#include <ev.h>
// 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_loop's to stop iterating
ev_unloop (EV_A_ EVUNLOOP_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_loop to stop iterating
ev_unloop (EV_A_ EVUNLOOP_ONE);
}
int
main (void)
{
// use the default event loop unless you have special needs
struct ev_loop *loop = ev_default_loop (0);
// 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_loop (loop, 0);
// unloop was called, so exit
return 0;
}
The newest version of this document is also available as an html-formatted web page you might find easier to navigate when reading it for the first time: <http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod>.
While this document tries to be as complete as possible in documenting libev, its usage and the rationale behind its design, it is not a tutorial on event-based programming, nor will it introduce event-based programming with libev.
Familarity with event based programming techniques in general is assumed throughout this document.
To do this, it must take more or less complete control over your process (or thread) by executing the event loop handler, and will then communicate events via a callback mechanism.
You register interest in certain events by registering so-called event watchers, which are relatively small C structures you initialise with the details of the event, and then hand it over to libev by starting the watcher.
It also is quite fast (see this <benchmark> comparing it to libevent for example).
When libev catches an operating system error it cannot handle (for example a system call indicating a condition libev cannot fix), it calls the callback set via "ev_set_syserr_cb", which is supposed to fix the problem or abort. The default is to print a diagnostic message and to call "abort ()".
When libev detects a usage error such as a negative timer interval, then it will print a diagnostic message and abort (via the "assert" mechanism, so "NDEBUG" will disable this checking): these are programming errors in the libev caller and need to be fixed there.
Libev also has a few internal error-checking "assert"ions, and also has extensive consistency checking code. These do not trigger under normal circumstances, as they indicate either a bug in libev or worse.
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.
assert (("libev version mismatch",
ev_version_major () == EV_VERSION_MAJOR
&& ev_version_minor () >= EV_VERSION_MINOR));
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));
See the description of "ev_embed" watchers for more info.
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);
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);
The library knows two types of such loops, the default loop, which supports signals and child events, and dynamically created loops which do not.
If you don't know what event loop to use, use the one returned from this function.
Note that this function is not thread-safe, so if you want to use it from multiple threads, you have to lock (note also that this is unlikely, as loops cannot be shared easily between threads anyway).
The default loop is the only loop that can handle "ev_signal" and "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" that doesn't do that, or you can simply overwrite the "SIGCHLD" signal handler after calling "ev_default_init".
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:
This works by calling "getpid ()" on every iteration of the loop, and thus this might slow down your event loop if you do a lot of loop iterations and little real work, but is usually not noticeable (on my GNU/Linux system for example, "getpid" is actually a simple 5-insn sequence without a system call and thus very fast, but my GNU/Linux system also has "pthread_atfork" which is even faster).
The big advantage of this flag is that you can forget about fork (and forget about forgetting to tell libev about forking) when you use this flag.
This flag setting cannot be overridden or specified in the "LIBEV_FLAGS" environment variable.
Signalfd will not be used by default as this changes your signal mask, and there are a lot of shoddy libraries and programs (glib's threadpool for example) that can't properly initialise their signal masks.
To get good performance out of this backend you need a high amount of parallelism (most of the file descriptors should be busy). If you are writing a server, you should "accept ()" in a loop to accept as many connections as possible during one iteration. You might also want to have a look at "ev_set_io_collect_interval ()" to increase the amount of readiness notifications you get per iteration.
This backend maps "EV_READ" to the "readfds" set and "EV_WRITE" to the "writefds" set (and to work around Microsoft Windows bugs, also onto the "exceptfds" set on that platform).
This backend maps "EV_READ" to "POLLIN | POLLERR | POLLHUP", and "EV_WRITE" to "POLLOUT | POLLERR | POLLHUP".
For few fds, this backend is a bit little slower than poll and select, but it scales phenomenally better. While poll and select usually scale like O(total_fds) where n is the total number of fds (or the highest fd), epoll scales either O(1) or O(active_fds).
The epoll mechanism deserves honorable mention as the most misdesigned of the more advanced event mechanisms: mere annoyances include silently dropping file descriptors, requiring a system call per change per file descriptor (and unnecessary guessing of parameters), problems with dup and so on. The biggest issue is fork races, however - if a program forks then both parent and child process have to recreate the epoll set, which can take considerable time (one syscall per file descriptor) and is of course hard to detect.
Epoll is also notoriously buggy - embedding epoll fds should work, but of course doesn't, and epoll just loves to report events for totally different file descriptors (even already closed ones, so one cannot even remove them from the set) than registered in the set (especially on SMP systems). Libev tries to counter these spurious notifications by employing an additional generation counter and comparing that against the events to filter out spurious ones, recreating the set when required.
While stopping, setting and starting an I/O watcher in the same iteration will result in some caching, there is still a system call per such incident (because the same file descriptor could point to a different file description now), so its best to avoid that. Also, "dup ()"'ed file descriptors might not work very well if you register events for both file descriptors.
Best performance from this backend is achieved by not unregistering all watchers for a file descriptor until it has been closed, if possible, i.e. keep at least one watcher active per fd at all times. Stopping and starting a watcher (without re-setting it) also usually doesn't cause extra overhead. A fork can both result in spurious notifications as well as in libev having to destroy and recreate the epoll object, which can take considerable time and thus should be avoided.
All this means that, in practice, "EVBACKEND_SELECT" can be as fast or faster than epoll for maybe up to a hundred file descriptors, depending on the usage. So sad.
While nominally embeddable in other event loops, this feature is broken in all kernel versions tested so far.
This backend maps "EV_READ" and "EV_WRITE" in the same way as "EVBACKEND_POLL".
You still can embed kqueue into a normal poll or select backend and use it only for sockets (after having made sure that sockets work with kqueue on the target platform). See "ev_embed" watchers for more info.
It scales in the same way as the epoll backend, but the interface to the kernel is more efficient (which says nothing about its actual speed, of course). While stopping, setting and starting an I/O watcher does never cause an extra system call as with "EVBACKEND_EPOLL", it still adds up to two event changes per incident. Support for "fork ()" is very bad (but sane, unlike epoll) and it drops fds silently in similarly hard-to-detect cases
This backend usually performs well under most conditions.
While nominally embeddable in other event loops, this doesn't work everywhere, so you might need to test for this. And since it is broken almost everywhere, you should only use it when you have a lot of sockets (for which it usually works), by embedding it into another event loop (e.g. "EVBACKEND_SELECT" or "EVBACKEND_POLL" (but "poll" is of course also broken on OS X)) and, did I mention it, using it only for sockets.
This backend maps "EV_READ" into an "EVFILT_READ" kevent with "NOTE_EOF", and "EV_WRITE" into an "EVFILT_WRITE" kevent with "NOTE_EOF".
Please note that Solaris event ports can deliver a lot of spurious notifications, so you need to use non-blocking I/O or other means to avoid blocking when no data (or space) is available.
While this backend scales well, it requires one system call per active file descriptor per loop iteration. For small and medium numbers of file descriptors a ``slow'' "EVBACKEND_SELECT" or "EVBACKEND_POLL" backend might perform better.
On the positive side, with the exception of the spurious readiness notifications, this backend actually performed fully to specification in all tests and is fully embeddable, which is a rare feat among the OS-specific backends (I vastly prefer correctness over speed hacks).
This backend maps "EV_READ" and "EV_WRITE" in the same way as "EVBACKEND_POLL".
It is definitely not recommended to use this flag.
If one or more of the backend flags are or'ed into the flags value, then only these backends will be tried (in the reverse order as listed here). If none are specified, all backends in "ev_recommended_backends ()" will be tried.
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);
Example: Use whatever libev has to offer, but make sure that kqueue is used if available (warning, breaks stuff, best use only with your own private event loop and only if you know the OS supports your types of fds):
ev_default_loop (ev_recommended_backends () | EVBACKEND_KQUEUE);
Note that this function is thread-safe, and the recommended 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.
Example: Try to create a event loop that uses epoll and nothing else.
struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
if (!epoller)
fatal ("no epoll found here, maybe it hides under your chair");
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.
In general it is not advisable to call this function except in the rare occasion where you really need to free e.g. the signal handling pipe fds. If you need dynamically allocated loops it is better to use "ev_loop_new" and "ev_loop_destroy".
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 library in the child. If you just fork+exec, you don't have to call it at all.
The function itself is quite fast and it's usually not a problem to call it just in case after a fork. To make this easy, the function will fit in quite nicely into a call to "pthread_atfork":
pthread_atfork (0, 0, ev_default_fork);
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.
Outside "ev_loop", this number is zero. In a callback, this number is 1, unless "ev_loop" was invoked recursively (or from another thread), in which case it is higher.
Leaving "ev_loop" abnormally (setjmp/longjmp, cancelling the thread etc.), doesn't count as exit.
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.
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 occured 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").
If the flags argument is specified as 0, it will not return until either no event watchers are active anymore or "ev_unloop" was called.
Please note that an explicit "ev_unloop" 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.
A flags value of "EVLOOP_NONBLOCK" will look for new events, will handle those events and any already outstanding ones, but will not block your process in case there are no events and will return after one iteration of the loop.
A flags value of "EVLOOP_ONESHOT" 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_loop""). 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_loop" does:
- Before the first iteration, call any pending watchers.
* 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 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, EVLOOP_NONBLOCK or not having
any active watchers at all will result in not sleeping).
- Sleep if the I/O and timer collect interval say so.
- 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.
- Unless any events are pending now, queue all idle watchers.
- 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_unloop has been called, or EVLOOP_ONESHOT or EVLOOP_NONBLOCK
were used, or there are no active watchers, return, otherwise
continue with step *.
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_loop (my_loop, 0); ... jobs done or somebody called unloop. yeah!
This ``unloop state'' will be cleared when entering "ev_loop" again.
It is safe to call "ev_unloop" from otuside any "ev_loop" calls.
This is useful when you have a watcher that you never intend to unregister, but that nevertheless should not keep "ev_loop" 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_loop" from exiting if no event watchers registered by it are active. It is also an excellent way to do this for generic recurring timers or from within 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_loop" running when nothing else is active.
ev_signal exitsig; ev_signal_init (&exitsig, sig_cb, SIGINT); ev_signal_start (loop, &exitsig); evf_unref (loop);
Example: For some weird reason, unregister the above signal handler again.
ev_ref (loop); ev_signal_stop (loop, &exitsig);
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 be not 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.
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 transations 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);
If you want to reset the callback, use "ev_invoke_pending" as new callback.
However, "ev_loop" 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 loop via "ev_unloop" and "av_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_loop" when you want it to take note of any changes you made.
In theory, threads executing "ev_loop" will be async-cancel safe between invocations of "release" and "acquire".
See also the locking example in the "THREADS" section later in this document.
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.
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.
A watcher is a structure that you create and register to record your interest in some event. For instance, if you want to wait for STDIN to become readable, you would create an "ev_io" watcher for that:
static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
{
ev_io_stop (w);
ev_unloop (loop, EVUNLOOP_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_loop (loop, 0);
As you can see, you are responsible for allocating the memory for your watcher structures (and it is usually a bad idea to do this on the stack).
Each watcher has an associated watcher structure (called "struct ev_TYPE" or simply "ev_TYPE", as typedefs are provided for all watcher structs).
Each watcher structure must be initialised by a call to "ev_init (watcher *, callback)", which expects a callback to be provided. This callback gets invoked each time the event occurs (or, in the case of I/O watchers, each time the event loop detects that the file descriptor given is readable and/or writable).
Each watcher type further has its own "ev_TYPE_set (watcher *, ...)" macro to configure it, with arguments specific to the watcher type. There is also a macro to combine initialisation and setting in one call: "ev_TYPE_init (watcher *, callback, ...)".
To make the watcher actually watch out for events, you have to start it with a watcher-specific start function ("ev_TYPE_start (loop, watcher *)"), and you can stop watching for events at any time by calling the corresponding stop function ("ev_TYPE_stop (loop, watcher *)".
As long as your watcher is active (has been started but not stopped) you must not touch the values stored in it. Most specifically you must never reinitialise it or call its "ev_TYPE_set" macro.
Each and every callback receives the event loop pointer as first, the registered watcher structure as second, and a bitset of received events as third argument.
The received events usually include a single bit per event type received (you can receive multiple events at the same time). The possible bit masks are:
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.
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);
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.
Example: Initialise and set an "ev_io" watcher in one step.
ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
Example: Start the "ev_io" watcher that is being abused as example in this whole section.
ev_io_start (EV_DEFAULT_UC, &w);
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.
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.
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.
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.
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);
And since your callback will be called with a pointer to the watcher, you can cast it back to your own type:
static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
{
struct my_io *w = (struct my_io *)w_;
...
}
More interesting and less C-conformant ways of casting your callback type instead have been omitted.
Another common scenario is to use some data structure with multiple embedded watchers:
struct my_biggy
{
int some_data;
ev_timer t1;
ev_timer t2;
}
In this case getting the pointer to "my_biggy" is a bit more complicated: Either you store the address of your "my_biggy" struct in the "data" member of the watcher (for woozies), or you need to use some pointer arithmetic using "offsetof" inside your watchers (for real programmers):
#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));
}
In libev, Watcher priorities can be set using "ev_set_priority". See its description for the more technical details such as the actual priority range.
There are two common ways how these these priorities are being interpreted by event loops:
In the more common lock-out model, higher priorities ``lock out'' invocation of lower priority watchers, which means as long as higher priority watchers receive events, lower priority watchers are not being invoked.
The less common only-for-ordering model uses priorities solely to order callback invocation within a single event loop iteration: Higher priority watchers are invoked before lower priority ones, but they all get invoked before polling for new events.
Libev uses the second (only-for-ordering) model for all its watchers except for idle watchers (which use the lock-out model).
The rationale behind this is that implementing the lock-out model for watchers is not well supported by most kernel interfaces, and most event libraries will just poll for the same events again and again as long as their callbacks have not been executed, which is very inefficient in the common case of one high-priority watcher locking out a mass of lower priority ones.
Static (ordering) priorities are most useful when you have two or more watchers handling the same resource: a typical usage example is having an "ev_io" watcher to receive data, and an associated "ev_timer" to handle timeouts. Under load, data might be received while the program handles other jobs, but since timers normally get invoked first, the timeout handler will be executed before checking for data. In that case, giving the timer a lower priority than the I/O watcher ensures that I/O will be handled first even under adverse conditions (which is usually, but not always, what you want).
Since idle watchers use the ``lock-out'' model, meaning that idle watchers will only be executed when no same or higher priority watchers have received events, they can be used to implement the ``lock-out'' model when required.
For example, to emulate how many other event libraries handle priorities, you can associate an "ev_idle" watcher to each such watcher, and in the normal watcher callback, you just start the idle watcher. The real processing is done in the idle watcher callback. This causes libev to continously poll and process kernel event data for the watcher, but when the lock-out case is known to be rare (which in turn is rare :), this is workable.
Usually, however, the lock-out model implemented that way will perform miserably under the type of load it was designed to handle. In that case, it might be preferable to stop the real watcher before starting the idle watcher, so the kernel will not have to process the event in case the actual processing will be delayed for considerable time.
Here is an example of an I/O watcher that should run at a strictly lower priority than the default, and which should only process data when no other events are pending:
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 ahndle 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);
In the ``real'' world, it might also be beneficial to start a timer, so that low-priority connections can not be locked out forever under load. This enables your program to keep a lower latency for important connections during short periods of high load, while not completely locking out less important ones.
Members are additionally marked with either [read-only], meaning that, while the watcher is active, you can look at the member and expect some sensible content, but you must not modify it (you can modify it while the watcher is stopped to your hearts content), or [read-write], which means you can expect it to have some sensible content while the watcher is active, but you can also modify it. Modifying it may not do something sensible or take immediate effect (or do anything at all), but libev will not crash or malfunction in any way.
In general you can register as many read and/or write event watchers per fd as you want (as long as you don't confuse yourself). Setting all file descriptors to non-blocking mode is also usually a good idea (but not required if you know what you are doing).
If you cannot use non-blocking mode, then force the use of a known-to-be-good backend (at the time of this writing, this includes only "EVBACKEND_SELECT" and "EVBACKEND_POLL"). The same applies to file descriptors for which non-blocking operation makes no sense (such as files) - libev doesn't guarentee any specific behaviour in that case.
Another thing you have to watch out for is that it is quite easy to receive ``spurious'' readiness notifications, that is your callback might be called with "EV_READ" but a subsequent "read"(2) will actually block because there is no data. Not only are some backends known to create a lot of those (for example Solaris ports), it is very easy to get into this situation even with a relatively standard program structure. Thus it is best to always use non-blocking I/O: An extra "read"(2) returning "EAGAIN" is far preferable to a program hanging until some data arrives.
If you cannot run the fd in non-blocking mode (for example you should not play around with an Xlib connection), then you have to separately re-test whether a file descriptor is really ready with a known-to-be good interface such as poll (fortunately in our Xlib example, Xlib already does this on its own, so its quite safe to use). Some people additionally use "SIGALRM" and an interval timer, just to be sure you won't block indefinitely.
But really, best use non-blocking mode.
The special problem of disappearing file descriptors
Some backends (e.g. kqueue, epoll) need to be told about closing a file descriptor (either due to calling "close" explicitly or any other means, such as "dup2"). The reason is that you register interest in some file descriptor, but when it goes away, the operating system will silently drop this interest. If another file descriptor with the same number then is registered with libev, there is no efficient way to see that this is, in fact, a different file descriptor.
To avoid having to explicitly tell libev about such cases, libev follows the following policy: Each time "ev_io_set" is being called, libev will assume that this is potentially a new file descriptor, otherwise it is assumed that the file descriptor stays the same. That means that you have to call "ev_io_set" (or "ev_io_init") when you change the descriptor even if the file descriptor number itself did not change.
This is how one would do it normally anyway, the important point is that the libev application should not optimise around libev but should leave optimisations to libev.
The special problem of dup'ed file descriptors
Some backends (e.g. epoll), cannot register events for file descriptors, but only events for the underlying file descriptions. That means when you have "dup ()"'ed file descriptors or weirder constellations, and register events for them, only one file descriptor might actually receive events.
There is no workaround possible except not registering events for potentially "dup ()"'ed file descriptors, or to resort to "EVBACKEND_SELECT" or "EVBACKEND_POLL".
The special problem of fork
Some backends (epoll, kqueue) do not support "fork ()" at all or exhibit useless behaviour. Libev fully supports fork, but needs to be told about it in the child.
To support fork in your programs, you either have to call "ev_default_fork ()" or "ev_loop_fork ()" after a fork in the child, enable "EVFLAG_FORKCHECK", or resort to "EVBACKEND_SELECT" or "EVBACKEND_POLL".
The special problem of SIGPIPE
While not really specific to libev, it is easy to forget about "SIGPIPE": when writing to a pipe whose other end has been closed, your program gets sent a SIGPIPE, which, by default, aborts your program. For most programs this is sensible behaviour, for daemons, this is usually undesirable.
So when you encounter spurious, unexplained daemon exits, make sure you ignore SIGPIPE (and maybe make sure you log the exit status of your daemon somewhere, as that would have given you a big clue).
Watcher-Specific Functions
Examples
Example: Call "stdin_readable_cb" when STDIN_FILENO has become, well readable, but only once. Since it is likely line-buffered, you could attempt to read a whole line in the callback.
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_loop (loop, 0);
The timers are based on real time, that is, if you register an event that times out after an hour and you reset your system clock to January last year, it will still time out after (roughly) one hour. ``Roughly'' because detecting time jumps is hard, and some inaccuracies are unavoidable (the monotonic clock option helps a lot here).
The callback is guaranteed to be invoked only after its timeout has passed (not at, so on systems with very low-resolution clocks this might introduce a small delay). If multiple timers become ready during the same loop iteration then the ones with earlier time-out values are invoked before ones of the same priority with later time-out values (but this is no longer true when a callback calls "ev_loop" recursively).
Be smart about timeouts
Many real-world problems involve some kind of timeout, usually for error recovery. A typical example is an HTTP request - if the other side hangs, you want to raise some error after a while.
What follows are some ways to handle this problem, from obvious and inefficient to smart and efficient.
In the following, a 60 second activity timeout is assumed - a timeout that gets reset to 60 seconds each time there is activity (e.g. each time some data or other life sign was received).
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.
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.
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 last_activity; // time of last activity
static void
callback (EV_P_ ev_timer *w, int revents)
{
ev_tstamp now = ev_now (EV_A);
ev_tstamp timeout = last_activity + 60.;
// if last_activity + 60. is older than now, we did time out
if (timeout < now)
{
// timeout occured, take action
}
else
{
// callback was invoked, but there was some activity, re-arm
// the watcher to fire in last_activity + 60, which is
// guaranteed to be in the future, so "again" is positive:
w->repeat = timeout - now;
ev_timer_again (EV_A_ w);
}
}
To summarise the callback: first calculate the real timeout (defined as ``60 seconds after the last activity''), then check if that time has been reached, which means something did, in fact, time out. Otherwise the callback was invoked too early ("timeout" is in the future), so re-schedule the timer to fire at that future time, to see if maybe we have a timeout then.
Note how "ev_timer_again" is used, taking advantage of the "ev_timer_again" optimisation when the timer is already running.
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 timer, simply initialise the watcher and set "last_activity" to the current time (meaning we just have some activity :), then call the callback, which will ``do the right thing'' and start the timer:
ev_init (timer, callback); last_activity = ev_now (loop); callback (loop, timer, EV_TIMEOUT);
And when there is some activity, simply store the current time in "last_activity", no libev calls at all:
last_actiivty = ev_now (loop);
This technique is slightly more complex, but in most cases where the time-out is unlikely to be triggered, much more efficient.
Changing the timeout is trivial as well (if it isn't hard-coded in the callback :) - just change the timeout and invoke the callback, which will fix things for you.
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.
So which method the best?
Method #2 is a simple no-brain-required solution that is adequate in most situations. Method #3 requires a bit more thinking, but handles many cases better, and isn't very complicated either. In most case, choosing either one is fine, with #3 being better in typical situations.
Method #1 is almost always a bad idea, and buys you nothing. Method #4 is rather complicated, but extremely efficient, something that really pays off after the first million or so of active timers, i.e. it's usually overkill :)
The special problem of time updates
Establishing the current time is a costly operation (it usually takes at least two system calls): EV therefore updates its idea of the current time only before and after "ev_loop" collects new events, which causes a growing difference between "ev_now ()" and "ev_time ()" when handling lots of events in one iteration.
The relative timeouts are calculated relative to the "ev_now ()" time. This is usually the right thing as this timestamp refers to the time of the event triggering whatever timeout you are modifying/starting. If you suspect event processing to be delayed and you need to base the timeout on the current time, use something like this to adjust for this:
ev_timer_set (&timer, after + ev_now () - ev_time (), 0.);
If the event loop is suspended for a long time, you can also force an update of the time returned by "ev_now ()" by calling "ev_now_update ()".
The special problems of suspended animation
When you leave the server world it is quite customary to hit machines that can suspend/hibernate - what happens to the clocks during such a suspend?
Some quick tests made with a Linux 2.6.28 indicate that a suspend freezes all processes, while the clocks ("times", "CLOCK_MONOTONIC") continue to run until the system is suspended, but they will not advance while the system is suspended. That means, on resume, it will be as if the program was frozen for a few seconds, but the suspend time will not be counted towards "ev_timer" when a monotonic clock source is used. The real time clock advanced as expected, but if it is used as sole clocksource, then a long suspend would be detected as a time jump by libev, and timers would be adjusted accordingly.
I would not be surprised to see different behaviour in different between operating systems, OS versions or even different hardware.
The other form of suspend (job control, or sending a SIGSTOP) will see a time jump in the monotonic clocks and the realtime clock. If the program is suspended for a very long time, and monotonic clock sources are in use, then you can expect "ev_timer"s to expire as the full suspension time will be counted towards the timers. When no monotonic clock source is in use, then libev will again assume a timejump and adjust accordingly.
It might be beneficial for this latter case to call "ev_suspend" and "ev_resume" in code that handles "SIGTSTP", to at least get deterministic behaviour in this case (you can do nothing against "SIGSTOP").
Watcher-Specific Functions and Data Members
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.
If the timer is pending, its pending status is cleared.
If the timer is started but non-repeating, stop it (as if it timed out).
If the timer is repeating, either start it if necessary (with the "repeat" value), or reset the running timer to the "repeat" value.
This sounds a bit complicated, see ``Be smart about timeouts'', above, for a usage example.
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_remain" 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.
Examples
Example: Create a timer that fires after 60 seconds.
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);
Example: Create a timeout timer that times out after 10 seconds of inactivity.
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_loop (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);
Unlike "ev_timer", periodic watchers are not based on real time (or relative time, the physical time that passes) but on wall clock time (absolute time, the thing you can read on your calender or clock). The difference is that wall clock time can run faster or slower than real time, and time jumps are not uncommon (e.g. when you adjust your wrist-watch).
You can tell a periodic watcher to trigger after some specific point in time: for example, if you tell a periodic watcher to trigger ``in 10 seconds'' (by specifying e.g. "ev_now () + 10.", that is, an absolute time not a delay) and then reset your system clock to January of the previous year, then it will take a year or more to trigger the event (unlike an "ev_timer", which would still trigger roughly 10 seconds after starting it, as it uses a relative timeout).
"ev_periodic" watchers can also be used to implement vastly more complex timers, such as triggering an event on each ``midnight, local time'', or other complicated rules. This cannot be done with "ev_timer" watchers, as those cannot react to time jumps.
As with timers, the callback is guaranteed to be invoked only when the point in time where it is supposed to trigger has passed. If multiple timers become ready during the same loop iteration then the ones with earlier time-out values are invoked before ones with later time-out values (but this is no longer true when a callback calls "ev_loop" recursively).
Watcher-Specific Functions and Data Members
In this configuration the watcher triggers an event after the wall clock time "offset" has passed. It will not repeat and will not adjust when a time jump occurs, that is, if it is to be run at January 1st 2011 then it will be stopped and invoked when the system clock reaches or surpasses this point in time.
In this mode the watcher will always be scheduled to time out at the next "offset + N * interval" time (for some integer N, which can also be negative) and then repeat, regardless of any time jumps. The "offset" argument is merely an offset into the "interval" periods.
This can be used to create timers that do not drift with respect to the system clock, for example, here is an "ev_periodic" that triggers each hour, on the hour (with respect to UTC):
ev_periodic_set (&periodic, 0., 3600., 0);
This doesn't mean there will always be 3600 seconds in between triggers, but only that the callback will be called when the system time shows a full hour (UTC), or more correctly, when the system time is evenly divisible by 3600.
Another way to think about it (for the mathematically inclined) is that "ev_periodic" will try to run the callback in this mode at the next possible time where "time = offset (mod interval)", regardless of any time jumps.
For numerical stability it is preferable that the "offset" value is near "ev_now ()" (the current time), but there is no range requirement for this value, and in fact is often specified as zero.
Note also that there is an upper limit to how often a timer can fire (CPU speed for example), so if "interval" is very small then timing stability will of course deteriorate. Libev itself tries to be exact to be about one millisecond (if the OS supports it and the machine is fast enough).
In this mode the values for "interval" and "offset" are both being ignored. Instead, each time the periodic watcher gets scheduled, the reschedule callback will be called with the watcher as first, and the current time as second argument.
NOTE: This callback MUST NOT stop or destroy any periodic watcher, ever, or make ANY other event loop modifications whatsoever, unless explicitly allowed by documentation here.
If you need to stop it, return "now + 1e30" (or so, fudge fudge) and stop it afterwards (e.g. by starting an "ev_prepare" watcher, which is the only event loop modification you are allowed to do).
The callback prototype is "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now)", e.g.:
static ev_tstamp
my_rescheduler (ev_periodic *w, ev_tstamp now)
{
return now + 60.;
}
It must return the next time to trigger, based on the passed time value (that is, the lowest time value larger than to the second argument). It will usually be called just before the callback will be triggered, but might be called at other times, too.
NOTE: This callback must always return a time that is higher than or equal to the passed "now" value.
This can be used to create very complex timers, such as a timer that triggers on ``next midnight, local time''. To do this, you would calculate the next midnight after "now" and return the timestamp value for this. How you do this is, again, up to you (but it is not trivial, which is the main reason I omitted it as an example).
Can be modified any time, but changes only take effect when the periodic timer fires or "ev_periodic_again" is being called.
Examples
Example: Call a callback every hour, or, more precisely, whenever the system time is divisible by 3600. The callback invocation times have potentially a lot of jitter, but good long-term stability.
static void
clock_cb (struct ev_loop *loop, ev_io *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);
Example: The same as above, but use a reschedule callback to do it:
#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);
Example: Call a callback every hour, starting now:
ev_periodic hourly_tick;
ev_periodic_init (&hourly_tick, clock_cb,
fmod (ev_now (loop), 3600.), 3600., 0);
ev_periodic_start (loop, &hourly_tick);
If you want signals to be delivered truly asynchronously, just use "sigaction" as you would do without libev and forget about sharing the signal. You can even use "ev_async" from a signal handler to synchronously wake up an event loop.
You can configure as many watchers as you like for the same signal, but only within the same loop, i.e. you can watch for "SIGINT" in your default loop and for "SIGIO" in another loop, but you cannot watch for "SIGINT" in both the default loop and another loop at the same time. At the moment, "SIGCHLD" is permanently tied to the default loop.
When the first watcher gets started will libev actually register something with the kernel (thus it coexists with your own signal handlers as long as you don't register any with libev for the same signal).
If possible and supported, libev will install its handlers with "SA_RESTART" (or equivalent) behaviour enabled, so system calls should not be unduly interrupted. If you have a problem with system calls getting interrupted by signals you can block all signals in an "ev_check" watcher and unblock them in an "ev_prepare" watcher.
The special problem of inheritance over fork/execve/pthread_create
Both the signal mask ("sigprocmask") and the signal disposition ("sigaction") are unspecified after starting a signal watcher (and after stopping it again), that is, libev might or might not block the signal, and might or might not set or restore the installed signal handler.
While this does not matter for the signal disposition (libev never sets signals to "SIG_IGN", so handlers will be reset to "SIG_DFL" on "execve"), this matters for the signal mask: many programs do not expect certain signals to be blocked.
This means that before calling "exec" (from the child) you should reset the signal mask to whatever ``default'' you expect (all clear is a good choice usually).
The simplest way to ensure that the signal mask is reset in the child is to install a fork handler with "pthread_atfork" that resets it. That will catch fork calls done by libraries (such as the libc) as well.
In current versions of libev, the signal will not be blocked indefinitely unless you use the "signalfd" API ("EV_SIGNALFD"). While this reduces the window of opportunity for problems, it will not go away, as libev has to modify the signal mask, at least temporarily.
So I can't stress this enough: If you do not reset your signal mask when you expect it to be empty, you have a race condition in your code. This is not a libev-specific thing, this is true for most event libraries.
Watcher-Specific Functions and Data Members
Examples
Example: Try to exit cleanly on SIGINT.
static void
sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
{
ev_unloop (loop, EVUNLOOP_ALL);
}
ev_signal signal_watcher;
ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
ev_signal_start (loop, &signal_watcher);
Only the default event loop is capable of handling signals, and therefore you can only register child watchers in the default event loop.
Due to some design glitches inside libev, child watchers will always be handled at maximum priority (their priority is set to "EV_MAXPRI" by libev)
Process Interaction
Libev grabs "SIGCHLD" as soon as the default event loop is initialised. This is necessary to guarantee proper behaviour even if the first child watcher is started after the child exits. The occurrence of "SIGCHLD" is recorded asynchronously, but child reaping is done synchronously as part of the event loop processing. Libev always reaps all children, even ones not watched.
Overriding the Built-In Processing
Libev offers no special support for overriding the built-in child processing, but if your application collides with libev's default child handler, you can override it easily by installing your own handler for "SIGCHLD" after initialising the default loop, and making sure the default loop never gets destroyed. You are encouraged, however, to use an event-based approach to child reaping and thus use libev's support for that, so other libev users can use "ev_child" watchers freely.
Stopping the Child Watcher
Currently, the child watcher never gets stopped, even when the child terminates, so normally one needs to stop the watcher in the callback. Future versions of libev might stop the watcher automatically when a child exit is detected (calling "ev_child_stop" twice is not a problem).
Watcher-Specific Functions and Data Members
Examples
Example: "fork()" a new process and install a child handler to wait for its completion.
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);
}
The path does not need to exist: changing from ``path exists'' to ``path does not exist'' is a status change like any other. The condition ``path does not exist'' (or more correctly ``path cannot be stat'ed'') is signified by the "st_nlink" field being zero (which is otherwise always forced to be at least one) and all the other fields of the stat buffer having unspecified contents.
The path must not end in a slash or contain special components such as "." or "..". The path should be absolute: If it is relative and your working directory changes, then the behaviour is undefined.
Since there is no portable change notification interface available, the portable implementation simply calls stat(2) regularly on the path to see if it changed somehow. You can specify a recommended polling interval for this case. If you specify a polling interval of 0 (highly recommended!) then a suitable, unspecified default value will be used (which you can expect to be around five seconds, although this might change dynamically). Libev will also impose a minimum interval which is currently around 0.1, but that's usually overkill.
This watcher type is not meant for massive numbers of stat watchers, as even with OS-supported change notifications, this can be resource-intensive.
At the time of this writing, the only OS-specific interface implemented is the Linux inotify interface (implementing kqueue support is left as an exercise for the reader. Note, however, that the author sees no way of implementing "ev_stat" semantics with kqueue, except as a hint).
ABI Issues (Largefile Support)
Libev by default (unless the user overrides this) uses the default compilation environment, which means that on systems with large file support disabled by default, you get the 32 bit version of the stat structure. When using the library from programs that change the ABI to use 64 bit file offsets the programs will fail. In that case you have to compile libev with the same flags to get binary compatibility. This is obviously the case with any flags that change the ABI, but the problem is most noticeably displayed with ev_stat and large file support.
The solution for this is to lobby your distribution maker to make large file interfaces available by default (as e.g. FreeBSD does) and not optional. Libev cannot simply switch on large file support because it has to exchange stat structures with application programs compiled using the default compilation environment.
Inotify and Kqueue
When "inotify (7)" support has been compiled into libev and present at runtime, it will be used to speed up change detection where possible. The inotify descriptor will be created lazily when the first "ev_stat" watcher is being started.
Inotify presence does not change the semantics of "ev_stat" watchers except that changes might be detected earlier, and in some cases, to avoid making regular "stat" calls. Even in the presence of inotify support there are many cases where libev has to resort to regular "stat" polling, but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too many bugs), the path exists (i.e. stat succeeds), and the path resides on a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and xfs are fully working) libev usually gets away without polling.
There is no support for kqueue, as apparently it cannot be used to implement this functionality, due to the requirement of having a file descriptor open on the object at all times, and detecting renames, unlinks etc. is difficult.
"stat ()" is a synchronous operation
Libev doesn't normally do any kind of I/O itself, and so is not blocking the process. The exception are "ev_stat" watchers - those call "stat ()", which is a synchronous operation.
For local paths, this usually doesn't matter: unless the system is very busy or the intervals between stat's are large, a stat call will be fast, as the path data is usually in memory already (except when starting the watcher).
For networked file systems, calling "stat ()" can block an indefinite time due to network issues, and even under good conditions, a stat call often takes multiple milliseconds.
Therefore, it is best to avoid using "ev_stat" watchers on networked paths, although this is fully supported by libev.
The special problem of stat time resolution
The "stat ()" system call only supports full-second resolution portably, and even on systems where the resolution is higher, most file systems still only support whole seconds.
That means that, if the time is the only thing that changes, you can easily miss updates: on the first update, "ev_stat" detects a change and calls your callback, which does something. When there is another update within the same second, "ev_stat" will be unable to detect unless the stat data does change in other ways (e.g. file size).
The solution to this is to delay acting on a change for slightly more than a second (or till slightly after the next full second boundary), using a roughly one-second-delay "ev_timer" (e.g. "ev_timer_set (w, 0., 1.02); ev_timer_again (loop, w)").
The .02 offset is added to work around small timing inconsistencies of some operating systems (where the second counter of the current time might be be delayed. One such system is the Linux kernel, where a call to "gettimeofday" might return a timestamp with a full second later than a subsequent "time" call - if the equivalent of "time ()" is used to update file times then there will be a small window where the kernel uses the previous second to update file times but libev might already execute the timer callback).
Watcher-Specific Functions and Data Members
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).
Examples
Example: Watch "/etc/passwd" for attribute changes.
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);
Example: Like above, but additionally use a one-second delay so we do not miss updates (however, frequent updates will delay processing, too, so one might do the work both on "ev_stat" callback invocation and on "ev_timer" callback invocation).
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);
That is, as long as your process is busy handling sockets or timeouts (or even signals, imagine) of the same or higher priority it will not be triggered. But when your process is idle (or only lower-priority watchers are pending), the idle watchers are being called once per event loop iteration - until stopped, that is, or your process receives more events and becomes busy again with higher priority stuff.
The most noteworthy effect is that as long as any idle watchers are active, the process will not block when waiting for new events.
Apart from keeping your process non-blocking (which is a useful effect on its own sometimes), idle watchers are a good place to do ``pseudo-background processing'', or delay processing stuff to after the event loop has handled all outstanding events.
Watcher-Specific Functions and Data Members
Examples
Example: Dynamically allocate an "ev_idle" watcher, start it, and in the callback, free it. Also, use no error checking, as usual.
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);
You must not call "ev_loop" or similar functions that enter the current event loop from either "ev_prepare" or "ev_check" watchers. Other loops than the current one are fine, however. The rationale behind this is that you do not need to check for recursion in those watchers, i.e. the sequence will always be "ev_prepare", blocking, "ev_check" so if you have one watcher of each kind they will always be called in pairs bracketing the blocking call.
Their main purpose is to integrate other event mechanisms into libev and their use is somewhat advanced. They could be used, for example, to track variable changes, implement your own watchers, integrate net-snmp or a coroutine library and lots more. They are also occasionally useful if you cache some data and want to flush it before blocking (for example, in X programs you might want to do an "XFlush ()" in an "ev_prepare" watcher).
This is done by examining in each prepare call which file descriptors need to be watched by the other library, registering "ev_io" watchers for them and starting an "ev_timer" watcher for any timeouts (many libraries provide exactly this functionality). Then, in the check watcher, you check for any events that occurred (by checking the pending status of all watchers and stopping them) and call back into the library. The I/O and timer callbacks will never actually be called (but must be valid nevertheless, because you never know, you know?).
As another example, the Perl Coro module uses these hooks to integrate coroutines into libev programs, by yielding to other active coroutines during each prepare and only letting the process block if no coroutines are ready to run (it's actually more complicated: it only runs coroutines with priority higher than or equal to the event loop and one coroutine of lower priority, but only once, using idle watchers to keep the event loop from blocking if lower-priority coroutines are active, thus mapping low-priority coroutines to idle/background tasks).
It is recommended to give "ev_check" watchers highest ("EV_MAXPRI") priority, to ensure that they are being run before any other watchers after the poll (this doesn't matter for "ev_prepare" watchers).
Also, "ev_check" watchers (and "ev_prepare" watchers, too) should not activate (``feed'') events into libev. While libev fully supports this, they might get executed before other "ev_check" watchers did their job. As "ev_check" watchers are often used to embed other (non-libev) event loops those other event loops might be in an unusable state until their "ev_check" watcher ran (always remind yourself to coexist peacefully with others).
Watcher-Specific Functions and Data Members
Examples
There are a number of principal ways to embed other event loops or modules into libev. Here are some ideas on how to include libadns into libev (there is a Perl module named "EV::ADNS" that does this, which you could use as a working example. Another Perl module named "EV::Glib" embeds a Glib main context into libev, and finally, "Glib::EV" embeds EV into the Glib event loop).
Method 1: Add IO watchers and a timeout watcher in a prepare handler, and in a check watcher, destroy them and call into libadns. What follows is pseudo-code only of course. This requires you to either use a low priority for the check watcher or use "ev_clear_pending" explicitly, as the callbacks for the IO/timeout watchers might not have been called yet.
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));
}
Method 2: This would be just like method 1, but you run "adns_afterpoll" in the prepare watcher and would dispose of the check watcher.
Method 3: If the module to be embedded supports explicit event notification (libadns does), you can also make use of the actual watcher callbacks, and only destroy/create the watchers in the prepare watcher.
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
Method 4: Do not use a prepare or check watcher because the module you want to embed is not flexible enough to support it. Instead, you can override their poll function. The drawback with this solution is that the main loop is now no longer controllable by EV. The "Glib::EV" module uses this approach, effectively embedding EV as a client into the horrible libglib event loop.
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_loop (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;
}
There are primarily two reasons you would want that: work around bugs and prioritise I/O.
As an example for a bug workaround, the kqueue backend might only support sockets on some platform, so it is unusable as generic backend, but you still want to make use of it because you have many sockets and it scales so nicely. In this case, you would create a kqueue-based loop and embed it into your default loop (which might use e.g. poll). Overall operation will be a bit slower because first libev has to call "poll" and then "kevent", but at least you can use both mechanisms for what they are best: "kqueue" for scalable sockets and "poll" if you want it to work :)
As for prioritising I/O: under rare circumstances you have the case where some fds have to be watched and handled very quickly (with low latency), and even priorities and idle watchers might have too much overhead. In this case you would put all the high priority stuff in one loop and all the rest in a second one, and embed the second one in the first.
As long as the watcher is active, the callback will be invoked every time there might be events pending in the embedded loop. The callback must then call "ev_embed_sweep (mainloop, watcher)" to make a single sweep and invoke their callbacks (the callback doesn't need to invoke the "ev_embed_sweep" function directly, it could also start an idle watcher to give the embedded loop strictly lower priority for example).
You can also set the callback to 0, in which case the embed watcher will automatically execute the embedded loop sweep whenever necessary.
Fork detection will be handled transparently while the "ev_embed" watcher is active, i.e., the embedded loop will automatically be forked when the embedding loop forks. In other cases, the user is responsible for calling "ev_loop_fork" on the embedded loop.
Unfortunately, not all backends are embeddable: only the ones returned by "ev_embeddable_backends" are, which, unfortunately, does not include any portable one.
So when you want to use this feature you will always have to be prepared that you cannot get an embeddable loop. The recommended way to get around this is to have a separate variables for your embeddable loop, try to create it, and if that fails, use the normal loop for everything.
"ev_embed" and fork
While the "ev_embed" watcher is running, forks in the embedding loop will automatically be applied to the embedded loop as well, so no special fork handling is required in that case. When the watcher is not running, however, it is still the task of the libev user to call "ev_loop_fork ()" as applicable.
Watcher-Specific Functions and Data Members
Examples
Example: Try to get an embeddable event loop and embed it into the default event loop. If that is not possible, use the default loop. The default loop is stored in "loop_hi", while the embeddable loop is stored in "loop_lo" (which is "loop_hi" in the case no embeddable loop can be used).
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;
Example: Check if kqueue is available but not recommended and create a kqueue backend for use with sockets (which usually work with any kqueue implementation). Store the kqueue/socket-only event loop in "loop_socket". (One might optionally use "EVFLAG_NOENV", too).
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
The special problem of life after fork - how is it possible?
Most uses of "fork()" consist of forking, then some simple calls to ste up/change the process environment, followed by a call to "exec()". This sequence should be handled by libev without any problems.
This changes when the application actually wants to do event handling in the child, or both parent in child, in effect ``continuing'' after the fork.
The default mode of operation (for libev, with application help to detect forks) is to duplicate all the state in the child, as would be expected when either the parent or the child process continues.
When both processes want to continue using libev, then this is usually the wrong result. In that case, usually one process (typically the parent) is supposed to continue with all watchers in place as before, while the other process typically wants to start fresh, i.e. without any active watchers.
The cleanest and most efficient way to achieve that with libev is to simply create a new event loop, which of course will be ``empty'', and use that for new watchers. This has the advantage of not touching more memory than necessary, and thus avoiding the copy-on-write, and the disadvantage of having to use multiple event loops (which do not support signal watchers).
When this is not possible, or you want to use the default loop for other reasons, then in the process that wants to start ``fresh'', call "ev_default_destroy ()" followed by "ev_default_loop (...)". Destroying the default loop will ``orphan'' (not stop) all registered watchers, so you have to be careful not to execute code that modifies those watchers. Note also that in that case, you have to re-register any signal watchers.
Watcher-Specific Functions and Data Members
Sometimes, however, you need to wake up another event loop you do not control, for example because it belongs to another thread. This is what "ev_async" watchers do: as long as the "ev_async" watcher is active, you can signal it by calling "ev_async_send", which is thread- and signal safe.
This functionality is very similar to "ev_signal" watchers, as signals, too, are asynchronous in nature, and signals, too, will be compressed (i.e. the number of callback invocations may be less than the number of "ev_async_sent" calls).
Unlike "ev_signal" watchers, "ev_async" works with any event loop, not just the default loop.
Queueing
"ev_async" does not support queueing of data in any way. The reason is that the author does not know of a simple (or any) algorithm for a multiple-writer-single-reader queue that works in all cases and doesn't need elaborate support such as pthreads or unportable memory access semantics.
That means that if you want to queue data, you have to provide your own queue. But at least I can tell you how to implement locking around your queue:
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...).
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);
}
Watcher-Specific Functions and Data Members
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, set on "ev_async_send", reset when the event loop detects that).
This call incurs the overhead of a system call only once per event loop iteration, so while the overhead might be noticeable, it doesn't apply to repeated calls to "ev_async_send" for the same 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.
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 gets passed an "revents" set like normal event callbacks (a combination of "EV_ERROR", "EV_READ", "EV_WRITE" or "EV_TIMEOUT") 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_TIMEOUT)
/* doh, nothing entered */;
}
ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
To use it,
#include <ev++.h>
This automatically includes ev.h and puts all of its definitions (many of them macros) into the global namespace. All C++ specific things are put into the "ev" namespace. It should support all the same embedding options as ev.h, most notably "EV_MULTIPLICITY".
Care has been taken to keep the overhead low. The only data member the C++ classes add (compared to plain C-style watchers) is the event loop pointer that the watcher is associated with (or no additional members at all if you disable "EV_MULTIPLICITY" when embedding libev).
Currently, functions, and static and non-static member functions can be used as callbacks. Other types should be easy to add as long as they only need one additional pointer for context. If you need support for other types of functors please contact the author (preferably after implementing it).
Here is a list of things available in the "ev" namespace:
All of those classes have these methods:
The constructor calls "ev_init" for you, which means you have to call the "set" method before starting it.
It will not set a callback, however: You have to call the templated "set" method to set a callback before you can start the watcher.
(The reason why you have to use a method is a limitation in C++ which does not allow explicit template arguments for constructors).
The destructor automatically stops the watcher if it is active.
This method synthesizes efficient thunking code to call your method from the C callback that libev requires. If your compiler can inline your callback (i.e. it is visible to it at the place of the "set" call and your compiler is good :), then the method will be fully inlined into the thunking function, making it as fast as a direct C callback.
Example: simple class declaration and watcher initialisation
struct myclass
{
void io_cb (ev::io &w, int revents) { }
}
myclass obj;
ev::io iow;
iow.set <myclass, &myclass::io_cb> (&obj);
This is a variation of a method callback - leaving out the method to call will default the method to "operator ()", which makes it possible to use functor objects without having to manually specify the "operator ()" all the time. Incidentally, you can then also leave out the template argument list.
The "operator ()" method prototype must be "void operator ()(watcher &w, int revents)".
See the method-"set" above for more details.
Example: use a functor object as callback.
struct myfunctor
{
void operator() (ev::io &w, int revents)
{
...
}
}
myfunctor f;
ev::io w;
w.set (&f);
The prototype of the "function" must be "void (*)(ev::TYPE &w, int)".
See the method-"set" above for more details.
Example: Use a plain function as callback.
static void io_cb (ev::io &w, int revents) { }
iow.set <io_cb> ();
Example: Define a class with an IO and idle watcher, start one of them in the constructor.
class myclass
{
ev::io io ; void io_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);
idle.set <myclass, &myclass::idle_cb> (this);
io.start (fd, ev::READ);
}
};
It can be found and installed via CPAN, its homepage is at <http://software.schmorp.de/pkg/EV>.
Roger Pack reports that using the link order "-lws2_32 -lmsvcrt-ruby-190" makes rev work even on mingw.
To make it easier to write programs that cope with either variant, the following macros are defined:
ev_unref (EV_A); ev_timer_add (EV_A_ watcher); ev_loop (EV_A_ 0);
It assumes the variable "loop" of type "struct ev_loop *" is in scope, which is often provided by the following macro.
// 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".
It is often prudent to use "EV_DEFAULT" when initialising the first watcher in a function but use "EV_DEFAULT_UC" afterwards.
Example: Declare and initialise a check watcher, utilising the above macros so it will work regardless of whether multiple loops are supported or not.
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_loop (EV_DEFAULT_ 0);
The goal is to enable you to just copy the necessary files into your source directory without having to change even a single line in them, so you can easily upgrade by simply copying (or having a checked-out copy of libev somewhere in your source tree).
CORE EVENT LOOP
To include only the libev core (all the "ev_*" functions), with manual configuration (no autoconf):
#define EV_STANDALONE 1 #include "ev.c"
This will automatically include ev.h, too, and should be done in a single C source file only to provide the function implementations. To use it, do the same for ev.h in all files wishing to use this API (best done by writing a wrapper around ev.h that you can include instead and where you can put other configuration options):
#define EV_STANDALONE 1 #include "ev.h"
Both header files and implementation files can be compiled with a C++ compiler (at least, that's a stated goal, and breakage will be treated as a bug).
You need the following files in your source tree, or in a directory in your include path (e.g. in libev/ when using -Ilibev):
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)
ev.c includes the backend files directly when enabled, so you only need to compile this single file.
LIBEVENT COMPATIBILITY API
To include the libevent compatibility API, also include:
#include "event.c"
in the file including ev.c, and:
#include "event.h"
in the files that want to use the libevent API. This also includes ev.h.
You need the following additional files for this:
event.h event.c
AUTOCONF SUPPORT
Instead of using "EV_STANDALONE=1" and providing your configuration in whatever way you want, you can also "m4_include([libev.m4])" in your configure.ac and leave "EV_STANDALONE" undefined. ev.c will then include config.h and configure itself accordingly.
For this of course you need the m4 file:
libev.m4
In standalone mode, libev will still try to automatically deduce the configuration, but has to be more conservative.
In the absence of this define, libev will use "sig_atomic_t volatile" (from signal.h), which is usually good enough on most platforms.
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.
You can save even more by disabling watcher types you do not need and setting "EV_MAXPRI" == "EV_MINPRI". Also, disabling "assert" ("-DNDEBUG") will usually reduce code size a lot.
Defining "EV_MINIMAL" to 2 will additionally reduce the core API to provide a bare-bones event library. See "ev.h" for details on what parts of the API are still available, and do not complain if this subset changes over time.
The default is 1 unless "EV_MINIMAL" is set in which case it is 0 (disabled).
The default is 1 unless "EV_MINIMAL" is set in which case it is 0 (disabled).
The default is 1, unless "EV_MINIMAL" is set, in which case it will be 0.
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 ";" */
Symbols.ev for libev proper Symbols.event for the libevent emulation
This can also be used to rename all public symbols to avoid clashes with multiple versions of libev linked together (which is obviously bad in itself, but sometimes it is inconvenient to avoid this).
A sed command like this will create wrapper "#define"'s that you need to include before including ev.h:
<Symbols.ev sed -e "s/.*/#define & myprefix_&/" >wrap.h
This would create a file wrap.h which essentially looks like this:
#define ev_backend myprefix_ev_backend #define ev_check_start myprefix_ev_check_start #define ev_check_stop myprefix_ev_check_stop ...
The usage in rxvt-unicode is simpler. It has a ev_cpp.h header file that everybody includes and which overrides some configure choices:
#define EV_MINIMAL 1 #define EV_USE_POLL 0 #define EV_MULTIPLICITY 0 #define EV_PERIODIC_ENABLE 0 #define EV_STAT_ENABLE 0 #define EV_FORK_ENABLE 0 #define EV_CONFIG_H <config.h> #define EV_MINPRI 0 #define EV_MAXPRI 0 #include "ev++.h"
And a ev_cpp.C implementation file that contains libev proper and is compiled:
#include "ev_cpp.h" #include "ev.c"
All libev functions are reentrant and thread-safe unless explicitly documented otherwise, but libev implements no locking itself. This means that you can use as many loops as you want in parallel, as long as there are no concurrent calls into any libev function with the same loop parameter ("ev_default_*" calls have an implicit default loop parameter, of course): libev guarantees that different event loops share no data structures that need any locking.
Or to put it differently: calls with different loop parameters can be done concurrently from multiple threads, calls with the same loop parameter must be done serially (but can be done from different threads, as long as only one thread ever is inside a call at any point in time, e.g. by using a mutex per loop).
Specifically to support threads (and signal handlers), libev implements so-called "ev_async" watchers, which allow some limited form of concurrency on the same event loop, namely waking it up ``from the outside''.
If you want to know which design (one loop, locking, or multiple loops without or something else still) is best for your problem, then I cannot help you, but here is some generic advice:
This helps integrating other libraries or software modules that use libev themselves and don't care/know about threading.
Doing this is almost never wrong, sometimes a better-performance model exists, but it is always a good start.
Choosing a model is hard - look around, learn, know that usually you can do better than you currently do :-)
"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.
THREAD LOCKING EXAMPLE
Here is a fictitious example of how to run an event loop in a different thread than where callbacks are being invoked and watchers are created/added/removed.
For a real-world example, see the "EV::Loop::Async" perl module, which uses exactly this technique (which is suited for many high-level languages).
The example uses a pthread mutex to protect the loop data, a condition variable to wait for callback invocations, an async watcher to notify the event loop thread and an unspecified mechanism to wake up the main thread.
First, you need to associate some data with the event loop:
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_loop
pthread_create (&u->tid, 0, l_run, EV_A);
}
The callback for the "ev_async" watcher does nothing: the watcher is used solely to wake up the event loop so it takes notice of any new watchers that might have been added:
static void
async_cb (EV_P_ ev_async *w, int revents)
{
// just used for the side effects
}
The "l_release" and "l_acquire" callbacks simply unlock/lock the mutex protecting the loop data, respectively.
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);
}
The event loop thread first acquires the mutex, and then jumps straight into "ev_loop":
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_loop (EV_A_ 0);
l_release (EV_A);
return 0;
}
Instead of invoking all pending watchers, the "l_invoke" callback will signal the main thread via some unspecified mechanism (signals? pipe writes? "Async::Interrupt"?) and then waits until all pending watchers have been called (in a while loop because a) spurious wakeups are possible and b) skipping inter-thread-communication when there are no pending watchers is very beneficial):
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);
}
}
Now, whenever the main thread gets told to invoke pending watchers, it will grab the lock, call "ev_invoke_pending" and then signal the loop thread to continue:
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);
}
Whenever you want to start/stop a watcher or do other modifications to an event loop, you will now have to 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);
Note that sending the "ev_async" watcher is required because otherwise an event loop currently blocking in the kernel will have no knowledge about the newly added timer. By waking up the loop it will pick up any new watchers in the next event loop iteration.
COROUTINES
Libev is very accommodating to coroutines (``cooperative threads''): libev fully supports nesting calls to its functions from different coroutines (e.g. you can call "ev_loop" on the same loop from two different coroutines, and switch freely between both coroutines running the loop, as long as you don't confuse yourself). The only exception is that you must not do this from "ev_periodic" reschedule callbacks.
Care has been taken to ensure that libev does not keep local state inside "ev_loop", and other calls do not usually allow for coroutine switches as they do not call any callbacks.
However, these are unavoidable for many reasons. For one, each compiler has different warnings, and each user has different tastes regarding warning options. ``Warn-free'' code therefore cannot be a goal except when targeting a specific compiler and compiler-version.
Another reason is that some compiler warnings require elaborate workarounds, or other changes to the code that make it less clear and less maintainable.
And of course, some compiler warnings are just plain stupid, or simply wrong (because they don't actually warn about the condition their message seems to warn about). For example, certain older gcc versions had some warnings that resulted an extreme number of false positives. These have been fixed, but some people still insist on making code warn-free with such buggy versions.
While libev is written to generate as few warnings as possible, ``warn-free'' code is not a goal, and it is recommended not to build libev with any compiler warnings enabled unless you are prepared to cope with them (e.g. by ignoring them). Remember that warnings are just that: warnings, not errors, or proof of bugs.
If you think you found a bug (memory leak, uninitialised data access etc.) in libev, then check twice: If valgrind reports something like:
==2274== definitely lost: 0 bytes in 0 blocks. ==2274== possibly lost: 0 bytes in 0 blocks. ==2274== still reachable: 256 bytes in 1 blocks.
Then there is no memory leak, just as memory accounted to global variables is not a memleak - the memory is still being referenced, and didn't leak.
Similarly, under some circumstances, valgrind might report kernel bugs as if it were a bug in libev (e.g. in realloc or in the poll backend, although an acceptable workaround has been found here), or it might be confused.
Keep in mind that valgrind is a very good tool, but only a tool. Don't make it into some kind of religion.
If you are unsure about something, feel free to contact the mailing list with the full valgrind report and an explanation on why you think this is a bug in libev (best check the archives, too :). However, don't be annoyed when you get a brisk ``this is no bug'' answer and take the chance of learning how to interpret valgrind properly.
If you need, for some reason, empty reports from valgrind for your project I suggest using suppression lists.
Lifting these limitations would basically require the full re-implementation of the I/O system. If you are into these kinds of things, then note that glib does exactly that for you in a very portable way (note also that glib is the slowest event library known to man).
There is no supported compilation method available on windows except embedding it into other applications.
Sensible signal handling is officially unsupported by Microsoft - libev tries its best, but under most conditions, signals will simply not work.
Not a libev limitation but worth mentioning: windows apparently doesn't accept large writes: instead of resulting in a partial write, windows will either accept everything or return "ENOBUFS" if the buffer is too large, so make sure you only write small amounts into your sockets (less than a megabyte seems safe, but this apparently depends on the amount of memory available).
Due to the many, low, and arbitrary limits on the win32 platform and the abysmal performance of winsockets, using a large number of sockets is not recommended (and not reasonable). If your program needs to use more than a hundred or so sockets, then likely it needs to use a totally different implementation for windows, as libev offers the POSIX readiness notification model, which cannot be implemented efficiently on windows (due to Microsoft monopoly games).
A typical way to use libev under windows is to embed it (see the embedding section for details) and use the following evwrap.h header file instead of ev.h:
#define EV_STANDALONE /* keeps ev from requiring config.h */ #define EV_SELECT_IS_WINSOCKET 1 /* configure libev for windows select */ #include "ev.h"
And compile the following evwrap.c file into your project (make sure you do not compile the ev.c or any other embedded source files!):
#include "evwrap.h" #include "ev.c"
The configuration for a ``naked'' win32 using the Microsoft runtime libraries and raw winsocket select is:
#define EV_USE_SELECT 1 #define EV_SELECT_IS_WINSOCKET 1 /* forces EV_SELECT_USE_FD_SET, too */
Note that winsockets handling of fd sets is O(n), so you can easily get a complexity in the O(nAX) range when using win32.
Early versions of winsocket's select only supported waiting for a maximum of 64 handles (probably owning to the fact that all windows kernels can only wait for 64 things at the same time internally; Microsoft recommends spawning a chain of threads and wait for 63 handles and the previous thread in each. Sounds great!).
Newer versions support more handles, but you need to define "FD_SETSIZE" to some high number (e.g. 2048) before compiling the winsocket select call (which might be in libev or elsewhere, for example, perl and many other interpreters do their own select emulation on windows).
Another limit is the number of file descriptors in the Microsoft runtime libraries, which by default is 64 (there must be a hidden 64 fetish or something like this inside Microsoft). You can increase this by calling "_setmaxstdio", which can increase this limit to 2048 (another arbitrary limit), but is broken in many versions of the Microsoft runtime libraries. This might get you to about 512 or 2048 sockets (depending on windows version and/or the phase of the moon). To get more, you need to wrap all I/O functions and provide your own fd management, but the cost of calling select (O(nAX)) will likely make this unworkable.
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.
If you know of other additional requirements drop me a note.
All of the following are about amortised time: If an array needs to be extended, libev needs to realloc and move the whole array, but this happens asymptotically rarer with higher number of elements, so O(1) might mean that libev does a lengthy realloc operation in rare cases, but on average it is much faster and asymptotically approaches constant time.
In libev, events are represented as single bits (such as "EV_READ" or "EV_TIMEOUT").
A watcher can be pending, but not active. Stopping a watcher also clears its pending status.