changes¶

ev_child_init (ev_child *, callback, int pid, int trace)

ev_child_set (ev_child *, int pid, int trace)

Configures the watcher to wait for status changes of process "pid" (or any process if "pid" is specified as 0). The callback can look at the "rstatus" member of the "ev_child" watcher structure to see the status word (use the macros from "sys/wait.h" and see your systems "waitpid" documentation). The "rpid" member contains the pid of the process causing the status change. "trace" must be either 0 (only activate the watcher when the process terminates) or 1 (additionally activate the watcher when the process is stopped or continued).

int pid [read-only]

The process id this watcher watches out for, or 0, meaning any process id.

int rpid [read-write]

The process id that detected a status change.

int rstatus [read-write]

The process exit/trace status caused by "rpid" (see your systems "waitpid" and "sys/wait.h" documentation for details).

   ev_child cw;
   static void
   child_cb (EV_P_ ev_child *w, int revents)
   {
     ev_child_stop (EV_A_ w);
     printf ("process %d exited with status %x\n", w->rpid, w->rstatus);
   }
   pid_t pid = fork ();
   if (pid < 0)
     // error
   else if (pid == 0)
     {
       // the forked child executes here
       exit (1);
     }
   else
     {
       ev_child_init (&cw, child_cb, pid, 0);
       ev_child_start (EV_DEFAULT_ &cw);
     }

"ev_stat" - did the file attributes just change?¶

ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)

ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)

Configures the watcher to wait for status changes of the given "path". The "interval" is a hint on how quickly a change is expected to be detected and should normally be specified as 0 to let libev choose a suitable value. The memory pointed to by "path" must point to the same path for as long as the watcher is active.

 

The callback will receive an "EV_STAT" event when a change was detected, relative to the attributes at the time the watcher was started (or the last change was detected).

ev_stat_stat (loop, ev_stat *)

Updates the stat buffer immediately with new values. If you change the watched path in your callback, you could call this function to avoid detecting this change (while introducing a race condition if you are not the only one changing the path). Can also be useful simply to find out the new values.

ev_statdata attr [read-only]

The most-recently detected attributes of the file. Although the type is "ev_statdata", this is usually the (or one of the) "struct stat" types suitable for your system, but you can only rely on the POSIX-standardised members to be present. If the "st_nlink" member is 0, then there was some error while "stat"ing the file.

ev_statdata prev [read-only]

The previous attributes of the file. The callback gets invoked whenever "prev" != "attr", or, more precisely, one or more of these members differ: "st_dev", "st_ino", "st_mode", "st_nlink", "st_uid", "st_gid", "st_rdev", "st_size", "st_atime", "st_mtime", "st_ctime".

ev_tstamp interval [read-only]

The specified interval.

const char *path [read-only]

The file system path that is being watched.

   static void
   passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
   {
     /* /etc/passwd changed in some way */
     if (w->attr.st_nlink)
       {
         printf ("passwd current size  %ld\n", (long)w->attr.st_size);
         printf ("passwd current atime %ld\n", (long)w->attr.st_mtime);
         printf ("passwd current mtime %ld\n", (long)w->attr.st_mtime);
       }
     else
       /* you shalt not abuse printf for puts */
       puts ("wow, /etc/passwd is not there, expect problems. "
             "if this is windows, they already arrived\n");
   }
   ...
   ev_stat passwd;
   ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
   ev_stat_start (loop, &passwd);

   static ev_stat passwd;
   static ev_timer timer;
   static void
   timer_cb (EV_P_ ev_timer *w, int revents)
   {
     ev_timer_stop (EV_A_ w);
     /* now it's one second after the most recent passwd change */
   }
   static void
   stat_cb (EV_P_ ev_stat *w, int revents)
   {
     /* reset the one-second timer */
     ev_timer_again (EV_A_ &timer);
   }
   ...
   ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
   ev_stat_start (loop, &passwd);
   ev_timer_init (&timer, timer_cb, 0., 1.02);

"ev_idle" - when you've got nothing better to do...¶

ev_idle_init (ev_idle *, callback)

Initialises and configures the idle watcher - it has no parameters of any kind. There is a "ev_idle_set" macro, but using it is utterly pointless, believe me.

   static void
   idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
   {
     free (w);
     // now do something you wanted to do when the program has
     // no longer anything immediate to do.
   }
   ev_idle *idle_watcher = malloc (sizeof (ev_idle));
   ev_idle_init (idle_watcher, idle_cb);
   ev_idle_start (loop, idle_watcher);

"ev_prepare" and "ev_check" - customise your event loop!¶

ev_prepare_init (ev_prepare *, callback)

ev_check_init (ev_check *, callback)

Initialises and configures the prepare or check watcher - they have no parameters of any kind. There are "ev_prepare_set" and "ev_check_set" macros, but using them is utterly, utterly, utterly and completely pointless.

   static ev_io iow [nfd];
   static ev_timer tw;
   static void
   io_cb (struct ev_loop *loop, ev_io *w, int revents)
   {
   }
   // create io watchers for each fd and a timer before blocking
   static void
   adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents)
   {
     int timeout = 3600000;
     struct pollfd fds [nfd];
     // actual code will need to loop here and realloc etc.
     adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));
     /* the callback is illegal, but won't be called as we stop during check */
     ev_timer_init (&tw, 0, timeout * 1e-3, 0.);
     ev_timer_start (loop, &tw);
     // create one ev_io per pollfd
     for (int i = 0; i < nfd; ++i)
       {
         ev_io_init (iow + i, io_cb, fds [i].fd,
           ((fds [i].events & POLLIN ? EV_READ : 0)
            | (fds [i].events & POLLOUT ? EV_WRITE : 0)));
         fds [i].revents = 0;
         ev_io_start (loop, iow + i);
       }
   }
   // stop all watchers after blocking
   static void
   adns_check_cb (struct ev_loop *loop, ev_check *w, int revents)
   {
     ev_timer_stop (loop, &tw);
     for (int i = 0; i < nfd; ++i)
       {
         // set the relevant poll flags
         // could also call adns_processreadable etc. here
         struct pollfd *fd = fds + i;
         int revents = ev_clear_pending (iow + i);
         if (revents & EV_READ ) fd->revents |= fd->events & POLLIN;
         if (revents & EV_WRITE) fd->revents |= fd->events & POLLOUT;
         // now stop the watcher
         ev_io_stop (loop, iow + i);
       }
     adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
   }

   static void
   timer_cb (EV_P_ ev_timer *w, int revents)
   {
     adns_state ads = (adns_state)w->data;
     update_now (EV_A);
     adns_processtimeouts (ads, &tv_now);
   }
   static void
   io_cb (EV_P_ ev_io *w, int revents)
   {
     adns_state ads = (adns_state)w->data;
     update_now (EV_A);
     if (revents & EV_READ ) adns_processreadable  (ads, w->fd, &tv_now);
     if (revents & EV_WRITE) adns_processwriteable (ads, w->fd, &tv_now);
   }
   // do not ever call adns_afterpoll

   static gint
   event_poll_func (GPollFD *fds, guint nfds, gint timeout)
   {
     int got_events = 0;
     for (n = 0; n < nfds; ++n)
       // create/start io watcher that sets the relevant bits in fds[n] and increment got_events
     if (timeout >= 0)
       // create/start timer
     // poll
     ev_run (EV_A_ 0);
     // stop timer again
     if (timeout >= 0)
       ev_timer_stop (EV_A_ &to);
     // stop io watchers again - their callbacks should have set
     for (n = 0; n < nfds; ++n)
       ev_io_stop (EV_A_ iow [n]);
     return got_events;
   }

"ev_embed" - when one backend isn't enough...¶

ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)

ev_embed_set (ev_embed *, callback, struct ev_loop *embedded_loop)

Configures the watcher to embed the given loop, which must be embeddable. If the callback is 0, then "ev_embed_sweep" will be invoked automatically, otherwise it is the responsibility of the callback to invoke it (it will continue to be called until the sweep has been done, if you do not want that, you need to temporarily stop the embed watcher).

ev_embed_sweep (loop, ev_embed *)

Make a single, non-blocking sweep over the embedded loop. This works similarly to "ev_run (embedded_loop, EVRUN_NOWAIT)", but in the most appropriate way for embedded loops.

struct ev_loop *other [read-only]

The embedded event loop.

   struct ev_loop *loop_hi = ev_default_init (0);
   struct ev_loop *loop_lo = 0;
   ev_embed embed;
   
   // see if there is a chance of getting one that works
   // (remember that a flags value of 0 means autodetection)
   loop_lo = ev_embeddable_backends () & ev_recommended_backends ()
     ? ev_loop_new (ev_embeddable_backends () & ev_recommended_backends ())
     : 0;
   // if we got one, then embed it, otherwise default to loop_hi
   if (loop_lo)
     {
       ev_embed_init (&embed, 0, loop_lo);
       ev_embed_start (loop_hi, &embed);
     }
   else
     loop_lo = loop_hi;

   struct ev_loop *loop = ev_default_init (0);
   struct ev_loop *loop_socket = 0;
   ev_embed embed;
   
   if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
     if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
       {
         ev_embed_init (&embed, 0, loop_socket);
         ev_embed_start (loop, &embed);
       }
   if (!loop_socket)
     loop_socket = loop;
   // now use loop_socket for all sockets, and loop for everything else

"ev_fork" - the audacity to resume the event loop after a fork¶

ev_fork_init (ev_fork *, callback)

Initialises and configures the fork watcher - it has no parameters of any kind. There is a "ev_fork_set" macro, but using it is utterly pointless, really.

"ev_cleanup" - even the best things end¶

ev_cleanup_init (ev_cleanup *, callback)

Initialises and configures the cleanup watcher - it has no parameters of any kind. There is a "ev_cleanup_set" macro, but using it is utterly pointless, I assure you.

   static void
   program_exits (void)
   {
     ev_loop_destroy (EV_DEFAULT_UC);
   }
   ...
   atexit (program_exits);

"ev_async" - how to wake up an event loop¶

queueing from a signal handler context

To implement race-free queueing, you simply add to the queue in the signal handler but you block the signal handler in the watcher callback. Here is an example that does that for some fictitious SIGUSR1 handler:

 

   static ev_async mysig;
   static void
   sigusr1_handler (void)
   {
     sometype data;
     // no locking etc.
     queue_put (data);
     ev_async_send (EV_DEFAULT_ &mysig);
   }
   static void
   mysig_cb (EV_P_ ev_async *w, int revents)
   {
     sometype data;
     sigset_t block, prev;
     sigemptyset (&block);
     sigaddset (&block, SIGUSR1);
     sigprocmask (SIG_BLOCK, &block, &prev);
     while (queue_get (&data))
       process (data);
     if (sigismember (&prev, SIGUSR1)
       sigprocmask (SIG_UNBLOCK, &block, 0);
   }
    

 

(Note: pthreads in theory requires you to use "pthread_setmask" instead of "sigprocmask" when you use threads, but libev doesn't do it either...).

queueing from a thread context

The strategy for threads is different, as you cannot (easily) block threads but you can easily preempt them, so to queue safely you need to employ a traditional mutex lock, such as in this pthread example:

 

   static ev_async mysig;
   static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
   static void
   otherthread (void)
   {
     // only need to lock the actual queueing operation
     pthread_mutex_lock (&mymutex);
     queue_put (data);
     pthread_mutex_unlock (&mymutex);
     ev_async_send (EV_DEFAULT_ &mysig);
   }
   static void
   mysig_cb (EV_P_ ev_async *w, int revents)
   {
     pthread_mutex_lock (&mymutex);
     while (queue_get (&data))
       process (data);
     pthread_mutex_unlock (&mymutex);
   }
    

ev_async_init (ev_async *, callback)

Initialises and configures the async watcher - it has no parameters of any kind. There is a "ev_async_set" macro, but using it is utterly pointless, trust me.

ev_async_send (loop, ev_async *)

Sends/signals/activates the given "ev_async" watcher, that is, feeds an "EV_ASYNC" event on the watcher into the event loop, and instantly returns.

 

Unlike "ev_feed_event", this call is safe to do from other threads, signal or similar contexts (see the discussion of "EV_ATOMIC_T" in the embedding section below on what exactly this means).

 

Note that, as with other watchers in libev, multiple events might get compressed into a single callback invocation (another way to look at this is that "ev_async" watchers are level-triggered: they are set on "ev_async_send", reset when the event loop detects that).

 

This call incurs the overhead of at most one extra system call per event loop iteration, if the event loop is blocked, and no syscall at all if the event loop (or your program) is processing events. That means that repeated calls are basically free (there is no need to avoid calls for performance reasons) and that the overhead becomes smaller (typically zero) under load.

bool = ev_async_pending (ev_async *)

Returns a non-zero value when "ev_async_send" has been called on the watcher but the event has not yet been processed (or even noted) by the event loop.

 

"ev_async_send" sets a flag in the watcher and wakes up the loop. When the loop iterates next and checks for the watcher to have become active, it will reset the flag again. "ev_async_pending" can be used to very quickly check whether invoking the loop might be a good idea.

 

Not that this does not check whether the watcher itself is pending, only whether it has been requested to make this watcher pending: there is a time window between the event loop checking and resetting the async notification, and the callback being invoked.

OTHER FUNCTIONS¶

ev_once (loop, int fd, int events, ev_tstamp timeout, callback)

This function combines a simple timer and an I/O watcher, calls your callback on whichever event happens first and automatically stops both watchers. This is useful if you want to wait for a single event on an fd or timeout without having to allocate/configure/start/stop/free one or more watchers yourself.

 

If "fd" is less than 0, then no I/O watcher will be started and the "events" argument is being ignored. Otherwise, an "ev_io" watcher for the given "fd" and "events" set will be created and started.

 

If "timeout" is less than 0, then no timeout watcher will be started. Otherwise an "ev_timer" watcher with after = "timeout" (and repeat = 0) will be started. 0 is a valid timeout.

 

The callback has the type "void (*cb)(int revents, void *arg)" and is passed an "revents" set like normal event callbacks (a combination of "EV_ERROR", "EV_READ", "EV_WRITE" or "EV_TIMER") and the "arg" value passed to "ev_once". Note that it is possible to receive both a timeout and an io event at the same time - you probably should give io events precedence.

 

Example: wait up to ten seconds for data to appear on STDIN_FILENO.

 

   static void stdin_ready (int revents, void *arg)
   {
     if (revents & EV_READ)
       /* stdin might have data for us, joy! */;
     else if (revents & EV_TIMER)
       /* doh, nothing entered */;
   }
   ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
    

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.

ev_feed_signal_event (loop, int signum)

Feed an event as if the given signal occurred. See also "ev_feed_signal", which is async-safe.

COMMON OR USEFUL IDIOMS (OR BOTH)¶

ASSOCIATING CUSTOM DATA WITH A WATCHER¶

   struct my_io
   {
     ev_io io;
     int otherfd;
     void *somedata;
     struct whatever *mostinteresting;
   };
   ...
   struct my_io w;
   ev_io_init (&w.io, my_cb, fd, EV_READ);

   static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
   {
     struct my_io *w = (struct my_io *)w_;
     ...
   }

BUILDING YOUR OWN COMPOSITE WATCHERS¶

   struct my_biggy
   {
     int some_data;
     ev_timer t1;
     ev_timer t2;
   }

   #include <stddef.h>
   static void
   t1_cb (EV_P_ ev_timer *w, int revents)
   {
     struct my_biggy big = (struct my_biggy *)
       (((char *)w) - offsetof (struct my_biggy, t1));
   }
   static void
   t2_cb (EV_P_ ev_timer *w, int revents)
   {
     struct my_biggy big = (struct my_biggy *)
       (((char *)w) - offsetof (struct my_biggy, t2));
   }

AVOIDING FINISHING BEFORE RETURNING¶

  callback ()
  {
    free (request);
  }
  request = start_new_request (..., callback);

   ev_set_cb (watcher, callback);
   ev_feed_event (EV_A_ watcher, 0);

MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS¶

   // main loop
   int exit_main_loop = 0;
   while (!exit_main_loop)
     ev_run (EV_DEFAULT_ EVRUN_ONCE);
   // in a modal watcher
   int exit_nested_loop = 0;
   while (!exit_nested_loop)
     ev_run (EV_A_ EVRUN_ONCE);

   // 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;

THREAD LOCKING EXAMPLE¶

   typedef struct {
     mutex_t lock; /* global loop lock */
     ev_async async_w;
     thread_t tid;
     cond_t invoke_cv;
   } userdata;
   void prepare_loop (EV_P)
   {
      // for simplicity, we use a static userdata struct.
      static userdata u;
      ev_async_init (&u->async_w, async_cb);
      ev_async_start (EV_A_ &u->async_w);
      pthread_mutex_init (&u->lock, 0);
      pthread_cond_init (&u->invoke_cv, 0);
      // now associate this with the loop
      ev_set_userdata (EV_A_ u);
      ev_set_invoke_pending_cb (EV_A_ l_invoke);
      ev_set_loop_release_cb (EV_A_ l_release, l_acquire);
      // then create the thread running ev_run
      pthread_create (&u->tid, 0, l_run, EV_A);
   }

   static void
   async_cb (EV_P_ ev_async *w, int revents)
   {
      // just used for the side effects
   }

   static void
   l_release (EV_P)
   {
     userdata *u = ev_userdata (EV_A);
     pthread_mutex_unlock (&u->lock);
   }
   static void
   l_acquire (EV_P)
   {
     userdata *u = ev_userdata (EV_A);
     pthread_mutex_lock (&u->lock);
   }

   void *
   l_run (void *thr_arg)
   {
     struct ev_loop *loop = (struct ev_loop *)thr_arg;
     l_acquire (EV_A);
     pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0);
     ev_run (EV_A_ 0);
     l_release (EV_A);
     return 0;
   }

   static void
   l_invoke (EV_P)
   {
     userdata *u = ev_userdata (EV_A);
     while (ev_pending_count (EV_A))
       {
         wake_up_other_thread_in_some_magic_or_not_so_magic_way ();
         pthread_cond_wait (&u->invoke_cv, &u->lock);
       }
   }

   static void
   real_invoke_pending (EV_P)
   {
     userdata *u = ev_userdata (EV_A);
     pthread_mutex_lock (&u->lock);
     ev_invoke_pending (EV_A);
     pthread_cond_signal (&u->invoke_cv);
     pthread_mutex_unlock (&u->lock);
   }

   ev_timer timeout_watcher;
   userdata *u = ev_userdata (EV_A);
   ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
   pthread_mutex_lock (&u->lock);
   ev_timer_start (EV_A_ &timeout_watcher);
   ev_async_send (EV_A_ &u->async_w);
   pthread_mutex_unlock (&u->lock);

THREADS, COROUTINES, CONTINUATIONS, QUEUES... INSTEAD OF CALLBACKS¶

   #define EV_CB_DECLARE(type)   struct my_coro *cb;
   #define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb)

   void
   wait_for_event (ev_watcher *w)
   {
     ev_cb_set (w) = current_coro;
     switch_to (libev_coro);
   }

   // my_ev.h
   #define EV_CB_DECLARE(type)   struct my_coro *cb;
   #define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb);
   #include "../libev/ev.h"
   // my_ev.c
   #define EV_H "my_ev.h"
   #include "../libev/ev.c"

LIBEVENT EMULATION¶

•

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.

•

Use it by including <event.h>, as usual.

•

The following members are fully supported: ev_base, ev_callback, ev_arg, ev_fd, ev_res, ev_events.

•

Avoid using ev_flags and the EVLIST_*-macros, while it is maintained by libev, it does not work exactly the same way as in libevent (consider it a private API).

•

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.

•

In libevent, the last base created gets the signals, in libev, the base that registered the signal gets the signals.

•

Other members are not supported.

•

The libev emulation is not ABI compatible to libevent, you need to use the libev header file and library.

C++ SUPPORT¶

   #include <ev++.h>

"ev::READ", "ev::WRITE" etc.

These are just enum values with the same values as the "EV_READ" etc. macros from ev.h.

"ev::tstamp", "ev::now"

Aliases to the same types/functions as with the "ev_" prefix.

"ev::io", "ev::timer", "ev::periodic", "ev::idle", "ev::sig" etc.

For each "ev_TYPE" watcher in ev.h there is a corresponding class of the same name in the "ev" namespace, with the exception of "ev_signal" which is called "ev::sig" to avoid clashes with the "signal" macro defined by many implementations.

 

All of those classes have these methods:

   class myclass
   {
     ev::io   io  ; void io_cb   (ev::io   &w, int revents);
     ev::io   io2 ; void io2_cb  (ev::io   &w, int revents);
     ev::idle idle; void idle_cb (ev::idle &w, int revents);
     myclass (int fd)
     {
       io  .set <myclass, &myclass::io_cb  > (this);
       io2 .set <myclass, &myclass::io2_cb > (this);
       idle.set <myclass, &myclass::idle_cb> (this);
       io.set (fd, ev::WRITE); // configure the watcher
       io.start ();            // start it whenever convenient
       io2.start (fd, ev::READ); // set + start in one call
     }
   };

OTHER LANGUAGE BINDINGS¶

Perl

The EV module implements the full libev API and is actually used to test libev. EV is developed together with libev. Apart from the EV core module, there are additional modules that implement libev-compatible interfaces to "libadns" ("EV::ADNS", but "AnyEvent::DNS" is preferred nowadays), "Net::SNMP" ("Net::SNMP::EV") and the "libglib" event core ("Glib::EV" and "EV::Glib").

 

It can be found and installed via CPAN, its homepage is at <http://software.schmorp.de/pkg/EV>.

Python

Python bindings can be found at <http://code.google.com/p/pyev/>. It seems to be quite complete and well-documented.

Ruby

Tony Arcieri has written a ruby extension that offers access to a subset of the libev API and adds file handle abstractions, asynchronous DNS and more on top of it. It can be found via gem servers. Its homepage is at <http://rev.rubyforge.org/>.

 

Roger Pack reports that using the link order "-lws2_32 -lmsvcrt-ruby-190" makes rev work even on mingw.

Haskell

A haskell binding to libev is available at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hlibev <http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hlibev>.

D

Leandro Lucarella has written a D language binding (ev.d) for libev, to be found at <http://www.llucax.com.ar/proj/ev.d/index.html>.

Ocaml

Erkki Seppala has written Ocaml bindings for libev, to be found at http://modeemi.cs.tut.fi/~flux/software/ocaml-ev/ <http://modeemi.cs.tut.fi/~flux/software/ocaml-ev/>.

Lua

Brian Maher has written a partial interface to libev for lua (at the time of this writing, only "ev_io" and "ev_timer"), to be found at http://github.com/brimworks/lua-ev <http://github.com/brimworks/lua-ev>.

MACRO MAGIC¶

"EV_A", "EV_A_"

This provides the loop argument for functions, if one is required ("ev loop argument"). The "EV_A" form is used when this is the sole argument, "EV_A_" is used when other arguments are following. Example:

 

   ev_unref (EV_A);
   ev_timer_add (EV_A_ watcher);
   ev_run (EV_A_ 0);
    

 

It assumes the variable "loop" of type "struct ev_loop *" is in scope, which is often provided by the following macro.

"EV_P", "EV_P_"

This provides the loop parameter for functions, if one is required ("ev loop parameter"). The "EV_P" form is used when this is the sole parameter, "EV_P_" is used when other parameters are following. Example:

 

   // this is how ev_unref is being declared
   static void ev_unref (EV_P);
   // this is how you can declare your typical callback
   static void cb (EV_P_ ev_timer *w, int revents)
    

 

It declares a parameter "loop" of type "struct ev_loop *", quite suitable for use with "EV_A".

"EV_DEFAULT", "EV_DEFAULT_"

Similar to the other two macros, this gives you the value of the default loop, if multiple loops are supported ("ev loop default"). The default loop will be initialised if it isn't already initialised.

 

For non-multiplicity builds, these macros do nothing, so you always have to initialise the loop somewhere.

"EV_DEFAULT_UC", "EV_DEFAULT_UC_"

Usage identical to "EV_DEFAULT" and "EV_DEFAULT_", but requires that the default loop has been initialised ("UC" == unchecked). Their behaviour is undefined when the default loop has not been initialised by a previous execution of "EV_DEFAULT", "EV_DEFAULT_" or "ev_default_init (...)".

 

It is often prudent to use "EV_DEFAULT" when initialising the first watcher in a function but use "EV_DEFAULT_UC" afterwards.

   static void
   check_cb (EV_P_ ev_timer *w, int revents)
   {
     ev_check_stop (EV_A_ w);
   }
   ev_check check;
   ev_check_init (&check, check_cb);
   ev_check_start (EV_DEFAULT_ &check);
   ev_run (EV_DEFAULT_ 0);

EMBEDDING¶

FILESETS¶

   #define EV_STANDALONE 1
   #include "ev.c"

   #define EV_STANDALONE 1
   #include "ev.h"

   ev.h
   ev.c
   ev_vars.h
   ev_wrap.h
   ev_win32.c      required on win32 platforms only
   ev_select.c     only when select backend is enabled (which is enabled by default)
   ev_poll.c       only when poll backend is enabled (disabled by default)
   ev_epoll.c      only when the epoll backend is enabled (disabled by default)
   ev_kqueue.c     only when the kqueue backend is enabled (disabled by default)
   ev_port.c       only when the solaris port backend is enabled (disabled by default)

   #include "event.c"

   #include "event.h"

   event.h
   event.c

   libev.m4

PREPROCESSOR SYMBOLS/MACROS¶

EV_COMPAT3 (h)

Backwards compatibility is a major concern for libev. This is why this release of libev comes with wrappers for the functions and symbols that have been renamed between libev version 3 and 4.

 

You can disable these wrappers (to test compatibility with future versions) by defining "EV_COMPAT3" to 0 when compiling your sources. This has the additional advantage that you can drop the "struct" from "struct ev_loop" declarations, as libev will provide an "ev_loop" typedef in that case.

 

In some future version, the default for "EV_COMPAT3" will become 0, and in some even more future version the compatibility code will be removed completely.

EV_STANDALONE (h)

Must always be 1 if you do not use autoconf configuration, which keeps libev from including 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 event.h that are not directly supported by the libev core alone.

 

In standalone mode, libev will still try to automatically deduce the configuration, but has to be more conservative.

EV_USE_FLOOR

If defined to be 1, libev will use the "floor ()" function for its periodic reschedule calculations, otherwise libev will fall back on a portable (slower) implementation. If you enable this, you usually have to link against libm or something equivalent. Enabling this when the "floor" function is not available will fail, so the safe default is to not enable this.

EV_USE_MONOTONIC

If defined to be 1, libev will try to detect the availability of the monotonic clock option at both compile time and runtime. Otherwise no use of the monotonic clock option will be attempted. If you enable this, you usually have to link against librt or something similar. Enabling it when the functionality isn't available is safe, though, although you have to make sure you link against any libraries where the "clock_gettime" function is hiding in (often -lrt). See also "EV_USE_CLOCK_SYSCALL".

EV_USE_REALTIME

If defined to be 1, libev will try to detect the availability of the real-time clock option at compile time (and assume its availability at runtime if successful). Otherwise no use of the real-time clock option will be attempted. This effectively replaces "gettimeofday" by "clock_get (CLOCK_REALTIME, ...)" and will not normally affect correctness. See the note about libraries in the description of "EV_USE_MONOTONIC", though. Defaults to the opposite value of "EV_USE_CLOCK_SYSCALL".

EV_USE_CLOCK_SYSCALL

If defined to be 1, libev will try to use a direct syscall instead of calling the system-provided "clock_gettime" function. This option exists because on GNU/Linux, "clock_gettime" is in "librt", but "librt" unconditionally pulls in "libpthread", slowing down single-threaded programs needlessly. Using a direct syscall is slightly slower (in theory), because no optimised vdso implementation can be used, but avoids the pthread dependency. Defaults to 1 on GNU/Linux with glibc 2.x or higher, as it simplifies linking (no need for "-lrt").

EV_USE_NANOSLEEP

If defined to be 1, libev will assume that "nanosleep ()" is available and will use it for delays. Otherwise it will use "select ()".

EV_USE_EVENTFD

If defined to be 1, then libev will assume that "eventfd ()" is available and will probe for kernel support at runtime. This will improve "ev_signal" and "ev_async" performance and reduce resource consumption. If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc 2.7 or newer, otherwise disabled.

EV_USE_SELECT

If undefined or defined to be 1, libev will compile in support for the "select"(2) backend. No attempt at auto-detection will be done: if no other method takes over, select will be it. Otherwise the select backend will not be compiled in.

EV_SELECT_USE_FD_SET

If defined to 1, then the select backend will use the system "fd_set" structure. This is useful if libev doesn't compile due to a missing "NFDBITS" or "fd_mask" definition or it mis-guesses the bitset layout on exotic systems. This usually limits the range of file descriptors to some low limit such as 1024 or might have other limitations (winsocket only allows 64 sockets). The "FD_SETSIZE" macro, set before compilation, configures the maximum size of the "fd_set".

EV_SELECT_IS_WINSOCKET

When defined to 1, the select backend will assume that select/socket/connect etc. don't understand file descriptors but wants osf handles on win32 (this is the case when the select to be used is the winsock select). This means that it will call "_get_osfhandle" on the fd to convert it to an OS handle. Otherwise, it is assumed that all these functions actually work on fds, even on win32. Should not be defined on non-win32 platforms.

EV_FD_TO_WIN32_HANDLE(fd)

If "EV_SELECT_IS_WINSOCKET" is enabled, then libev needs a way to map file descriptors to socket handles. When not defining this symbol (the default), then libev will call "_get_osfhandle", which is usually correct. In some cases, programs use their own file descriptor management, in which case they can provide this function to map fds to socket handles.

EV_WIN32_HANDLE_TO_FD(handle)

If "EV_SELECT_IS_WINSOCKET" then libev maps handles to file descriptors using the standard "_open_osfhandle" function. For programs implementing their own fd to handle mapping, overwriting this function makes it easier to do so. This can be done by defining this macro to an appropriate value.

EV_WIN32_CLOSE_FD(fd)

If programs implement their own fd to handle mapping on win32, then this macro can be used to override the "close" function, useful to unregister file descriptors again. Note that the replacement function has to close the underlying OS handle.

EV_USE_POLL

If defined to be 1, libev will compile in support for the "poll"(2) backend. Otherwise it will be enabled on non-win32 platforms. It takes precedence over select.

EV_USE_EPOLL

If defined to be 1, libev will compile in support for the Linux "epoll"(7) backend. Its availability will be detected at runtime, otherwise another method will be used as fallback. This is the preferred backend for GNU/Linux systems. If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.

EV_USE_KQUEUE

If defined to be 1, libev will compile in support for the BSD style "kqueue"(2) backend. Its actual availability will be detected at runtime, otherwise another method will be used as fallback. This is the preferred backend for BSD and BSD-like systems, although on most BSDs kqueue only supports some types of fds correctly (the only platform we found that supports ptys for example was NetBSD), so kqueue might be compiled in, but not be used unless explicitly requested. The best way to use it is to find out whether kqueue supports your type of fd properly and use an embedded kqueue loop.

EV_USE_PORT

If defined to be 1, libev will compile in support for the Solaris 10 port style backend. Its availability will be detected at runtime, otherwise another method will be used as fallback. This is the preferred backend for Solaris 10 systems.

EV_USE_DEVPOLL

Reserved for future expansion, works like the USE symbols above.

EV_USE_INOTIFY

If defined to be 1, libev will compile in support for the Linux inotify interface to speed up "ev_stat" watchers. Its actual availability will be detected at runtime. If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.

EV_NO_SMP

If defined to be 1, libev will assume that memory is always coherent between threads, that is, threads can be used, but threads never run on different cpus (or different cpu cores). This reduces dependencies and makes libev faster.

EV_NO_THREADS

If defined to be 1, libev will assume that it will never be called from different threads, which is a stronger assumption than "EV_NO_SMP", above. This reduces dependencies and makes libev faster.

EV_ATOMIC_T

Libev requires an integer type (suitable for storing 0 or 1) whose access is atomic and serialised with respect to other threads or signal contexts. No such type is easily found in the C language, so you can provide your own type that you know is safe for your purposes. It is used both for signal handler "locking" as well as for signal and thread safety in "ev_async" watchers.

 

In the absence of this define, libev will use "sig_atomic_t volatile" (from signal.h), which is usually good enough on most platforms, although strictly speaking using a type that also implies a memory fence is required.

EV_H (h)

The name of the ev.h header file used to include it. The default if undefined is "ev.h" in event.h, ev.c and ev++.h. This can be used to virtually rename the ev.h header file in case of conflicts.

EV_CONFIG_H (h)

If "EV_STANDALONE" isn't 1, this variable can be used to override ev.c's idea of where to find the config.h file, similarly to "EV_H", above.

EV_EVENT_H (h)

Similarly to "EV_H", this macro can be used to override event.c's idea of how the event.h header can be found, the default is "event.h".

EV_PROTOTYPES (h)

If defined to be 0, then 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.

EV_MULTIPLICITY

If undefined or defined to 1, then all event-loop-specific functions will have the "struct ev_loop *" as first argument, and you can create additional independent event loops. Otherwise there will be no support for multiple event loops and there is no first event loop pointer argument. Instead, all functions act on the single default loop.

 

Note that "EV_DEFAULT" and "EV_DEFAULT_" will no longer provide a default loop when multiplicity is switched off - you always have to initialise the loop manually in this case.

EV_MINPRI

EV_MAXPRI

The range of allowed priorities. "EV_MINPRI" must be smaller or equal to "EV_MAXPRI", but otherwise there are no non-obvious limitations. You can provide for more priorities by overriding those symbols (usually defined to be "-2" and 2, respectively).

 

When doing priority-based operations, libev usually has to linearly search all the priorities, so having many of them (hundreds) uses a lot of space and time, so using the defaults of five priorities (-2 .. +2) is usually fine.

 

If your embedding application does not need any priorities, defining these both to 0 will save some memory and CPU.

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 1 (and the platform supports it), then the respective watcher type is supported. If defined to be 0, then it is not. Disabling watcher types mainly saves code size.

EV_FEATURES

If you need to shave off some kilobytes of code at the expense of some speed (but with the full API), you can define this symbol to request certain subsets of functionality. The default is to enable all features that can be enabled on the platform.

 

A typical way to use this symbol is to define it to 0 (or to a bitset with some broad features you want) and then selectively re-enable additional parts you want, for example if you want everything minimal, but multiple event loop support, async and child watchers and the poll backend, use this:

 

   #define EV_FEATURES 0
   #define EV_MULTIPLICITY 1
   #define EV_USE_POLL 1
   #define EV_CHILD_ENABLE 1
   #define EV_ASYNC_ENABLE 1
    

 

The actual value is a bitset, it can be a combination of the following values:

EV_API_STATIC

If this symbol is defined (by default it is not), then all identifiers will have static linkage. This means that libev will not export any identifiers, and you cannot link against libev anymore. This can be useful when you embed libev, only want to use libev functions in a single file, and do not want its identifiers to be visible.

 

To use this, define "EV_API_STATIC" and include ev.c in the file that wants to use libev.

 

This option only works when libev is compiled with a C compiler, as C++ doesn't support the required declaration syntax.

EV_AVOID_STDIO

If this is set to 1 at compiletime, then libev will avoid using stdio functions (printf, scanf, perror etc.). This will increase the code size 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.

EV_NSIG

The highest supported signal number, +1 (or, the number of signals): Normally, libev tries to deduce the maximum number of signals automatically, but sometimes this fails, in which case it can be specified. Also, using a lower number than detected (32 should be good for about any system in existence) can save some memory, as libev statically allocates some 12-24 bytes per signal number.

EV_PID_HASHSIZE

"ev_child" watchers use a small hash table to distribute workload by pid. The default size is 16 (or 1 with "EV_FEATURES" disabled), usually more than enough. If you need to manage thousands of children you might want to increase this value ( must be a power of two).

EV_INOTIFY_HASHSIZE

"ev_stat" watchers use a small hash table to distribute workload by inotify watch id. The default size is 16 (or 1 with "EV_FEATURES" disabled), usually more than enough. If you need to manage thousands of "ev_stat" watchers you might want to increase this value ( must be a power of two).

EV_USE_4HEAP

Heaps are not very cache-efficient. To improve the cache-efficiency of the timer and periodics heaps, libev uses a 4-heap when this symbol is defined to 1. The 4-heap uses more complicated (longer) code but has noticeably faster performance with many (thousands) of watchers.

 

The default is 1, unless "EV_FEATURES" overrides it, in which case it will be 0.

EV_HEAP_CACHE_AT

Heaps are not very cache-efficient. To improve the cache-efficiency of the timer and periodics heaps, libev can cache the timestamp ( at) within the heap structure (selected by defining "EV_HEAP_CACHE_AT" to 1), which uses 8-12 bytes more per watcher and a few hundred bytes more code, but avoids random read accesses on heap changes. This improves performance noticeably with many (hundreds) of watchers.

 

The default is 1, unless "EV_FEATURES" overrides it, in which case it will be 0.

EV_VERIFY

Controls how much internal verification (see "ev_verify ()") will be done: If set to 0, no internal verification code will be compiled in. If set to 1, then verification code will be compiled in, but not called. If set to 2, then the internal verification code will be called once per loop, which can slow down libev. If set to 3, then the verification code will be called very frequently, which will slow down libev considerably.

 

The default is 1, unless "EV_FEATURES" overrides it, in which case it will be 0.

EV_COMMON

By default, all watchers have a "void *data" member. By redefining this macro to something else you can include more and other types of members. You have to define it each time you include one of the files, though, and it must be identical each time.

 

For example, the perl EV module uses something like this:

 

   #define EV_COMMON                       \
     SV *self; /* contains this struct */  \
     SV *cb_sv, *fh /* note no trailing ";" */
    

EV_CB_DECLARE (type)

EV_CB_INVOKE (watcher, revents)

ev_set_cb (ev, cb)

Can be used to change the callback member declaration in each watcher, and the way callbacks are invoked and set. Must expand to a struct member definition and a statement, respectively. See the ev.h header file for their default definitions. One possible use for overriding these is to avoid the "struct ev_loop *" as first argument in all cases, or to use method calls instead of plain function calls in C++.

EXPORTED API SYMBOLS¶

   Symbols.ev      for libev proper
   Symbols.event   for the libevent emulation

   <Symbols.ev sed -e "s/.*/#define & myprefix_&/" >wrap.h

   #define ev_backend     myprefix_ev_backend
   #define ev_check_start myprefix_ev_check_start
   #define ev_check_stop  myprefix_ev_check_stop
   ...

EXAMPLES¶

   #define EV_FEATURES 8
   #define EV_USE_SELECT 1
   #define EV_PREPARE_ENABLE 1
   #define EV_IDLE_ENABLE 1
   #define EV_SIGNAL_ENABLE 1
   #define EV_CHILD_ENABLE 1
   #define EV_USE_STDEXCEPT 0
   #define EV_CONFIG_H <config.h>
   #include "ev++.h"

   #include "ev_cpp.h"
   #include "ev.c"

INTERACTION WITH OTHER PROGRAMS, LIBRARIES OR THE ENVIRONMENT¶

THREADS AND COROUTINES¶

•

most applications have a main thread: use the default libev loop in that thread, or create a separate thread running only the default loop.

 

This helps integrating other libraries or software modules that use libev themselves and don't care/know about threading.

•

one loop per thread is usually a good model.

 

Doing this is almost never wrong, sometimes a better-performance model exists, but it is always a good start.

•

other models exist, such as the leader/follower pattern, where one loop is handed through multiple threads in a kind of round-robin fashion.

 

Choosing a model is hard - look around, learn, know that usually you can do better than you currently do :-)

•

often you need to talk to some other thread which blocks in the event loop.

 

"ev_async" watchers can be used to wake them up from other threads safely (or from signal contexts...).

 

An example use would be to communicate signals or other events that only work in the default loop by registering the signal watcher with the default loop and triggering an "ev_async" watcher from the default loop watcher callback into the event loop interested in the signal.

COMPILER WARNINGS¶

VALGRIND¶

   ==2274==    definitely lost: 0 bytes in 0 blocks.
   ==2274==      possibly lost: 0 bytes in 0 blocks.
   ==2274==    still reachable: 256 bytes in 1 blocks.

PORTABILITY NOTES¶

GNU/LINUX 32 BIT LIMITATIONS¶

OS/X AND DARWIN BUGS¶

SOLARIS PROBLEMS AND WORKAROUNDS¶

AIX POLL BUG¶

WIN32 PLATFORM LIMITATIONS AND WORKAROUNDS¶

   #define EV_STANDALONE              /* keeps ev from requiring config.h */
   #define EV_SELECT_IS_WINSOCKET 1   /* configure libev for windows select */
   #include "ev.h"

   #include "evwrap.h"
   #include "ev.c"

   #define EV_USE_SELECT 1
   #define EV_SELECT_IS_WINSOCKET 1   /* forces EV_SELECT_USE_FD_SET, too */

PORTABILITY REQUIREMENTS¶

"void (*)(ev_watcher_type *, int revents)" must have compatible calling conventions regardless of "ev_watcher_type *".

Libev assumes not only that all watcher pointers have the same internal structure (guaranteed by POSIX but not by ISO C for example), but it also assumes that the same (machine) code can be used to call any watcher callback: The watcher callbacks have different type signatures, but libev calls them using an "ev_watcher *" internally.

pointer accesses must be thread-atomic

Accessing a pointer value must be atomic, it must both be readable and writable in one piece - this is the case on all current architectures.

"sig_atomic_t volatile" must be thread-atomic as well

The type "sig_atomic_t volatile" (or whatever is defined as "EV_ATOMIC_T") must be atomic with respect to accesses from different threads. This is not part of the specification for "sig_atomic_t", but is believed to be sufficiently portable.

"sigprocmask" must work in a threaded environment

Libev uses "sigprocmask" to temporarily block signals. This is not allowed in a threaded program ("pthread_sigmask" has to be used). Typical pthread implementations will either allow "sigprocmask" in the "main thread" or will block signals process-wide, both behaviours would be compatible with libev. Interaction between "sigprocmask" and "pthread_sigmask" could complicate things, however.

 

The most portable way to handle signals is to block signals in all threads except the initial one, and run the default loop in the initial thread as well.

"long" must be large enough for common memory allocation sizes

To improve portability and simplify its API, libev uses "long" internally instead of "size_t" when allocating its data structures. On non-POSIX systems (Microsoft...) this might be unexpectedly low, but is still at least 31 bits everywhere, which is enough for hundreds of millions of watchers.

"double" must hold a time value in seconds with enough accuracy

The type "double" is used to represent timestamps. It is required to have at least 51 bits of mantissa (and 9 bits of exponent), which is good enough for at least into the year 4000 with millisecond accuracy (the design goal for libev). This requirement is overfulfilled by implementations using IEEE 754, which is basically all existing ones.

 

With IEEE 754 doubles, you get microsecond accuracy until at least the year 2255 (and millisecond accuracy till the year 287396 - by then, libev is either obsolete or somebody patched it to use "long double" or something like that, just kidding).

ALGORITHMIC COMPLEXITIES¶

Starting and stopping timer/periodic watchers: O(log skipped_other_timers)

This means that, when you have a watcher that triggers in one hour and there are 100 watchers that would trigger before that, then inserting will have to skip roughly seven ("ld 100") of these watchers.

Changing timer/periodic watchers (by autorepeat or calling again): O(log skipped_other_timers)

That means that changing a timer costs less than removing/adding them, as only the relative motion in the event queue has to be paid for.

Starting io/check/prepare/idle/signal/child/fork/async watchers: O(1)

These just add the watcher into an array or at the head of a list.

Stopping check/prepare/idle/fork/async watchers: O(1)

Stopping an io/signal/child watcher: O(number_of_watchers_for_this_(fd/signal/pid % EV_PID_HASHSIZE))

These watchers are stored in lists, so they need to be walked to find the correct watcher to remove. The lists are usually short (you don't usually have many watchers waiting for the same fd or signal: one is typical, two is rare).

Finding the next timer in each loop iteration: O(1)

By virtue of using a binary or 4-heap, the next timer is always found at a fixed position in the storage array.

Each change on a file descriptor per loop iteration: O(number_of_watchers_for_this_fd)

A change means an I/O watcher gets started or stopped, which requires libev to recalculate its status (and possibly tell the kernel, depending on backend and whether "ev_io_set" was used).

Activating one watcher (putting it into the pending state): O(1)

Priority handling: O(number_of_priorities)

Priorities are implemented by allocating some space for each priority. When doing priority-based operations, libev usually has to linearly search all the priorities, but starting/stopping and activating watchers becomes O(1) with respect to priority handling.

Sending an ev_async: O(1)

Processing ev_async_send: O(number_of_async_watchers)

Processing signals: O(max_signal_number)

Sending involves a system call iff there were no other "ev_async_send" calls in the current loop iteration and the loop is currently blocked. Checking for async and signal events involves iterating over all running async watchers or all signal numbers.

PORTING FROM LIBEV 3.X TO 4.X¶

"EV_COMPAT3" backwards compatibility mechanism

The backward compatibility mechanism can be controlled by "EV_COMPAT3". See "MACROS" in PREPROCESSOR SYMBOLS in the EMBEDDING section.

"ev_default_destroy" and "ev_default_fork" have been removed

These calls can be replaced easily by their "ev_loop_xxx" counterparts:

 

   ev_loop_destroy (EV_DEFAULT_UC);
   ev_loop_fork (EV_DEFAULT);
    

function/symbol renames

A number of functions and symbols have been renamed:

 

  ev_loop         => ev_run
  EVLOOP_NONBLOCK => EVRUN_NOWAIT
  EVLOOP_ONESHOT  => EVRUN_ONCE
  ev_unloop       => ev_break
  EVUNLOOP_CANCEL => EVBREAK_CANCEL
  EVUNLOOP_ONE    => EVBREAK_ONE
  EVUNLOOP_ALL    => EVBREAK_ALL
  EV_TIMEOUT      => EV_TIMER
  ev_loop_count   => ev_iteration
  ev_loop_depth   => ev_depth
  ev_loop_verify  => ev_verify
    

 

Most functions working on "struct ev_loop" objects don't have an "ev_loop_" prefix, so it was removed; "ev_loop", "ev_unloop" and associated constants have been renamed to not collide with the "struct ev_loop" anymore and "EV_TIMER" now follows the same naming scheme as all other watcher types. Note that "ev_loop_fork" is still called "ev_loop_fork" because it would otherwise clash with the "ev_fork" typedef.

"EV_MINIMAL" mechanism replaced by "EV_FEATURES"

The preprocessor symbol "EV_MINIMAL" has been replaced by a different mechanism, "EV_FEATURES". Programs using "EV_MINIMAL" usually compile and work, but the library code will of course be larger.

GLOSSARY¶

active

A watcher is active as long as it has been started and not yet stopped. See "WATCHER STATES" for details.

application

In this document, an application is whatever is using libev.

backend

The part of the code dealing with the operating system interfaces.

callback

The address of a function that is called when some event has been detected. Callbacks are being passed the event loop, the watcher that received the event, and the actual event bitset.

callback/watcher invocation

The act of calling the callback associated with a watcher.

event

A change of state of some external event, such as data now being available for reading on a file descriptor, time having passed or simply not having any other events happening anymore.

 

In libev, events are represented as single bits (such as "EV_READ" or "EV_TIMER").

event library

A software package implementing an event model and loop.

event loop

An entity that handles and processes external events and converts them into callback invocations.

event model

The model used to describe how an event loop handles and processes watchers and events.

pending

A watcher is pending as soon as the corresponding event has been detected. See "WATCHER STATES" for details.

real time

The physical time that is observed. It is apparently strictly monotonic :)

wall-clock time

The time and date as shown on clocks. Unlike real time, it can actually be wrong and jump forwards and backwards, e.g. when you adjust your clock.

watcher

A data structure that describes interest in certain events. Watchers need to be started (attached to an event loop) before they can receive events.

AUTHOR¶