ev_io - is this file descriptor readable or writable?
ev_timer - relative and optionally repeating timeouts
ev_periodic - to cron or not to cron?
ev_signal - signal me when a signal gets signalled!
ev_child - watch out for process status changes
ev_stat - did the file attributes just change?
ev_idle - when you've got nothing better to do...
ev_prepare and ev_check - customise your event loop!
ev_embed - when one backend isn't enough...
ev_fork - the audacity to resume the event loop after a fork
ev_async - how to wake up another event loop
libev - a high performance full-featured event loop written in C
#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;
}
This document documents the libev software package.
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.
Libev is an event loop: you register interest in certain events (such as a file descriptor being readable or a timeout occurring), and it will manage these event sources and provide your program with events.
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.
Libev supports select, poll, the Linux-specific epoll, the
BSD-specific kqueue and the Solaris-specific event port mechanisms
for file descriptor events (ev_io), the Linux inotify interface
(for ev_stat), relative timers (ev_timer), absolute timers
with customised rescheduling (ev_periodic), synchronous signals
(ev_signal), process status change events (ev_child), and event
watchers dealing with the event loop mechanism itself (ev_idle,
ev_embed, ev_prepare and ev_check watchers) as well as
file watchers (ev_stat) and even limited support for fork events
(ev_fork).
It also is quite fast (see this benchmark comparing it to libevent for example).
Libev is very configurable. In this manual the default (and most common)
configuration will be described, which supports multiple event loops. For
more info about various configuration options please have a look at
EMBED section in this manual. If libev was configured without support
for multiple event loops, then all functions taking an initial argument of
name loop (which is always of type ev_loop *) will not have
this argument.
Libev represents time as a single floating point number, representing
the (fractional) number of seconds since the (POSIX) epoch (somewhere
near the beginning of 1970, details are complicated, don't ask). This
type is called ev_tstamp, which is what you should use too. It usually
aliases to the double type in C. When you need to do any calculations
on it, you should treat it as some floating point value. Unlike the name
component stamp might indicate, it is also used for time differences
throughout libev.
Libev knows three classes of errors: operating system errors, usage errors and internal errors (bugs).
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 assertions, 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 functions can be called anytime, even before initialising the library in any way.
ev_now function is usually faster and also often returns the timestamp
you actually want to know.
sleep ().
You can find out the major and minor ABI version numbers of the library
you linked against by calling the functions ev_version_major and
ev_version_minor. If you want, you can compare against the global
symbols EV_VERSION_MAJOR and EV_VERSION_MINOR, which specify the
version of the library your program was compiled against.
These version numbers refer to the ABI version of the library, not the release version.
Usually, it's a good idea to terminate if the major versions mismatch, as this indicates an incompatible change. Minor versions are usually compatible to older versions, so a larger minor version alone is usually not a problem.
Example: Make sure we haven't accidentally been linked against the wrong version.
assert (("libev version mismatch",
ev_version_major () == EV_VERSION_MAJOR
&& ev_version_minor () >= EV_VERSION_MINOR));
Return the set of all backends (i.e. their corresponding EV_BACKEND_*
value) compiled into this binary of libev (independent of their
availability on the system you are running on). See ev_default_loop for
a description of the set values.
Example: make sure we have the epoll method, because yeah this is cool and a must have and can we have a torrent of it please!!!11
assert (("sorry, no epoll, no sex",
ev_supported_backends () & EVBACKEND_EPOLL));
ev_supported_backends, as for example kqueue is broken on
most BSDs and will not be auto-detected unless you explicitly request it
(assuming you know what you are doing). This is the set of backends that
libev will probe for if you specify no backends explicitly.
Returns the set of backends that are embeddable in other event loops. This
is the theoretical, all-platform, value. To find which backends
might be supported on the current system, you would need to look at
ev_embeddable_backends () & ev_supported_backends (), likewise for
recommended ones.
See the description of ev_embed watchers for more info.
Sets the allocation function to use (the prototype is similar - the
semantics are identical to the realloc C89/SuS/POSIX function). It is
used to allocate and free memory (no surprises here). If it returns zero
when memory needs to be allocated (size != 0), the library might abort
or take some potentially destructive action.
Since some systems (at least OpenBSD and Darwin) fail to implement
correct realloc semantics, libev will use a wrapper around the system
realloc and free functions by default.
You could override this function in high-availability programs to, say, free some memory if it cannot allocate memory, to use a special allocator, or even to sleep a while and retry until some memory is available.
Example: Replace the libev allocator with one that waits a bit and then
retries (example requires a standards-compliant realloc).
static void *
persistent_realloc (void *ptr, size_t size)
{
for (;;)
{
void *newptr = realloc (ptr, size);
if (newptr)
return newptr;
sleep (60);
}
}
...
ev_set_allocator (persistent_realloc);
Set the callback function to call on a retryable system call error (such as failed select, poll, epoll_wait). The message is a printable string indicating the system call or subsystem causing the problem. If this callback is set, then libev will expect it to remedy the situation, no matter what, when it returns. That is, libev will generally retry the requested operation, or, if the condition doesn't go away, do bad stuff (such as abort).
Example: This is basically the same thing that libev does internally, too.
static void
fatal_error (const char *msg)
{
perror (msg);
abort ();
}
...
ev_set_syserr_cb (fatal_error);
An event loop is described by a struct ev_loop * (the struct
is not optional in this case, as there is also an ev_loop
function).
The library knows two types of such loops, the default loop, which supports signals and child events, and dynamically created loops which do not.
This will initialise the default event loop if it hasn't been initialised
yet and return it. If the default loop could not be initialised, returns
false. If it already was initialised it simply returns it (and ignores the
flags. If that is troubling you, check ev_backend () afterwards).
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:
EVFLAG_AUTO
EVFLAG_NOENV
LIBEV_FLAGS. Otherwise (the default), this environment variable will
override the flags completely if it is found in the environment. This is
useful to try out specific backends to test their performance, or to work
around bugs.
EVFLAG_FORKCHECK
Instead of calling ev_default_fork or ev_loop_fork manually after
a fork, you can also make libev check for a fork in each iteration by
enabling this flag.
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.
EVBACKEND_SELECT (value 1, portable select backend)
This is your standard select(2) backend. Not completely standard, as libev tries to roll its own fd_set with no limits on the number of fds, but if that fails, expect a fairly low limit on the number of fds when using this backend. It doesn't scale too well (O(highest_fd)), but its usually the fastest backend for a low number of (low-numbered :) fds.
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).
EVBACKEND_POLL (value 2, poll backend, available everywhere except on windows)
And this is your standard poll(2) backend. It's more complicated
than select, but handles sparse fds better and has no artificial
limit on the number of fds you can use (except it will slow down
considerably with a lot of inactive fds). It scales similarly to select,
i.e. O(total_fds). See the entry for EVBACKEND_SELECT, above, for
performance tips.
This backend maps EV_READ to POLLIN | POLLERR | POLLHUP, and
EV_WRITE to POLLOUT | POLLERR | POLLHUP.
EVBACKEND_EPOLL (value 4, Linux)
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.
EVBACKEND_KQUEUE (value 8, most BSD clones)
Kqueue deserves special mention, as at the time of this writing, it
was broken on all BSDs except NetBSD (usually it doesn't work reliably
with anything but sockets and pipes, except on Darwin, where of course
it's completely useless). Unlike epoll, however, whose brokenness
is by design, these kqueue bugs can (and eventually will) be fixed
without API changes to existing programs. For this reason it's not being
"auto-detected" unless you explicitly specify it in the flags (i.e. using
EVBACKEND_KQUEUE) or libev was compiled on a known-to-be-good (-enough)
system like NetBSD.
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.
EVBACKEND_DEVPOLL (value 16, Solaris 8)
/dev/poll only supports sockets
and is not embeddable, which would limit the usefulness of this backend
immensely.
EVBACKEND_PORT (value 32, Solaris 10)
This uses the Solaris 10 event port mechanism. As with everything on Solaris, it's really slow, but it still scales very well (O(active_fds)).
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.
EVBACKEND_ALL
Try all backends (even potentially broken ones that wouldn't be tried
with EVFLAG_AUTO). Since this is a mask, you can do stuff such as
EVBACKEND_ALL & ~EVBACKEND_KQUEUE.
It is definitely not recommended to use this flag.
If one or more of these 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);
Similar to ev_default_loop, but always creates a new event loop that is
always distinct from the default loop. Unlike the default loop, it cannot
handle signal and child watchers, and attempts to do so will be greeted by
undefined behaviour (or a failed assertion if assertions are enabled).
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");
Destroys the default loop again (frees all memory and kernel state
etc.). None of the active event watchers will be stopped in the normal
sense, so e.g. ev_is_active might still return true. It is your
responsibility to either stop all watchers cleanly yourself before
calling this function, or cope with the fact afterwards (which is usually
the easiest thing, you can just ignore the watchers and/or free () them
for example).
Note that certain global state, such as signal state (and installed signal handlers), will not be freed by this function, and related watchers (such as signal and child watchers) would need to be stopped manually.
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).
ev_default_destroy, but destroys an event loop created by an
earlier call to ev_loop_new.
This function sets a flag that causes subsequent ev_loop iterations
to reinitialise the kernel state for backends that have one. Despite the
name, you can call it anytime, but it makes most sense after forking, in
the child process (or both child and parent, but that again makes little
sense). You must call it in the child before using any of the libev
functions, and it will only take effect at the next ev_loop iteration.
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);
ev_default_fork, but acts on an event loop created by
ev_loop_new. Yes, you have to call this on every allocated event loop
after fork that you want to re-use in the child, and how you do this is
entirely your own problem.
Returns the count of loop iterations for the loop, which is identical to
the number of times libev did poll for new events. It starts at 0 and
happily wraps around with enough iterations.
This value can sometimes be useful as a generation counter of sorts (it
"ticks" the number of loop iterations), as it roughly corresponds with
ev_prepare and ev_check calls.
EVBACKEND_* flags indicating the event backend in
use.
Establishes the current time by querying the kernel, updating the time
returned by ev_now () in the progress. This is a costly operation and
is usually done automatically within ev_loop ().
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.
These two functions suspend and resume a loop, for use when the loop is not used for a while and timeouts should not be processed.
A typical use case would be an interactive program such as a game: When
the user presses ^Z to suspend the game and resumes it an hour later it
would be best to handle timeouts as if no time had actually passed while
the program was suspended. This can be achieved by calling ev_suspend
in your SIGTSTP handler, sending yourself a SIGSTOP and calling
ev_resume directly afterwards to resume timer processing.
Effectively, all ev_timer watchers will be delayed by the time spend
between ev_suspend and ev_resume, and all ev_periodic watchers
will be rescheduled (that is, they will lose any events that would have
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).
Finally, this is it, the event handler. This function usually is called after you initialised all your watchers and you want to start handling events.
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!
Can be used to make a call to ev_loop return early (but only after it
has processed all outstanding events). The how argument must be either
EVUNLOOP_ONE, which will make the innermost ev_loop call return, or
EVUNLOOP_ALL, which will make all nested ev_loop calls return.
This "unloop state" will be cleared when entering ev_loop again.
It is safe to call ev_unloop from otuside any ev_loop calls.
Ref/unref can be used to add or remove a reference count on the event
loop: Every watcher keeps one reference, and as long as the reference
count is nonzero, ev_loop will not return on its own.
If you have a watcher you never unregister that should not keep ev_loop
from returning, 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);
These advanced functions influence the time that libev will spend waiting
for events. Both time intervals are by default 0, meaning that libev
will try to invoke timer/periodic callbacks and I/O callbacks with minimum
latency.
Setting these to a higher value (the interval must be >= 0)
allows libev to delay invocation of I/O and timer/periodic callbacks
to increase efficiency of loop iterations (or to increase power-saving
opportunities).
The idea is that sometimes your program runs just fast enough to handle
one (or very few) event(s) per loop iteration. While this makes the
program responsive, it also wastes a lot of CPU time to poll for new
events, especially with backends like select () which have a high
overhead for the actual polling but can deliver many events at once.
By setting a higher io collect interval you allow libev to spend more
time collecting I/O events, so you can handle more events per iteration,
at the cost of increasing latency. Timeouts (both ev_periodic and
ev_timer) will be not affected. Setting this to a non-null value will
introduce an additional ev_sleep () call into most loop iterations.
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.
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.
This function only does something when EV_VERIFY support has been
compiled in, which is the default for non-minimal builds. It tries to go
through all internal structures and checks them for validity. If anything
is found to be inconsistent, it will print an error message to standard
error and call abort ().
This can be used to catch bugs inside libev itself: under normal circumstances, this function will never abort as of course libev keeps its data structures consistent.
In the following description, uppercase TYPE in names stands for the
watcher type, e.g. ev_TYPE_start can mean ev_timer_start for timer
watchers and ev_io_start for I/O watchers.
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:
EV_READ
EV_WRITE
ev_io watcher has become readable and/or
writable.
EV_TIMEOUT
ev_timer watcher has timed out.
EV_PERIODIC
ev_periodic watcher has timed out.
EV_SIGNAL
ev_signal watcher has been received by a thread.
EV_CHILD
ev_child watcher has received a status change.
EV_STAT
ev_stat watcher changed its attributes somehow.
EV_IDLE
ev_idle watcher has determined that you have nothing better to do.
EV_PREPARE
EV_CHECK
ev_prepare watchers are invoked just before ev_loop starts
to gather new events, and all ev_check watchers are invoked just after
ev_loop has gathered them, but before it invokes any callbacks for any
received events. Callbacks of both watcher types can start and stop as
many watchers as they want, and all of them will be taken into account
(for example, a ev_prepare watcher might start an idle watcher to keep
ev_loop from blocking).
EV_EMBED
ev_embed watcher needs attention.
EV_FORK
ev_fork).
EV_ASYNC
ev_async).
EV_CUSTOM
ev_feed_event).
EV_ERROR
An unspecified error has occurred, the watcher has been stopped. This might happen because the watcher could not be properly started because libev ran out of memory, a file descriptor was found to be closed or any other problem. Libev considers these application bugs.
You best act on it by reporting the problem and somehow coping with the watcher being stopped. Note that well-written programs should not receive an error ever, so when your watcher receives it, this usually indicates a bug in your program.
Libev will usually signal a few "dummy" events together with an error, for example it might indicate that a fd is readable or writable, and if your callbacks is well-written it can just attempt the operation and cope with the error from read() or write(). This will not work in multi-threaded programs, though, as the fd could already be closed and reused for another thing, so beware.
ev_init (ev_TYPE *watcher, callback)
This macro initialises the generic portion of a watcher. The contents
of the watcher object can be arbitrary (so malloc will do). Only
the generic parts of the watcher are initialised, you need to call
the type-specific ev_TYPE_set macro afterwards to initialise the
type-specific parts. For each type there is also a ev_TYPE_init macro
which rolls both calls into one.
You can reinitialise a watcher at any time as long as it has been stopped (or never started) and there are no pending events outstanding.
The callback is always of type void (*)(struct ev_loop *loop, ev_TYPE *watcher,
int revents).
Example: Initialise an ev_io watcher in two steps.
ev_io w; ev_init (&w, my_cb); ev_io_set (&w, STDIN_FILENO, EV_READ);
ev_TYPE_set (ev_TYPE *, [args])
This macro initialises the type-specific parts of a watcher. You need to
call ev_init at least once before you call this macro, but you can
call ev_TYPE_set any number of times. You must not, however, call this
macro on a watcher that is active (it can be pending, however, which is a
difference to the ev_init macro).
Although some watcher types do not have type-specific arguments
(e.g. ev_prepare) you still need to call its set macro.
See ev_init, above, for an example.
ev_TYPE_init (ev_TYPE *watcher, callback, [args])
This convenience macro rolls both ev_init and ev_TYPE_set macro
calls into a single call. This is the most convenient method to initialise
a watcher. The same limitations apply, of course.
Example: Initialise and set an ev_io watcher in one step.
ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
ev_TYPE_start (loop *, ev_TYPE *watcher)
Starts (activates) the given watcher. Only active watchers will receive events. If the watcher is already active nothing will happen.
Example: Start the ev_io watcher that is being abused as example in this
whole section.
ev_io_start (EV_DEFAULT_UC, &w);
ev_TYPE_stop (loop *, ev_TYPE *watcher)
Stops the given watcher if active, and clears the pending status (whether the watcher was active or not).
It is possible that stopped watchers are pending - for example,
non-repeating timers are being stopped when they become pending - but
calling ev_TYPE_stop ensures that the watcher is neither active nor
pending. If you want to free or reuse the memory used by the watcher it is
therefore a good idea to always call its ev_TYPE_stop function.
ev_TYPE_set is safe), you must not change its priority, and you must
make sure the watcher is available to libev (e.g. you cannot free ()
it).
Set and query the priority of the watcher. The priority is a small
integer between EV_MAXPRI (default: 2) and EV_MINPRI
(default: -2). Pending watchers with higher priority will be invoked
before watchers with lower priority, but priority will not keep watchers
from being executed (except for ev_idle watchers).
If you need to suppress invocation when higher priority events are pending
you need to look at ev_idle watchers, which provide this functionality.
You must not change the priority of a watcher as long as it is active or pending.
Setting a priority outside the range of EV_MINPRI to EV_MAXPRI is
fine, as long as you do not mind that the priority value you query might
or might not have been clamped to the valid range.
The default priority used by watchers when no priority has been set is
always 0, which is supposed to not be too high and not be too low :).
See WATCHER PRIORITY MODELS, below, for a more thorough treatment of priorities.
watcher with the given loop and revents. Neither
loop nor revents need to be valid as long as the watcher callback
can deal with that fact, as both are simply passed through to the
callback.
If the watcher is pending, this function clears its pending status and
returns its revents bitset (as if its callback was invoked). If the
watcher isn't pending it does nothing and returns 0.
Sometimes it can be useful to "poll" a watcher instead of waiting for its callback to be invoked, which can be accomplished with this function.
Each watcher has, by default, a member void *data that you can change
and read at any time: libev will completely ignore it. This can be used
to associate arbitrary data with your watcher. If you need more data and
don't want to allocate memory and store a pointer to it in that data
member, you can also "subclass" the watcher type and provide your own
data:
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));
}
Many event loops support watcher priorities, which are usually small integers that influence the ordering of event callback invocation between watchers in some way, all else being equal.
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.
This section describes each watcher in detail, but will not repeat information given in the last section. Any initialisation/set macros, functions and members specific to the watcher type are explained.
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.
ev_io - is this file descriptor readable or writable?
I/O watchers check whether a file descriptor is readable or writable in each iteration of the event loop, or, more precisely, when reading would not block the process and writing would at least be able to write some data. This behaviour is called level-triggering because you keep receiving events as long as the condition persists. Remember you can stop the watcher if you don't want to act on the event and neither want to receive future events.
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.
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.
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.
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.
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).
ev_io watcher. The fd is the file descriptor to
receive events for and events is either EV_READ, EV_WRITE or
EV_READ | EV_WRITE, to express the desire to receive the given events.
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);
ev_timer - relative and optionally repeating timeouts
Timer watchers are simple relative timers that generate an event after a given time, and optionally repeating in regular intervals after that.
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 with later time-out values (but this is no longer true when a
callback calls ev_loop recursively).
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).
This is the most obvious, but not the most simple way: In the beginning, start the watcher:
ev_timer_init (timer, callback, 60., 0.); ev_timer_start (loop, timer);
Then, each time there is some activity, ev_timer_stop it, initialise it
and start it again:
ev_timer_stop (loop, timer); ev_timer_set (timer, 60., 0.); ev_timer_start (loop, timer);
This is relatively simple to implement, but means that each time there is some activity, libev will first have to remove the timer from its internal data structure and then add it again. Libev tries to be fast, but it's still not a constant-time operation.
ev_timer_again inactivity.
This is the easiest way, and involves using ev_timer_again instead of
ev_timer_start.
To implement this, configure an ev_timer with a repeat value
of 60 and then call ev_timer_again at start and each time you
successfully read or write some data. If you go into an idle state where
you do not expect data to travel on the socket, you can ev_timer_stop
the timer, and ev_timer_again will automatically restart it if need be.
That means you can ignore both the ev_timer_start function and the
after argument to ev_timer_set, and only ever use the repeat
member and ev_timer_again.
At start:
ev_timer_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.
This method is more tricky, but usually most efficient: Most timeouts are relatively long compared to the intervals between other activity - in our example, within 60 seconds, there are usually many I/O events with associated activity resets.
In this case, it would be more efficient to leave the ev_timer alone,
but remember the time of last activity, and check for a real timeout only
within the callback:
ev_tstamp 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_timer_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.
If there is not one request, but many thousands (millions...), all employing some kind of timeout with the same timeout value, then one can do even better:
When starting the timeout, calculate the timeout value and put the timeout at the end of the list.
Then use an ev_timer to fire when the timeout at the beginning of
the list is expected to fire (for example, using the technique #3).
When there is some activity, remove the timer from the list, recalculate
the timeout, append it to the end of the list again, and make sure to
update the ev_timer if it was taken from the beginning of the list.
This way, one can manage an unlimited number of timeouts in O(1) time for starting, stopping and updating the timers, at the expense of a major complication, and having to use a constant timeout. The constant timeout ensures that the list stays sorted.
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 :)
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
().
Configure the timer to trigger after after seconds. If repeat
is 0., then it will automatically be stopped once the timeout is
reached. If it is positive, then the timer will automatically be
configured to trigger again repeat seconds later, again, and again,
until stopped manually.
The timer itself will do a best-effort at avoiding drift, that is, if you configure a timer to trigger every 10 seconds, then it will normally trigger at exactly 10 second intervals. If, however, your program cannot keep up with the timer (because it takes longer than those 10 seconds to do stuff) the timer will not fire more than once per event loop iteration.
This will act as if the timer timed out and restart it again if it is repeating. The exact semantics are:
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.
repeat value. Will be used each time the watcher times out
or ev_timer_again is called, and determines the next timeout (if any),
which is also when any modifications are taken into account.
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);
ev_periodic - to cron or not to cron?
Periodic watchers are also timers of a kind, but they are very versatile (and unfortunately a bit complex).
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).
Lots of arguments, let's sort it out... There are basically three modes of operation, and we will explain them from simplest to most complex:
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).
offset argument to
ev_periodic_set, but indeed works even in interval and manual
rescheduling modes.
When repeating, this contains the offset value, otherwise this is the
absolute point in time (the offset value passed to ev_periodic_set,
although libev might modify this value for better numerical stability).
Can be modified any time, but changes only take effect when the periodic
timer fires or ev_periodic_again is being called.
ev_periodic_again is being
called.
0, if this functionality is
switched off. Can be changed any time, but changes only take effect when
the periodic timer fires or ev_periodic_again is being called.
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);
ev_signal - signal me when a signal gets signalled!
Signal watchers will trigger an event when the process receives a specific signal one or more times. Even though signals are very asynchronous, libev will try it's best to deliver signals synchronously, i.e. as part of the normal event processing, like any other event.
If you want signals 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 per signal. Only when the first watcher gets started will libev actually register a signal handler 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). Similarly, when the last signal watcher for a signal is stopped, libev will reset the signal handler to SIG_DFL (regardless of what it was set to before).
If possible and supported, libev will install its handlers with
SA_RESTART 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.
SIGxxx constants).
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);
ev_child - watch out for process status changes
Child watchers trigger when your process receives a SIGCHLD in response to some child status changes (most typically when a child of yours dies or exits). It is permissible to install a child watcher after the child has been forked (which implies it might have already exited), as long as the event loop isn't entered (or is continued from a watcher), i.e., forking and then immediately registering a watcher for the child is fine, but forking and registering a watcher a few event loop iterations later is not.
Only the default event loop is capable of handling signals, and therefore you can only register child watchers in the default event loop.
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.
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.
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.
pid (or
any process if pid is specified as 0). The callback can look
at the rstatus member of the ev_child watcher structure to see
the status word (use the macros from sys/wait.h and see your systems
waitpid documentation). The rpid member contains the pid of the
process causing the status change. trace must be either 0 (only
activate the watcher when the process terminates) or 1 (additionally
activate the watcher when the process is stopped or continued).
0, meaning any process id.
rpid (see your systems
waitpid and sys/wait.h documentation for details).
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);
}
ev_stat - did the file attributes just change?
This watches a file system path for attribute changes. That is, it calls
stat on that path in regular intervals (or when the OS says it changed)
and sees if it changed compared to the last time, invoking the callback if
it did.
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).
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.
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 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).
Configures the watcher to wait for status changes of the given
path. The interval is a hint on how quickly a change is expected to
be detected and should normally be specified as 0 to let libev choose
a suitable value. The memory pointed to by path must point to the same
path for as long as the watcher is active.
The callback will receive an EV_STAT event when a change was detected,
relative to the attributes at the time the watcher was started (or the
last change was detected).
ev_statdata, this is usually the (or one of the) struct stat types
suitable for your system, but you can only rely on the POSIX-standardised
members to be present. If the st_nlink member is 0, then there was
some error while stating the file.
prev != attr, or, more precisely, one or more of these members
differ: st_dev, st_ino, st_mode, st_nlink, st_uid,
st_gid, st_rdev, st_size, st_atime, st_mtime, st_ctime.
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);
ev_idle - when you've got nothing better to do...
Idle watchers trigger events when no other events of the same or higher priority are pending (prepare, check and other idle watchers do not count as receiving "events").
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.
ev_idle_set macro, but using it is utterly pointless,
believe me.
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_cb);
ev_prepare and ev_check - customise your event loop!
Prepare and check watchers are usually (but not always) used in pairs: prepare watchers get invoked before the process blocks and check watchers afterwards.
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).
ev_prepare_set and ev_check_set
macros, but using them is utterly, utterly, utterly and completely
pointless.
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);
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;
}
ev_embed - when one backend isn't enough...
This is a rather advanced watcher type that lets you embed one event loop
into another (currently only ev_io events are supported in the embedded
loop, other types of watchers might be handled in a delayed or incorrect
fashion and must not be used).
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.
0, then ev_embed_sweep will be
invoked automatically, otherwise it is the responsibility of the callback
to invoke it (it will continue to be called until the sweep has been done,
if you do not want that, you need to temporarily stop the embed watcher).
ev_loop (embedded_loop, EVLOOP_NONBLOCK), but in the most
appropriate way for embedded loops.
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
ev_fork - the audacity to resume the event loop after a fork
Fork watchers are called when a fork () was detected (usually because
whoever is a good citizen cared to tell libev about it by calling
ev_default_fork or ev_loop_fork). The invocation is done before the
event loop blocks next and before ev_check watchers are being called,
and only in the child after the fork. If whoever good citizen calling
ev_default_fork cheats and calls it in the wrong process, the fork
handlers will be invoked, too, of course.
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.
ev_fork_set macro, but using it is utterly pointless,
believe me.
ev_async - how to wake up another event loop
In general, you cannot use an ev_loop from multiple threads or other
asynchronous sources such as signal handlers (as opposed to multiple event
loops - those are of course safe to use in different threads).
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.
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.
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:
To implement race-free queueing, you simply add to the queue in the signal handler but you block the signal handler in the watcher callback. Here is an example that does that for some fictitious SIGUSR1 handler:
static ev_async mysig;
static void
sigusr1_handler (void)
{
sometype data;
// no locking etc.
queue_put (data);
ev_async_send (EV_DEFAULT_ &mysig);
}
static void
mysig_cb (EV_P_ ev_async *w, int revents)
{
sometype data;
sigset_t block, prev;
sigemptyset (&block);
sigaddset (&block, SIGUSR1);
sigprocmask (SIG_BLOCK, &block, &prev);
while (queue_get (&data))
process (data);
if (sigismember (&prev, SIGUSR1)
sigprocmask (SIG_UNBLOCK, &block, 0);
}
(Note: pthreads in theory requires you to use pthread_setmask
instead of sigprocmask when you use threads, but libev doesn't do it
either...).
The strategy for threads is different, as you cannot (easily) block threads but you can easily preempt them, so to queue safely you need to employ a traditional mutex lock, such as in this pthread example:
static ev_async mysig;
static pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
static void
otherthread (void)
{
// only need to lock the actual queueing operation
pthread_mutex_lock (&mymutex);
queue_put (data);
pthread_mutex_unlock (&mymutex);
ev_async_send (EV_DEFAULT_ &mysig);
}
static void
mysig_cb (EV_P_ ev_async *w, int revents)
{
pthread_mutex_lock (&mymutex);
while (queue_get (&data))
process (data);
pthread_mutex_unlock (&mymutex);
}
ev_async_set macro, but using it is utterly pointless,
trust me.
Sends/signals/activates the given ev_async watcher, that is, feeds
an EV_ASYNC event on the watcher into the event loop. Unlike
ev_feed_event, this call is safe to do from other threads, signal or
similar contexts (see the discussion of EV_ATOMIC_T in the embedding
section below on what exactly this means).
Note that, as with other watchers in libev, multiple events might get
compressed into a single callback invocation (another way to look at this
is that ev_async watchers are level-triggered, 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.
Returns a non-zero value when ev_async_send has been called on the
watcher but the event has not yet been processed (or even noted) by the
event loop.
ev_async_send sets a flag in the watcher and wakes up the loop. When
the loop iterates next and checks for the watcher to have become active,
it will reset the flag again. ev_async_pending can be used to very
quickly check whether invoking the loop might be a good idea.
Not that this does not check whether the watcher itself is pending, only whether it has been requested to make this watcher pending: there is a time window between the event loop checking and resetting the async notification, and the callback being invoked.
There are some other functions of possible interest. Described. Here. Now.
This function combines a simple timer and an I/O watcher, calls your callback on whichever event happens first and automatically stops both watchers. This is useful if you want to wait for a single event on an fd or timeout without having to allocate/configure/start/stop/free one or more watchers yourself.
If fd is less than 0, then no I/O watcher will be started and the
events argument is being ignored. Otherwise, an ev_io watcher for
the given fd and events set will be created and started.
If timeout is less than 0, then no timeout watcher will be
started. Otherwise an ev_timer watcher with after = timeout (and
repeat = 0) will be started. 0 is a valid timeout.
The callback has the type void (*cb)(int revents, void *arg) and 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);
loop must be the default
loop!).
Libev offers a compatibility emulation layer for libevent. It cannot emulate the internals of libevent, so here are some usage hints:
Libev comes with some simplistic wrapper classes for C++ that mainly allow you to use some convenience methods to start/stop watchers and also change the callback model to a model using method callbacks on objects.
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:
ev::READ, ev::WRITE etc.
EV_READ etc.
macros from ev.h.
ev::tstamp, ev::now
ev_ prefix.
ev::io, ev::timer, ev::periodic, ev::idle, ev::sig etc.
For each ev_TYPE watcher in ev.h there is a corresponding class of
the same name in the ev namespace, with the exception of ev_signal
which is called ev::sig to avoid clashes with the signal macro
defines by many implementations.
All of those classes have these methods:
The constructor (optionally) takes an event loop to associate the watcher
with. If it is omitted, it will use EV_DEFAULT.
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 sets the callback method to call. The method has to have a
signature of void (*)(ev_TYPE &, int), it receives the watcher as
first argument and the revents as second. The object must be given as
parameter and is stored in the data member of the watcher.
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 an experimental feature that might go away in a future version.
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);
Also sets a callback, but uses a static method or plain function as
callback. The optional data argument will be stored in the watcher's
data member and is free for you to use.
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> ();
struct ev_loop with this watcher. You can only
do this when the watcher is inactive (and not pending either).
ev_TYPE_set, with the same arguments. Must be
called at least once. Unlike the C counterpart, an active watcher gets
automatically stopped and restarted when reconfiguring it with this
method.
loop argument, as the
constructor already stores the event loop.
loop argument.
ev::timer, ev::periodic only)
ev::timer and ev::periodic, this invokes the corresponding
ev_TYPE_again function.
ev::embed only)
ev_embed_sweep.
ev::stat only)
ev_stat_stat.
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);
}
};
Libev does not offer other language bindings itself, but bindings for a number of languages exist in the form of third-party packages. If you know any interesting language binding in addition to the ones listed here, drop me a note.
The EV module implements the full libev API and is actually used to test
libev. EV is developed together with libev. Apart from the EV core module,
there are additional modules that implement libev-compatible interfaces
to libadns (EV::ADNS, but AnyEvent::DNS is preferred nowadays),
Net::SNMP (Net::SNMP::EV) and the libglib event core (Glib::EV
and EV::Glib).
It can be found and installed via CPAN, its homepage is at http://software.schmorp.de/pkg/EV.
Tony Arcieri has written a ruby extension that offers access to a subset of the libev API and adds file handle abstractions, asynchronous DNS and more on top of it. It can be found via gem servers. Its homepage is at http://rev.rubyforge.org/.
Roger Pack reports that using the link order -lws2_32 -lmsvcrt-ruby-190
makes rev work even on mingw.
Libev can be compiled with a variety of options, the most fundamental
of which is EV_MULTIPLICITY. This option determines whether (most)
functions and callbacks have an initial struct ev_loop * argument.
To make it easier to write programs that cope with either variant, the following macros are defined:
EV_A, EV_A_
This provides the loop argument for functions, if one is required ("ev
loop argument"). The EV_A form is used when this is the sole argument,
EV_A_ is used when other arguments are following. Example:
ev_unref (EV_A); ev_timer_add (EV_A_ watcher); ev_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.
EV_P, EV_P_
This provides the loop parameter for functions, if one is required ("ev
loop parameter"). The EV_P form is used when this is the sole parameter,
EV_P_ is used when other parameters are following. Example:
// this is how ev_unref is being declared static void ev_unref (EV_P); // this is how you can declare your typical callback static void cb (EV_P_ ev_timer *w, int revents)
It declares a parameter loop of type struct ev_loop *, quite
suitable for use with EV_A.
EV_DEFAULT, EV_DEFAULT_
EV_DEFAULT_UC, EV_DEFAULT_UC_
Usage identical to EV_DEFAULT and EV_DEFAULT_, but requires that the
default loop has been initialised (UC == unchecked). Their behaviour
is undefined when the default loop has not been initialised by a previous
execution of EV_DEFAULT, EV_DEFAULT_ or ev_default_init (...).
It is often prudent to use EV_DEFAULT when initialising the first
watcher in a function but use EV_DEFAULT_UC afterwards.
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);
Libev can (and often is) directly embedded into host applications. Examples of applications that embed it include the Deliantra Game Server, the EV perl module, the GNU Virtual Private Ethernet (gvpe) and rxvt-unicode.
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).
Depending on what features you need you need to include one or more sets of files in your application.
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.
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
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
Libev can be configured via a variety of preprocessor symbols you have to define before including any of its files. The default in the absence of autoconf is documented for every option.
Must always be 1 if you do not use autoconf configuration, which
keeps libev from including config.h, and it also defines dummy
implementations for some libevent functions (such as logging, which is not
supported). It will also not define any of the structs usually found in
event.h that are not directly supported by the libev core alone.
In stanbdalone mode, libev will still try to automatically deduce the configuration, but has to be more conservative.
1, libev will try to detect the availability of the
monotonic clock option at both compile time and runtime. Otherwise no
use of the monotonic clock option will be attempted. If you enable this,
you usually have to link against librt or something similar. Enabling it
when the functionality isn't available is safe, though, although you have
to make sure you link against any libraries where the clock_gettime
function is hiding in (often -lrt). See also EV_USE_CLOCK_SYSCALL.
1, libev will try to detect the availability of the
real-time clock option at compile time (and assume its availability
at runtime if successful). Otherwise no use of the real-time clock
option will be attempted. This effectively replaces gettimeofday
by clock_get (CLOCK_REALTIME, ...) and will not normally affect
correctness. See the note about libraries in the description of
EV_USE_MONOTONIC, though. Defaults to the opposite value of
EV_USE_CLOCK_SYSCALL.
1, libev will try to use a direct syscall instead
of calling the system-provided clock_gettime function. This option
exists because on GNU/Linux, clock_gettime is in librt, but librt
unconditionally pulls in libpthread, slowing down single-threaded
programs needlessly. Using a direct syscall is slightly slower (in
theory), because no optimised vdso implementation can be used, but avoids
the pthread dependency. Defaults to 1 on GNU/Linux with glibc 2.x or
higher, as it simplifies linking (no need for -lrt).
1, libev will assume that nanosleep () is available
and will use it for delays. Otherwise it will use select ().
1, then libev will assume that eventfd () is
available and will probe for kernel support at runtime. This will improve
ev_signal and ev_async performance and reduce resource consumption.
If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
2.7 or newer, otherwise disabled.
1, libev will compile in support for the
select(2) backend. No attempt at auto-detection will be done: if no
other method takes over, select will be it. Otherwise the select backend
will not be compiled in.
1, then the select backend will use the system fd_set
structure. This is useful if libev doesn't compile due to a missing
NFDBITS or fd_mask definition or it mis-guesses the bitset layout
on exotic systems. This usually limits the range of file descriptors to
some low limit such as 1024 or might have other limitations (winsocket
only allows 64 sockets). The FD_SETSIZE macro, set before compilation,
configures the maximum size of the fd_set.
1, the select backend will assume that
select/socket/connect etc. don't understand file descriptors but
wants osf handles on win32 (this is the case when the select to
be used is the winsock select). This means that it will call
_get_osfhandle on the fd to convert it to an OS handle. Otherwise,
it is assumed that all these functions actually work on fds, even
on win32. Should not be defined on non-win32 platforms.
EV_SELECT_IS_WINSOCKET is enabled, then libev needs a way to map
file descriptors to socket handles. When not defining this symbol (the
default), then libev will call _get_osfhandle, which is usually
correct. In some cases, programs use their own file descriptor management,
in which case they can provide this function to map fds to socket handles.
1, libev will compile in support for the poll(2)
backend. Otherwise it will be enabled on non-win32 platforms. It
takes precedence over select.
1, libev will compile in support for the Linux
epoll(7) backend. Its availability will be detected at runtime,
otherwise another method will be used as fallback. This is the preferred
backend for GNU/Linux systems. If undefined, it will be enabled if the
headers indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
1, libev will compile in support for the BSD style
kqueue(2) backend. Its actual availability will be detected at runtime,
otherwise another method will be used as fallback. This is the preferred
backend for BSD and BSD-like systems, although on most BSDs kqueue only
supports some types of fds correctly (the only platform we found that
supports ptys for example was NetBSD), so kqueue might be compiled in, but
not be used unless explicitly requested. The best way to use it is to find
out whether kqueue supports your type of fd properly and use an embedded
kqueue loop.
1, libev will compile in support for the Solaris
10 port style backend. Its availability will be detected at runtime,
otherwise another method will be used as fallback. This is the preferred
backend for Solaris 10 systems.
1, libev will compile in support for the Linux inotify
interface to speed up ev_stat watchers. Its actual availability will
be detected at runtime. If undefined, it will be enabled if the headers
indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
Libev requires an integer type (suitable for storing 0 or 1) whose
access is atomic with respect to other threads or signal contexts. No such
type is easily found in the C language, so you can provide your own type
that you know is safe for your purposes. It is used both for signal handler "locking"
as well as for signal and thread safety in ev_async watchers.
In the absence of this define, libev will use sig_atomic_t volatile
(from signal.h), which is usually good enough on most platforms.
"ev.h" in event.h, ev.c and ev++.h. This can be
used to virtually rename the ev.h header file in case of conflicts.
EV_STANDALONE isn't 1, this variable can be used to override
ev.c's idea of where to find the config.h file, similarly to
EV_H, above.
EV_H, this macro can be used to override event.c's idea
of how the event.h header can be found, the default is "event.h".
0, then ev.h will not define any function
prototypes, but still define all the structs and other symbols. This is
occasionally useful if you want to provide your own wrapper functions
around libev functions.
1, then all event-loop-specific functions
will have the struct ev_loop * as first argument, and you can create
additional independent event loops. Otherwise there will be no support
for multiple event loops and there is no first event loop pointer
argument. Instead, all functions act on the single default loop.
The range of allowed priorities. EV_MINPRI must be smaller or equal to
EV_MAXPRI, but otherwise there are no non-obvious limitations. You can
provide for more priorities by overriding those symbols (usually defined
to be -2 and 2, respectively).
When doing priority-based operations, libev usually has to linearly search all the priorities, so having many of them (hundreds) uses a lot of space and time, so using the defaults of five priorities (-2 .. +2) is usually fine.
If your embedding application does not need any priorities, defining these
both to 0 will save some memory and CPU.
1, then periodic timers are supported. If
defined to be 0, then they are not. Disabling them saves a few kB of
code.
1, then idle watchers are supported. If
defined to be 0, then they are not. Disabling them saves a few kB of
code.
1, then embed watchers are supported. If
defined to be 0, then they are not. Embed watchers rely on most other
watcher types, which therefore must not be disabled.
1, then stat watchers are supported. If
defined to be 0, then they are not.
1, then fork watchers are supported. If
defined to be 0, then they are not.
1, then async watchers are supported. If
defined to be 0, then they are not.
1. Currently this is used to override some
inlining decisions, saves roughly 30% code size on amd64. It also selects a
much smaller 2-heap for timer management over the default 4-heap.
ev_child watchers use a small hash table to distribute workload by
pid. The default size is 16 (or 1 with EV_MINIMAL), usually more
than enough. If you need to manage thousands of children you might want to
increase this value (must be a power of two).
ev_stat watchers use a small hash table to distribute workload by
inotify watch id. The default size is 16 (or 1 with EV_MINIMAL),
usually more than enough. If you need to manage thousands of ev_stat
watchers you might want to increase this value (must be a power of
two).
Heaps are not very cache-efficient. To improve the cache-efficiency of the
timer and periodics heaps, libev uses a 4-heap when this symbol is defined
to 1. The 4-heap uses more complicated (longer) code but has noticeably
faster performance with many (thousands) of watchers.
The default is 1 unless EV_MINIMAL is set in which case it is 0
(disabled).
Heaps are not very cache-efficient. To improve the cache-efficiency of the
timer and periodics heaps, libev can cache the timestamp (at) within
the heap structure (selected by defining EV_HEAP_CACHE_AT to 1),
which uses 8-12 bytes more per watcher and a few hundred bytes more code,
but avoids random read accesses on heap changes. This improves performance
noticeably with many (hundreds) of watchers.
The default is 1 unless EV_MINIMAL is set in which case it is 0
(disabled).
Controls how much internal verification (see ev_loop_verify ()) will
be done: If set to 0, no internal verification code will be compiled
in. If set to 1, then verification code will be compiled in, but not
called. If set to 2, then the internal verification code will be
called once per loop, which can slow down libev. If set to 3, then the
verification code will be called very frequently, which will slow down
libev considerably.
The default is 1, unless EV_MINIMAL is set, in which case it will be
0.
By default, all watchers have a void *data member. By redefining
this macro to a something else you can include more and other types of
members. You have to define it each time you include one of the files,
though, and it must be identical each time.
For example, the perl EV module uses something like this:
#define EV_COMMON \
SV *self; /* contains this struct */ \
SV *cb_sv, *fh /* note no trailing ";" */
struct ev_loop * as first argument in all cases, or to use
method calls instead of plain function calls in C++.
If you need to re-export the API (e.g. via a DLL) and you need a list of exported symbols, you can use the provided Symbol.* files which list all public symbols, one per line:
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 ...
For a real-world example of a program the includes libev verbatim, you can have a look at the EV perl module (http://software.schmorp.de/pkg/EV.html). It has the libev files in the libev/ subdirectory and includes them in the EV/EVAPI.h (public interface) and EV.xs (implementation) files. Only the EV.xs file will be compiled. It is pretty complex because it provides its own header file.
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:
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.
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.
Depending on your compiler and compiler settings, you might get no or a lot of warnings when compiling libev code. Some people are apparently scared by this.
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.
Valgrind has a special section here because it is a popular tool that is highly useful. Unfortunately, valgrind reports are very hard to interpret.
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.
Win32 doesn't support any of the standards (e.g. POSIX) that libev
requires, and its I/O model is fundamentally incompatible with the POSIX
model. Libev still offers limited functionality on this platform in
the form of the EVBACKEND_SELECT backend, and only supports socket
descriptors. This only applies when using Win32 natively, not when using
e.g. cygwin.
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 winsocket select function doesn't follow POSIX in that it
requires socket handles and not socket file descriptors (it is
also extremely buggy). This makes select very inefficient, and also
requires a mapping from file descriptors to socket handles (the Microsoft
C runtime provides the function _open_osfhandle for this). See the
discussion of the EV_SELECT_USE_FD_SET, EV_SELECT_IS_WINSOCKET and
EV_FD_TO_WIN32_HANDLE preprocessor symbols for more info.
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(n²) range when using win32.
Windows has numerous arbitrary (and low) limits on things.
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(n²)) will likely make this unworkable.
In addition to a working ISO-C implementation and of course the backend-specific APIs, libev relies on a few additional extensions:
void (*)(ev_watcher_type *, int revents) must have compatible
calling conventions regardless of ev_watcher_type *.
ev_watcher * internally.
sig_atomic_t volatile must be thread-atomic as well
sig_atomic_t volatile (or whatever is defined as
EV_ATOMIC_T) must be atomic with respect to accesses from different
threads. This is not part of the specification for sig_atomic_t, but is
believed to be sufficiently portable.
sigprocmask must work in a threaded environment
Libev uses sigprocmask to temporarily block signals. This is not
allowed in a threaded program (pthread_sigmask has to be used). Typical
pthread implementations will either allow sigprocmask in the "main
thread" or will block signals process-wide, both behaviours would
be compatible with libev. Interaction between sigprocmask and
pthread_sigmask could complicate things, however.
The most portable way to handle signals is to block signals in all threads except the initial one, and run the default loop in the initial thread as well.
long must be large enough for common memory allocation sizes
long internally
instead of size_t when allocating its data structures. On non-POSIX
systems (Microsoft...) this might be unexpectedly low, but is still at
least 31 bits everywhere, which is enough for hundreds of millions of
watchers.
double must hold a time value in seconds with enough accuracy
double is used to represent timestamps. It is required to
have at least 51 bits of mantissa (and 9 bits of exponent), which is good
enough for at least into the year 4000. This requirement is fulfilled by
implementations implementing IEEE 754 (basically all existing ones).
If you know of other additional requirements drop me a note.
In this section the complexities of (many of) the algorithms used inside
libev will be documented. For complexity discussions about backends see
the documentation for ev_default_init.
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.
ld 100) of these watchers.
ev_io_set was used).
ev_async_send
calls in the current loop iteration. Checking for async and signal events
involves iterating over all running async watchers or all signal numbers.
A change of state of some external event, such as data now being available for reading on a file descriptor, time having passed or simply not having any other events happening anymore.
In libev, events are represented as single bits (such as EV_READ or
EV_TIMEOUT).
A watcher is pending as soon as the corresponding event has been detected, and stops being pending as soon as the watcher will be invoked or its pending status is explicitly cleared by the application.
A watcher can be pending, but not active. Stopping a watcher also clears its pending status.
Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael Magnusson.