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_cleanup
- even the best things end
ev_async
- how to wake up an 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_run's to stop iterating ev_break (EV_A_ EVBREAK_ALL); } // another callback, this time for a time-out static void timeout_cb (EV_P_ ev_timer *w, int revents) { puts ("timeout"); // this causes the innermost ev_run to stop iterating ev_break (EV_A_ EVBREAK_ONE); } int main (void) { // use the default event loop unless you have special needs struct ev_loop *loop = EV_DEFAULT; // initialise an io watcher, then start it // this one will watch for stdin to become readable ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ); ev_io_start (loop, &stdin_watcher); // initialise a timer watcher, then start it // simple non-repeating 5.5 second timeout ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.); ev_timer_start (loop, &timeout_watcher); // now wait for events to arrive ev_run (loop, 0); // break was called, so exit return 0; }
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.
Familiarity with event based programming techniques in general is assumed throughout this document.
This manual tries to be very detailed, but unfortunately, this also makes
it very long. If you just want to know the basics of libev, I suggest
reading ANATOMY OF A WATCHER, then the EXAMPLE PROGRAM above and
look up the missing functions in GLOBAL FUNCTIONS and the ev_io
and
ev_timer
sections in WATCHER TYPES.
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 aio and epoll
interfaces, the BSD-specific kqueue
and the Solaris-specific event port
mechanisms for file descriptor events (ev_io
), the Linux inotify
interface (for ev_stat
), Linux eventfd/signalfd (for faster and cleaner
inter-thread wakeup (ev_async
)/signal handling (ev_signal
)) 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 struct 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 (in practice
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 (e.g. delays) 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.
Via the EV_FREQUENT
macro you can compile in and/or enable extensive
consistency checking code inside libev that can be used to check for
internal inconsistencies, suually caused by application bugs.
Libev also has a few internal error-checking assert
ions. 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.
Returns the current time as libev would use it. Please note that the
ev_now
function is usually faster and also often returns the timestamp
you actually want to know. Also interesting is the combination of
ev_now_update
and ev_now
.
Sleep for the given interval: The current thread will be blocked
until either it is interrupted or the given time interval has
passed (approximately - it might return a bit earlier even if not
interrupted). Returns immediately if interval <= 0
.
Basically this is a sub-second-resolution sleep ()
.
The range of the interval
is limited - libev only guarantees to work
with sleep times of up to one day (interval <= 86400
).
You can find out the major and minor ABI version numbers of the library
you linked against by calling the functions ev_version_major
and
ev_version_minor
. If you want, you can compare against the global
symbols EV_VERSION_MAJOR
and EV_VERSION_MINOR
, which specify the
version of the library your program was compiled against.
These version numbers refer to the ABI version of the library, not the release version.
Usually, it's a good idea to terminate if the major versions mismatch, as this indicates an incompatible change. Minor versions are usually compatible to older versions, so a larger minor version alone is usually not a problem.
Example: Make sure we haven't accidentally been linked against the wrong version (note, however, that this will not detect other ABI mismatches, such as LFS or reentrancy).
assert (("libev version mismatch", ev_version_major () == EV_VERSION_MAJOR && ev_version_minor () >= EV_VERSION_MINOR));
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));
Return the set of all backends compiled into this binary of libev and
also recommended for this platform, meaning it will work for most file
descriptor types. This set is often smaller than the one returned by
ev_supported_backends
, as for example kqueue is broken on most BSDs
and will not be auto-detected unless you explicitly request it (assuming
you know what you are doing). This is the set of backends that libev will
probe for if you specify no backends explicitly.
Returns the set of backends that are embeddable in other event loops. This
value is platform-specific but can include backends not available on the
current system. To find which embeddable backends might be supported on
the current system, you would need to look at ev_embeddable_backends ()
& ev_supported_backends ()
, likewise for recommended ones.
See the description of ev_embed
watchers for more info.
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: The following is the realloc
function that libev itself uses
which should work with realloc
and free
functions of all kinds and
is probably a good basis for your own implementation.
static void * ev_realloc_emul (void *ptr, long size) EV_NOEXCEPT { if (size) return realloc (ptr, size); free (ptr); return 0; }
Example: Replace the libev allocator with one that waits a bit and then retries.
static void * persistent_realloc (void *ptr, size_t size) { if (!size) { free (ptr); return 0; } 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);
This function can be used to "simulate" a signal receive. It is completely safe to call this function at any time, from any context, including signal handlers or random threads.
Its main use is to customise signal handling in your process, especially
in the presence of threads. For example, you could block signals
by default in all threads (and specifying EVFLAG_NOSIGMASK
when
creating any loops), and in one thread, use sigwait
or any other
mechanism to wait for signals, then "deliver" them to libev by calling
ev_feed_signal
.
An event loop is described by a struct ev_loop *
(the struct
is
not optional in this case unless libev 3 compatibility is disabled, as
libev 3 had an ev_loop
function colliding with the struct name).
The library knows two types of such loops, the default loop, which supports child process events, and dynamically created event loops which do not.
This returns the "default" event loop object, which is what you should
normally use when you just need "the event loop". Event loop objects and
the flags
parameter are described in more detail in the entry for
ev_loop_new
.
If the default loop is already initialised then this function simply
returns it (and ignores the flags. If that is troubling you, check
ev_backend ()
afterwards). Otherwise it will create it with the given
flags, which should almost always be 0
, unless the caller is also the
one calling ev_run
or otherwise qualifies as "the main program".
If you don't know what event loop to use, use the one returned from this
function (or via the EV_DEFAULT
macro).
Note that this function is not thread-safe, so if you want to use it from multiple threads, you have to employ some kind of mutex (note also that this case is unlikely, as loops cannot be shared easily between threads anyway).
The default loop is the only loop that can handle ev_child
watchers,
and to do this, it always registers a handler for SIGCHLD
. If this is
a problem for your application you can either create a dynamic loop with
ev_loop_new
which doesn't do that, or you can simply overwrite the
SIGCHLD
signal handler after calling ev_default_init
.
Example: This is the most typical usage.
if (!ev_default_loop (0)) fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");
Example: Restrict libev to the select and poll backends, and do not allow environment settings to be taken into account:
ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);
This will create and initialise a new event loop object. If the loop could not be initialised, returns false.
This function is thread-safe, and one common way to use libev with threads is indeed to create one loop per thread, and using the default loop in the "main" or "initial" thread.
The flags argument can be used to specify special behaviour or specific
backends to use, and is usually specified as 0
(or EVFLAG_AUTO
).
The following flags are supported:
EVFLAG_AUTO
The default flags value. Use this if you have no clue (it's the right thing, believe me).
EVFLAG_NOENV
If this flag bit is or'ed into the flag value (or the program runs setuid
or setgid) then libev will not look at the environment variable
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, to work
around bugs, or to make libev threadsafe (accessing environment variables
cannot be done in a threadsafe way, but usually it works if no other
thread modifies them).
EVFLAG_FORKCHECK
Instead of calling 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). (Update: glibc
versions 2.25 apparently removed the getpid
optimisation again).
The big advantage of this flag is that you can forget about fork (and
forget about forgetting to tell libev about forking, although you still
have to ignore SIGPIPE
) when you use this flag.
This flag setting cannot be overridden or specified in the LIBEV_FLAGS
environment variable.
EVFLAG_NOINOTIFY
When this flag is specified, then libev will not attempt to use the
inotify API for its ev_stat
watchers. Apart from debugging and
testing, this flag can be useful to conserve inotify file descriptors, as
otherwise each loop using ev_stat
watchers consumes one inotify handle.
EVFLAG_SIGNALFD
When this flag is specified, then libev will attempt to use the
signalfd API for its ev_signal
(and ev_child
) watchers. This API
delivers signals synchronously, which makes it both faster and might make
it possible to get the queued signal data. It can also simplify signal
handling with threads, as long as you properly block signals in your
threads that are not interested in handling them.
Signalfd will not be used by default as this changes your signal mask, and there are a lot of shoddy libraries and programs (glib's threadpool for example) that can't properly initialise their signal masks.
EVFLAG_NOSIGMASK
When this flag is specified, then libev will avoid to modify the signal mask. Specifically, this means you have to make sure signals are unblocked when you want to receive them.
This behaviour is useful when you want to do your own signal handling, or want to handle signals only in specific threads and want to avoid libev unblocking the signals.
It's also required by POSIX in a threaded program, as libev calls
sigprocmask
, whose behaviour is officially unspecified.
EVFLAG_NOTIMERFD
When this flag is specified, the libev will avoid using a timerfd
to
detect time jumps. It will still be able to detect time jumps, but takes
longer and has a lower accuracy in doing so, but saves a file descriptor
per loop.
The current implementation only tries to use a timerfd
when the first
ev_periodic
watcher is started and falls back on other methods if it
cannot be created, but this behaviour might change in the future.
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)Use the Linux-specific epoll(7) interface (for both pre- and post-2.6.9 kernels).
For few fds, this backend is a bit little slower than poll and select, but it scales phenomenally better. While poll and select usually scale like O(total_fds) where total_fds 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, returning before the timeout value, resulting in additional iterations (and only giving 5ms accuracy while select on the same platform gives 0.1ms) 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. Epoll also erroneously rounds down timeouts, but gives you
no way to know when and by how much, so sometimes you have to busy-wait
because epoll returns immediately despite a nonzero timeout. And last
not least, it also refuses to work with some file descriptors which work
perfectly fine with select
(files, many character devices...).
Epoll is truly the train wreck among event poll mechanisms, a frankenpoll, cobbled together in a hurry, no thought to design or interaction with others. Oh, the pain, will it ever stop...
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 a lot of kernel revisions, but probably(!) works in current versions.
This backend maps EV_READ
and EV_WRITE
in the same way as
EVBACKEND_POLL
.
EVBACKEND_LINUXAIO
(value 64, Linux)Use the Linux-specific Linux AIO (not aio(7)
but io_submit(2)
) event interface available in post-4.18 kernels (but libev
only tries to use it in 4.19+).
This is another Linux train wreck of an event interface.
If this backend works for you (as of this writing, it was very experimental), it is the best event interface available on Linux and might be well worth enabling it - if it isn't available in your kernel this will be detected and this backend will be skipped.
This backend can batch oneshot requests and supports a user-space ring buffer to receive events. It also doesn't suffer from most of the design problems of epoll (such as not being able to remove event sources from the epoll set), and generally sounds too good to be true. Because, this being the Linux kernel, of course it suffers from a whole new set of limitations, forcing you to fall back to epoll, inheriting all its design issues.
For one, it is not easily embeddable (but probably could be done using an event fd at some extra overhead). It also is subject to a system wide limit that can be configured in /proc/sys/fs/aio-max-nr. If no AIO requests are left, this backend will be skipped during initialisation, and will switch to epoll when the loop is active.
Most problematic in practice, however, is that not all file descriptors work with it. For example, in Linux 5.1, TCP sockets, pipes, event fds, files, /dev/null and many others are supported, but ttys do not work properly (a known bug that the kernel developers don't care about, see https://lore.kernel.org/patchwork/patch/1047453/), so this is not (yet?) a generic event polling interface.
Overall, it seems the Linux developers just don't want it to have a
generic event handling mechanism other than select
or poll
.
To work around all these problem, the current version of libev uses its epoll backend as a fallback for file descriptor types that do not work. Or falls back completely to epoll if the kernel acts up.
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 this backend was
implemented, 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 be (and mostly have been)
fixed without API changes to existing programs. For this reason it's not
being "auto-detected" on all platforms 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 (you
might have to leak fds on fork, but it's more sane than 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)This is not implemented yet (and might never be, unless you send me an
implementation). According to reports, /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)).
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, 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).
On the negative side, the interface is bizarre - so bizarre that even sun itself gets it wrong in their code examples: The event polling function sometimes returns events to the caller even though an error occurred, but with no indication whether it has done so or not (yes, it's even documented that way) - deadly for edge-triggered interfaces where you absolutely have to know whether an event occurred or not because you have to re-arm the watcher.
Fortunately libev seems to be able to work around these idiocies.
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, use whatever
ev_recommended_backends ()
returns, or simply do not specify a backend
at all.
EVBACKEND_MASK
Not a backend at all, but a mask to select all backend bits from a
flags
value, in case you want to mask out any backends from a flags
value (e.g. when modifying the LIBEV_FLAGS
environment variable).
If one or more of the backend flags are or'ed into the flags value,
then only these backends will be tried (in the reverse order as listed
here). If none are specified, all backends in ev_recommended_backends
()
will be tried.
Example: 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");
Example: Use whatever libev has to offer, but make sure that kqueue is used if available.
struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);
Example: Similarly, on linux, you mgiht want to take advantage of the linux aio backend if possible, but fall back to something else if that isn't available.
struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_LINUXAIO);
Destroys an event loop object (frees all memory and kernel state
etc.). None of the active event watchers will be stopped in the normal
sense, so e.g. ev_is_active
might still return true. It is your
responsibility to either stop all watchers cleanly yourself before
calling this function, or cope with the fact afterwards (which is usually
the easiest thing, you can just ignore the watchers and/or free ()
them
for example).
Note that certain global state, such as signal state (and installed signal handlers), will not be freed by this function, and related watchers (such as signal and child watchers) would need to be stopped manually.
This function is normally used on loop objects allocated by
ev_loop_new
, but it can also be used on the default loop returned by
ev_default_loop
, in which case it is not thread-safe.
Note that it is not advisable to call this function on the default loop
except in the rare occasion where you really need to free its resources.
If you need dynamically allocated loops it is better to use ev_loop_new
and ev_loop_destroy
.
This function sets a flag that causes subsequent ev_run
iterations
to reinitialise the kernel state for backends that have one. Despite
the name, you can call it anytime you are allowed to start or stop
watchers (except inside an ev_prepare
callback), but it makes most
sense after forking, in the child process. You must call it (or use
EVFLAG_FORKCHECK
) in the child before resuming or calling ev_run
.
In addition, if you want to reuse a loop (via this function or
EVFLAG_FORKCHECK
), you also have to ignore SIGPIPE
.
Again, you have to call it on any loop that you want to re-use after a fork, even if you do not plan to use the loop in the parent. This is because some kernel interfaces *cough* kqueue *cough* do funny things during fork.
On the other hand, you only need to call this function in the child
process if and only if you want to use the event loop in the child. If
you just fork+exec or create a new loop in the child, you don't have to
call it at all (in fact, epoll
is so badly broken that it makes a
difference, but libev will usually detect this case on its own and do a
costly reset of the backend).
The function itself is quite fast and it's usually not a problem to call it just in case after a fork.
Example: Automate calling ev_loop_fork
on the default loop when
using pthreads.
static void post_fork_child (void) { ev_loop_fork (EV_DEFAULT); } ... pthread_atfork (0, 0, post_fork_child);
Returns true when the given loop is, in fact, the default loop, and false otherwise.
Returns the current iteration count for the event loop, which is identical
to the number of times libev did poll for new events. It starts at 0
and happily wraps around with enough iterations.
This value can sometimes be useful as a generation counter of sorts (it
"ticks" the number of loop iterations), as it roughly corresponds with
ev_prepare
and ev_check
calls - and is incremented between the
prepare and check phases.
Returns the number of times ev_run
was entered minus the number of
times ev_run
was exited normally, in other words, the recursion depth.
Outside ev_run
, this number is zero. In a callback, this number is
1
, unless ev_run
was invoked recursively (or from another thread),
in which case it is higher.
Leaving ev_run
abnormally (setjmp/longjmp, cancelling the thread,
throwing an exception etc.), doesn't count as "exit" - consider this
as a hint to avoid such ungentleman-like behaviour unless it's really
convenient, in which case it is fully supported.
Returns one of the EVBACKEND_*
flags indicating the event backend in
use.
Returns the current "event loop time", which is the time the event loop received events and started processing them. This timestamp does not change as long as callbacks are being processed, and this is also the base time used for relative timers. You can treat it as the timestamp of the event occurring (or more correctly, libev finding out about it).
Establishes the current time by querying the kernel, updating the time
returned by ev_now ()
in the progress. This is a costly operation and
is usually done automatically within ev_run ()
.
This function is rarely useful, but when some event callback runs for a very long time without entering the event loop, updating libev's idea of the current time is a good idea.
See also The special problem of time updates in the ev_timer
section.
These two functions suspend and resume an event loop, for use when the loop is not used for a while and timeouts should not be processed.
A typical use case would be an interactive program such as a game: When
the user presses ^Z
to suspend the game and resumes it an hour later it
would be best to handle timeouts as if no time had actually passed while
the program was suspended. This can be achieved by calling ev_suspend
in your SIGTSTP
handler, sending yourself a SIGSTOP
and calling
ev_resume
directly afterwards to resume timer processing.
Effectively, all ev_timer
watchers will be delayed by the time spend
between ev_suspend
and ev_resume
, and all ev_periodic
watchers
will be rescheduled (that is, they will lose any events that would have
occurred while suspended).
After calling ev_suspend
you must not call any function on the
given loop other than ev_resume
, and you must not call ev_resume
without a previous call to ev_suspend
.
Calling ev_suspend
/ev_resume
has the side effect of updating the
event loop time (see ev_now_update
).
Finally, this is it, the event handler. This function usually is called after you have initialised all your watchers and you want to start handling events. It will ask the operating system for any new events, call the watcher callbacks, and then repeat the whole process indefinitely: This is why event loops are called loops.
If the flags argument is specified as 0
, it will keep handling events
until either no event watchers are active anymore or ev_break
was
called.
The return value is false if there are no more active watchers (which
usually means "all jobs done" or "deadlock"), and true in all other cases
(which usually means " you should call ev_run
again").
Please note that an explicit ev_break
is usually better than
relying on all watchers to be stopped when deciding when a program has
finished (especially in interactive programs), but having a program
that automatically loops as long as it has to and no longer by virtue
of relying on its watchers stopping correctly, that is truly a thing of
beauty.
This function is mostly exception-safe - you can break out of a
ev_run
call by calling longjmp
in a callback, throwing a C++
exception and so on. This does not decrement the ev_depth
value, nor
will it clear any outstanding EVBREAK_ONE
breaks.
A flags value of EVRUN_NOWAIT
will look for new events, will handle
those events and any already outstanding ones, but will not wait and
block your process in case there are no events and will return after one
iteration of the loop. This is sometimes useful to poll and handle new
events while doing lengthy calculations, to keep the program responsive.
A flags value of EVRUN_ONCE
will look for new events (waiting if
necessary) and will handle those and any already outstanding ones. It
will block your process until at least one new event arrives (which could
be an event internal to libev itself, so there is no guarantee that a
user-registered callback will be called), and will return after one
iteration of the loop.
This is useful if you are waiting for some external event in conjunction
with something not expressible using other libev watchers (i.e. "roll your
own ev_run
"). However, a pair of ev_prepare
/ev_check
watchers is
usually a better approach for this kind of thing.
Here are the gory details of what ev_run
does (this is for your
understanding, not a guarantee that things will work exactly like this in
future versions):
- Increment loop depth. - Reset the ev_break status. - Before the first iteration, call any pending watchers. LOOP: - If EVFLAG_FORKCHECK was used, check for a fork. - If a fork was detected (by any means), queue and call all fork watchers. - Queue and call all prepare watchers. - If ev_break was called, goto FINISH. - If we have been forked, detach and recreate the kernel state as to not disturb the other process. - Update the kernel state with all outstanding changes. - Update the "event loop time" (ev_now ()). - Calculate for how long to sleep or block, if at all (active idle watchers, EVRUN_NOWAIT or not having any active watchers at all will result in not sleeping). - Sleep if the I/O and timer collect interval say so. - Increment loop iteration counter. - Block the process, waiting for any events. - Queue all outstanding I/O (fd) events. - Update the "event loop time" (ev_now ()), and do time jump adjustments. - Queue all expired timers. - Queue all expired periodics. - Queue all idle watchers with priority higher than that of pending events. - Queue all check watchers. - Call all queued watchers in reverse order (i.e. check watchers first). Signals, async and child watchers are implemented as I/O watchers, and will be handled here by queueing them when their watcher gets executed. - If ev_break has been called, or EVRUN_ONCE or EVRUN_NOWAIT were used, or there are no active watchers, goto FINISH, otherwise continue with step LOOP. FINISH: - Reset the ev_break status iff it was EVBREAK_ONE. - Decrement the loop depth. - Return.
Example: Queue some jobs and then loop until no events are outstanding anymore.
... queue jobs here, make sure they register event watchers as long ... as they still have work to do (even an idle watcher will do..) ev_run (my_loop, 0); ... jobs done or somebody called break. yeah!
Can be used to make a call to ev_run
return early (but only after it
has processed all outstanding events). The how
argument must be either
EVBREAK_ONE
, which will make the innermost ev_run
call return, or
EVBREAK_ALL
, which will make all nested ev_run
calls return.
This "break state" will be cleared on the next call to ev_run
.
It is safe to call ev_break
from outside any ev_run
calls, too, in
which case it will have no effect.
Ref/unref can be used to add or remove a reference count on the event
loop: Every watcher keeps one reference, and as long as the reference
count is nonzero, ev_run
will not return on its own.
This is useful when you have a watcher that you never intend to
unregister, but that nevertheless should not keep ev_run
from
returning. In such a case, call ev_unref
after starting, and ev_ref
before stopping it.
As an example, libev itself uses this for its internal signal pipe: It
is not visible to the libev user and should not keep ev_run
from
exiting if no event watchers registered by it are active. It is also an
excellent way to do this for generic recurring timers or from within
third-party libraries. Just remember to unref after start and ref
before stop (but only if the watcher wasn't active before, or was active
before, respectively. Note also that libev might stop watchers itself
(e.g. non-repeating timers) in which case you have to ev_ref
in the callback).
Example: Create a signal watcher, but keep it from keeping ev_run
running when nothing else is active.
ev_signal exitsig; ev_signal_init (&exitsig, sig_cb, SIGINT); ev_signal_start (loop, &exitsig); ev_unref (loop);
Example: For some weird reason, unregister the above signal handler again.
ev_ref (loop); ev_signal_stop (loop, &exitsig);
These advanced functions influence the time that libev will spend waiting
for events. Both time intervals are by default 0
, meaning that libev
will try to invoke timer/periodic callbacks and I/O callbacks with minimum
latency.
Setting these to a higher value (the interval
must be >= 0
)
allows libev to delay invocation of I/O and timer/periodic callbacks
to increase efficiency of loop iterations (or to increase power-saving
opportunities).
The idea is that sometimes your program runs just fast enough to handle
one (or very few) event(s) per loop iteration. While this makes the
program responsive, it also wastes a lot of CPU time to poll for new
events, especially with backends like select ()
which have a high
overhead for the actual polling but can deliver many events at once.
By setting a higher io collect interval you allow libev to spend more
time collecting I/O events, so you can handle more events per iteration,
at the cost of increasing latency. Timeouts (both ev_periodic
and
ev_timer
) will not be affected. Setting this to a non-null value will
introduce an additional ev_sleep ()
call into most loop iterations. The
sleep time ensures that libev will not poll for I/O events more often then
once per this interval, on average (as long as the host time resolution is
good enough).
Likewise, by setting a higher timeout collect interval you allow libev
to spend more time collecting timeouts, at the expense of increased
latency/jitter/inexactness (the watcher callback will be called
later). ev_io
watchers will not be affected. Setting this to a non-null
value will not introduce any overhead in libev.
Many (busy) programs can usually benefit by setting the I/O collect
interval to a value near 0.1
or so, which is often enough for
interactive servers (of course not for games), likewise for timeouts. It
usually doesn't make much sense to set it to a lower value than 0.01
,
as this approaches the timing granularity of most systems. Note that if
you do transactions with the outside world and you can't increase the
parallelity, then this setting will limit your transaction rate (if you
need to poll once per transaction and the I/O collect interval is 0.01,
then you can't do more than 100 transactions per second).
Setting the timeout collect interval can improve the opportunity for
saving power, as the program will "bundle" timer callback invocations that
are "near" in time together, by delaying some, thus reducing the number of
times the process sleeps and wakes up again. Another useful technique to
reduce iterations/wake-ups is to use ev_periodic
watchers and make sure
they fire on, say, one-second boundaries only.
Example: we only need 0.1s timeout granularity, and we wish not to poll more often than 100 times per second:
ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1); ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01);
This call will simply invoke all pending watchers while resetting their
pending state. Normally, ev_run
does this automatically when required,
but when overriding the invoke callback this call comes handy. This
function can be invoked from a watcher - this can be useful for example
when you want to do some lengthy calculation and want to pass further
event handling to another thread (you still have to make sure only one
thread executes within ev_invoke_pending
or ev_run
of course).
Returns the number of pending watchers - zero indicates that no watchers are pending.
This overrides the invoke pending functionality of the loop: Instead of
invoking all pending watchers when there are any, ev_run
will call
this callback instead. This is useful, for example, when you want to
invoke the actual watchers inside another context (another thread etc.).
If you want to reset the callback, use ev_invoke_pending
as new
callback.
Sometimes you want to share the same loop between multiple threads. This can be done relatively simply by putting mutex_lock/unlock calls around each call to a libev function.
However, ev_run
can run an indefinite time, so it is not feasible
to wait for it to return. One way around this is to wake up the event
loop via ev_break
and ev_async_send
, another way is to set these
release and acquire callbacks on the loop.
When set, then release
will be called just before the thread is
suspended waiting for new events, and acquire
is called just
afterwards.
Ideally, release
will just call your mutex_unlock function, and
acquire
will just call the mutex_lock function again.
While event loop modifications are allowed between invocations of
release
and acquire
(that's their only purpose after all), no
modifications done will affect the event loop, i.e. adding watchers will
have no effect on the set of file descriptors being watched, or the time
waited. Use an ev_async
watcher to wake up ev_run
when you want it
to take note of any changes you made.
In theory, threads executing ev_run
will be async-cancel safe between
invocations of release
and acquire
.
See also the locking example in the THREADS
section later in this
document.
Set and retrieve a single void *
associated with a loop. When
ev_set_userdata
has never been called, then ev_userdata
returns
0
.
These two functions can be used to associate arbitrary data with a loop,
and are intended solely for the invoke_pending_cb
, release
and
acquire
callbacks described above, but of course can be (ab-)used for
any other purpose as well.
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 an opaque structure that you allocate and register to record
your interest in some event. To make a concrete example, imagine 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_break (loop, EVBREAK_ALL); } struct ev_loop *loop = ev_default_loop (0); ev_io stdin_watcher; ev_init (&stdin_watcher, my_cb); ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ); ev_io_start (loop, &stdin_watcher); ev_run (loop, 0);
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 is
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 except when explicitly documented
otherwise. 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
The file descriptor in the ev_io
watcher has become readable and/or
writable.
EV_TIMER
The ev_timer
watcher has timed out.
EV_PERIODIC
The ev_periodic
watcher has timed out.
EV_SIGNAL
The signal specified in the ev_signal
watcher has been received by a thread.
EV_CHILD
The pid specified in the ev_child
watcher has received a status change.
EV_STAT
The path specified in the ev_stat
watcher changed its attributes somehow.
EV_IDLE
The ev_idle
watcher has determined that you have nothing better to do.
EV_PREPARE
EV_CHECK
All ev_prepare
watchers are invoked just before ev_run
starts to
gather new events, and all ev_check
watchers are queued (not invoked)
just after ev_run
has gathered them, but before it queues any callbacks
for any received events. That means ev_prepare
watchers are the last
watchers invoked before the event loop sleeps or polls for new events, and
ev_check
watchers will be invoked before any other watchers of the same
or lower priority within an event loop iteration.
Callbacks of both watcher types can start and stop as many watchers as
they want, and all of them will be taken into account (for example, a
ev_prepare
watcher might start an idle watcher to keep ev_run
from
blocking).
EV_EMBED
The embedded event loop specified in the ev_embed
watcher needs attention.
EV_FORK
The event loop has been resumed in the child process after fork (see
ev_fork
).
EV_CLEANUP
The event loop is about to be destroyed (see ev_cleanup
).
EV_ASYNC
The given async watcher has been asynchronously notified (see ev_async
).
EV_CUSTOM
Not ever sent (or otherwise used) by libev itself, but can be freely used
by libev users to signal watchers (e.g. via ev_feed_event
).
EV_ERROR
An unspecified error has occurred, the watcher has been stopped. This might happen because the watcher could not be properly started because libev ran out of memory, a file descriptor was found to be closed or any other problem. Libev considers these application bugs.
You best act on it by reporting the problem and somehow coping with the watcher being stopped. Note that well-written programs should not receive an error ever, so when your watcher receives it, this usually indicates a bug in your program.
Libev will usually signal a few "dummy" events together with an error, for example it might indicate that a fd is readable or writable, and if your callbacks is well-written it can just attempt the operation and cope with the error from read() or write(). This will not work in multi-threaded programs, though, as the fd could already be closed and reused for another thing, so beware.
ev_init
(ev_TYPE *watcher, callback)This macro initialises the generic portion of a watcher. The contents
of the watcher object can be arbitrary (so malloc
will do). Only
the generic parts of the watcher are initialised, you need to call
the type-specific ev_TYPE_set
macro afterwards to initialise the
type-specific parts. For each type there is also a ev_TYPE_init
macro
which rolls both calls into one.
You can reinitialise a watcher at any time as long as it has been stopped (or never started) and there are no pending events outstanding.
The callback is always of type void (*)(struct ev_loop *loop, ev_TYPE *watcher,
int revents)
.
Example: Initialise an ev_io
watcher in two steps.
ev_io w; ev_init (&w, my_cb); ev_io_set (&w, STDIN_FILENO, EV_READ);
ev_TYPE_set
(ev_TYPE *watcher, [args])This macro initialises the type-specific parts of a watcher. You need to
call ev_init
at least once before you call this macro, but you can
call ev_TYPE_set
any number of times. You must not, however, call this
macro on a watcher that is active (it can be pending, however, which is a
difference to the ev_init
macro).
Although some watcher types do not have type-specific arguments
(e.g. ev_prepare
) you still need to call its set
macro.
See ev_init
, above, for an example.
ev_TYPE_init
(ev_TYPE *watcher, callback, [args])This convenience macro rolls both ev_init
and ev_TYPE_set
macro
calls into a single call. This is the most convenient method to initialise
a watcher. The same limitations apply, of course.
Example: Initialise and set an ev_io
watcher in one step.
ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
ev_TYPE_start
(loop, ev_TYPE *watcher)Starts (activates) the given watcher. Only active watchers will receive events. If the watcher is already active nothing will happen.
Example: Start the ev_io
watcher that is being abused as example in this
whole section.
ev_io_start (EV_DEFAULT_UC, &w);
ev_TYPE_stop
(loop, ev_TYPE *watcher)Stops the given watcher if active, and clears the pending status (whether the watcher was active or not).
It is possible that stopped watchers are pending - for example,
non-repeating timers are being stopped when they become pending - but
calling ev_TYPE_stop
ensures that the watcher is neither active nor
pending. If you want to free or reuse the memory used by the watcher it is
therefore a good idea to always call its ev_TYPE_stop
function.
Returns a true value iff the watcher is active (i.e. it has been started and not yet been stopped). As long as a watcher is active you must not modify it unless documented otherwise.
Obviously, it is safe to call this on an active watcher, or actually any watcher that is initialised.
Returns a true value iff the watcher is pending, (i.e. it has outstanding
events but its callback has not yet been invoked). As long as a watcher
is pending (but not active) you must not call an init function on it (but
ev_TYPE_set
is safe), you must not change its priority, and you must
make sure the watcher is available to libev (e.g. you cannot free ()
it).
It is safe to call this on any watcher in any state as long as it is initialised.
Returns the callback currently set on the watcher.
Change the callback. You can change the callback at virtually any time (modulo threads).
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. Reading the priority with ev_priority
is fine in any state.
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.
Invoke the watcher
with the given loop
and revents
. Neither
loop
nor revents
need to be valid as long as the watcher callback
can deal with that fact, as both are simply passed through to the
callback.
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.
Feeds the given event set into the event loop, as if the specified event had happened for the specified watcher (which must be a pointer to an initialised but not necessarily started event watcher, though it can be active). Obviously you must not free the watcher as long as it has pending events.
Stopping the watcher, letting libev invoke it, or calling
ev_clear_pending
will clear the pending event, even if the watcher was
not started in the first place.
See also ev_feed_fd_event
and ev_feed_signal_event
for related
functions that do not need a watcher.
See also the ASSOCIATING CUSTOM DATA WITH A WATCHER and BUILDING YOUR OWN COMPOSITE WATCHERS idioms.
There are various watcher states mentioned throughout this manual - active, pending and so on. In this section these states and the rules to transition between them will be described in more detail - and while these rules might look complicated, they usually do "the right thing".
Before a watcher can be registered with the event loop it has to be
initialised. This can be done with a call to ev_TYPE_init
, or calls to
ev_init
followed by the watcher-specific ev_TYPE_set
function.
In this state it is simply some block of memory that is suitable for
use in an event loop. It can be moved around, freed, reused etc. at
will - as long as you either keep the memory contents intact, or call
ev_TYPE_init
again.
Once a watcher has been started with a call to ev_TYPE_start
it becomes
property of the event loop, and is actively waiting for events. While in
this state it cannot be accessed (except in a few documented ways, such as
stoping it), moved, freed or anything else - the only legal thing is to
keep a pointer to it, and call libev functions on it that are documented
to work on active watchers.
As a rule of thumb, before accessing a member or calling any function on a watcher, it should be stopped (or freshly initialised). If that is not convenient, you can check the documentation for that function or member to see if it is safe to use on an active watcher.
If a watcher is active and libev determines that an event it is interested in has occurred (such as a timer expiring), it will become pending. It will stay in this pending state until either it is explicitly stopped or its callback is about to be invoked, so it is not normally pending inside the watcher callback.
Generally, the watcher might or might not be active while it is pending
(for example, an expired non-repeating timer can be pending but no longer
active). If it is pending but not active, it can be freely accessed (e.g.
by calling ev_TYPE_set
), but it is still property of the event loop at
this time, so cannot be moved, freed or reused. And if it is active the
rules described in the previous item still apply.
Explicitly stopping a watcher will also clear the pending state unconditionally, so it is safe to stop a watcher and then free it.
It is also possible to feed an event on a watcher that is not active (e.g.
via ev_feed_event
), in which case it becomes pending without being
active.
A watcher can be stopped implicitly by libev (in which case it might still
be pending), or explicitly by calling its ev_TYPE_stop
function. The
latter will clear any pending state the watcher might be in, regardless
of whether it was active or not, so stopping a watcher explicitly before
freeing it is often a good idea.
While stopped (and not pending) the watcher is essentially in the
initialised state, that is, it can be reused, moved, modified in any way
you wish (but when you trash the memory block, you need to ev_TYPE_init
it again).
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
continuously 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 handle the actual event. // it will not be executed as long as other watchers // with the default priority are receiving events. ev_idle_start (EV_A_ &idle); } static void idle_cb (EV_P_ ev_idle *w, int revents) { // actual processing read (STDIN_FILENO, ...); // have to start the I/O watcher again, as // we have handled the event ev_io_start (EV_P_ &io); } // initialisation ev_idle_init (&idle, idle_cb); ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ); ev_io_start (EV_DEFAULT_ &io);
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.
Most 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 (within the same thread as the event loop, i.e. without creating data races). Modifying it may not do something sensible or take immediate effect (or do anything at all), but libev will not crash or malfunction in any way.
In any case, the documentation for each member will explain what the effects are, and if there are any additional access restrictions.
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).
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. 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 the case of Xlib, it 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, linuxaio) 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
.
Many people try to use select
(or libev) on file descriptors
representing files, and expect it to become ready when their program
doesn't block on disk accesses (which can take a long time on their own).
However, this cannot ever work in the "expected" way - you get a readiness notification as soon as the kernel knows whether and how much data is there, and in the case of open files, that's always the case, so you always get a readiness notification instantly, and your read (or possibly write) will still block on the disk I/O.
Another way to view it is that in the case of sockets, pipes, character devices and so on, there is another party (the sender) that delivers data on its own, but in the case of files, there is no such thing: the disk will not send data on its own, simply because it doesn't know what you wish to read - you would first have to request some data.
Since files are typically not-so-well supported by advanced notification
mechanism, libev tries hard to emulate POSIX behaviour with respect
to files, even though you should not use it. The reason for this is
convenience: sometimes you want to watch STDIN or STDOUT, which is
usually a tty, often a pipe, but also sometimes files or special devices
(for example, epoll
on Linux works with /dev/random but not with
/dev/urandom), and even though the file might better be served with
asynchronous I/O instead of with non-blocking I/O, it is still useful when
it "just works" instead of freezing.
So avoid file descriptors pointing to files when you know it (e.g. use libeio), but use them when it is convenient, e.g. for STDIN/STDOUT, or when you rarely read from a file instead of from a socket, and want to reuse the same code path.
Some backends (epoll, kqueue, linuxaio, iouring) do not support fork ()
at all or exhibit useless behaviour. Libev fully supports fork, but needs
to be told about it in the child if you want to continue to use it in the
child.
To support fork in your child processes, you have to call 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).
Many implementations of the POSIX accept
function (for example,
found in post-2004 Linux) have the peculiar behaviour of not removing a
connection from the pending queue in all error cases.
For example, larger servers often run out of file descriptors (because
of resource limits), causing accept
to fail with ENFILE
but not
rejecting the connection, leading to libev signalling readiness on
the next iteration again (the connection still exists after all), and
typically causing the program to loop at 100% CPU usage.
Unfortunately, the set of errors that cause this issue differs between operating systems, there is usually little the app can do to remedy the situation, and no known thread-safe method of removing the connection to cope with overload is known (to me).
One of the easiest ways to handle this situation is to just ignore it - when the program encounters an overload, it will just loop until the situation is over. While this is a form of busy waiting, no OS offers an event-based way to handle this situation, so it's the best one can do.
A better way to handle the situation is to log any errors other than
EAGAIN
and EWOULDBLOCK
, making sure not to flood the log with such
messages, and continue as usual, which at least gives the user an idea of
what could be wrong ("raise the ulimit!"). For extra points one could stop
the ev_io
watcher on the listening fd "for a while", which reduces CPU
usage.
If your program is single-threaded, then you could also keep a dummy file
descriptor for overload situations (e.g. by opening /dev/null), and
when you run into ENFILE
or EMFILE
, close it, run accept
,
close that fd, and create a new dummy fd. This will gracefully refuse
clients under typical overload conditions.
The last way to handle it is to simply log the error and exit
, as
is often done with malloc
failures, but this results in an easy
opportunity for a DoS attack.
Configures an ev_io
watcher. The fd
is the file descriptor to
receive events for and events
is either EV_READ
, EV_WRITE
, both
EV_READ | EV_WRITE
or 0
, to express the desire to receive the given
events.
Note that setting the events
to 0
and starting the watcher is
supported, but not specially optimized - if your program sometimes happens
to generate this combination this is fine, but if it is easy to avoid
starting an io watcher watching for no events you should do so.
Similar to ev_io_set
, but only changes the requested events. Using this
might be faster with some backends, as libev can assume that the fd
still refers to the same underlying file description, something it cannot
do when using ev_io_set
.
The file descriptor being watched. While it can be read at any time, you
must not modify this member even when the watcher is stopped - always use
ev_io_set
for that.
The set of events the fd is being watched for, among other flags. Remember
that this is a bit set - to test for EV_READ
, use w->events &
EV_READ
, and similarly for EV_WRITE
.
As with fd
, you must not modify this member even when the watcher is
stopped, always use ev_io_set
or ev_io_modify
for that.
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_run (loop, 0);
ev_timer
- relative and optionally repeating timeoutsTimer 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, see "the special problem of being too
early", below). If multiple timers become ready during the same loop
iteration then the ones with earlier time-out values are invoked before
ones of the same priority with later time-out values (but this is no
longer true when a callback calls ev_run
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_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 timeout = 60.; ev_tstamp last_activity; // time of last activity ev_timer timer; static void callback (EV_P_ ev_timer *w, int revents) { // calculate when the timeout would happen ev_tstamp after = last_activity - ev_now (EV_A) + timeout; // if negative, it means we the timeout already occurred if (after < 0.) { // timeout occurred, take action } else { // callback was invoked, but there was some recent // activity. simply restart the timer to time out // after "after" seconds, which is the earliest time // the timeout can occur. ev_timer_set (w, after, 0.); ev_timer_start (EV_A_ w); } }
To summarise the callback: first calculate in how many seconds the
timeout will occur (by calculating the absolute time when it would occur,
last_activity + timeout
, and subtracting the current time, ev_now
(EV_A)
from that).
If this value is negative, then we are already past the timeout, i.e. we timed out, and need to do whatever is needed in this case.
Otherwise, we now the earliest time at which the timeout would trigger, and simply start the timer with this timeout value.
In other words, each time the callback is invoked it will check whether the timeout occurred. If not, it will simply reschedule itself to check again at the earliest time it could time out. Rinse. Repeat.
This scheme causes more callback invocations (about one every 60 seconds minus half the average time between activity), but virtually no calls to libev to change the timeout.
To start the machinery, simply initialise the watcher and set
last_activity
to the current time (meaning there was some activity just
now), then call the callback, which will "do the right thing" and start
the timer:
last_activity = ev_now (EV_A); ev_init (&timer, callback); callback (EV_A_ &timer, 0);
When there is some activity, simply store the current time in
last_activity
, no libev calls at all:
if (activity detected) last_activity = ev_now (EV_A);
When your timeout value changes, then the timeout can be changed by simply providing a new value, stopping the timer and calling the callback, which will again do the right thing (for example, time out immediately :).
timeout = new_value; ev_timer_stop (EV_A_ &timer); callback (EV_A_ &timer, 0);
This technique is slightly more complex, but in most cases where the time-out is unlikely to be triggered, much more efficient.
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 :)
If you ask a timer to call your callback after three seconds, then you expect it to be invoked after three seconds - but of course, this cannot be guaranteed to infinite precision. Less obviously, it cannot be guaranteed to any precision by libev - imagine somebody suspending the process with a STOP signal for a few hours for example.
So, libev tries to invoke your callback as soon as possible after the delay has occurred, but cannot guarantee this.
A less obvious failure mode is calling your callback too early: many event loops compare timestamps with a "elapsed delay >= requested delay", but this can cause your callback to be invoked much earlier than you would expect.
To see why, imagine a system with a clock that only offers full second resolution (think windows if you can't come up with a broken enough OS yourself). If you schedule a one-second timer at the time 500.9, then the event loop will schedule your timeout to elapse at a system time of 500 (500.9 truncated to the resolution) + 1, or 501.
If an event library looks at the timeout 0.1s later, it will see "501 >= 501" and invoke the callback 0.1s after it was started, even though a one-second delay was requested - this is being "too early", despite best intentions.
This is the reason why libev will never invoke the callback if the elapsed delay equals the requested delay, but only when the elapsed delay is larger than the requested delay. In the example above, libev would only invoke the callback at system time 502, or 1.1s after the timer was started.
So, while libev cannot guarantee that your callback will be invoked exactly when requested, it can and does guarantee that the requested delay has actually elapsed, or in other words, it always errs on the "too late" side of things.
Establishing the current time is a costly operation (it usually takes
at least one system call): EV therefore updates its idea of the current
time only before and after ev_run
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 the following to adjust
for it:
ev_timer_set (&timer, after + (ev_time () - ev_now ()), 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
()
, although that will push the event time of all outstanding events
further into the future.
Modern systems have a variety of clocks - libev itself uses the normal "wall clock" clock and, if available, the monotonic clock (to avoid time jumps).
Neither of these clocks is synchronised with each other or any other clock
on the system, so ev_time ()
might return a considerably different time
than gettimeofday ()
or time ()
. On a GNU/Linux system, for example,
a call to gettimeofday
might return a second count that is one higher
than a directly following call to time
.
The moral of this is to only compare libev-related timestamps with
ev_time ()
and ev_now ()
, at least if you want better precision than
a second or so.
One more problem arises due to this lack of synchronisation: if libev uses
the system monotonic clock and you compare timestamps from ev_time
or ev_now
from when you started your timer and when your callback is
invoked, you will find that sometimes the callback is a bit "early".
This is because ev_timer
s work in real time, not wall clock time, so
libev makes sure your callback is not invoked before the delay happened,
measured according to the real time, not the system clock.
If your timeouts are based on a physical timescale (e.g. "time out this connection after 100 seconds") then this shouldn't bother you as it is exactly the right behaviour.
If you want to compare wall clock/system timestamps to your timers, then
you need to use ev_periodic
s, as these are based on the wall clock
time, where your comparisons will always generate correct results.
When you leave the server world it is quite customary to hit machines that can suspend/hibernate - what happens to the clocks during such a suspend?
Some quick tests made with a Linux 2.6.28 indicate that a suspend freezes
all processes, while the clocks (times
, CLOCK_MONOTONIC
) continue
to run until the system is suspended, but they will not advance while the
system is suspended. That means, on resume, it will be as if the program
was frozen for a few seconds, but the suspend time will not be counted
towards ev_timer
when a monotonic clock source is used. The real time
clock advanced as expected, but if it is used as sole clocksource, then a
long suspend would be detected as a time jump by libev, and timers would
be adjusted accordingly.
I would not be surprised to see different behaviour in different between operating systems, OS versions or even different hardware.
The other form of suspend (job control, or sending a SIGSTOP) will see a
time jump in the monotonic clocks and the realtime clock. If the program
is suspended for a very long time, and monotonic clock sources are in use,
then you can expect ev_timer
s to expire as the full suspension time
will be counted towards the timers. When no monotonic clock source is in
use, then libev will again assume a timejump and adjust accordingly.
It might be beneficial for this latter case to call ev_suspend
and ev_resume
in code that handles SIGTSTP
, to at least get
deterministic behaviour in this case (you can do nothing against
SIGSTOP
).
Configure the timer to trigger after after
seconds (fractional and
negative values are supported). 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 restarts it again if it is
repeating. It basically works like calling ev_timer_stop
, updating the
timeout to the repeat
value and calling ev_timer_start
.
The exact semantics are as in the following rules, all of which will be applied to the watcher:
repeat
value the new timeout
and start the timer, if necessary.This sounds a bit complicated, see Be smart about timeouts, above, for a usage example.
Returns the remaining time until a timer fires. If the timer is active, then this time is relative to the current event loop time, otherwise it's the timeout value currently configured.
That is, after an ev_timer_set (w, 5, 7)
, ev_timer_remaining
returns
5
. When the timer is started and one second passes, ev_timer_remaining
will return 4
. When the timer expires and is restarted, it will return
roughly 7
(likely slightly less as callback invocation takes some time,
too), and so on.
The current repeat
value. Will be used each time the watcher times out
or ev_timer_again
is called, and determines the next timeout (if any),
which is also when any modifications are taken into account.
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_run (loop, 0); // and in some piece of code that gets executed on any "activity": // reset the timeout to start ticking again at 10 seconds ev_timer_again (&mytimer);
ev_periodic
- to cron or not to cron?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 calendar 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 easily 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_run
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:
In this configuration the watcher triggers an event after the wall clock
time offset
has passed. It will not repeat and will not adjust when a
time jump occurs, that is, if it is to be run at January 1st 2011 then it
will be stopped and invoked when the system clock reaches or surpasses
this point in time.
In this mode the watcher will always be scheduled to time out at the next
offset + N * interval
time (for some integer N, which can also be
negative) and then repeat, regardless of any time jumps. The offset
argument is merely an offset into the interval
periods.
This can be used to create timers that do not drift with respect to the
system clock, for example, here is an ev_periodic
that triggers each
hour, on the hour (with respect to UTC):
ev_periodic_set (&periodic, 0., 3600., 0);
This doesn't mean there will always be 3600 seconds in between triggers, but only that the callback will be called when the system time shows a full hour (UTC), or more correctly, when the system time is evenly divisible by 3600.
Another way to think about it (for the mathematically inclined) is that
ev_periodic
will try to run the callback in this mode at the next possible
time where time = offset (mod interval)
, regardless of any time jumps.
The interval
MUST be positive, and for numerical stability, the
interval value should be higher than 1/8192
(which is around 100
microseconds) and offset
should be higher than 0
and should have
at most a similar magnitude as the current time (say, within a factor of
ten). Typical values for offset are, in fact, 0
or something between
0
and interval
, which is also the recommended range.
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. Here is a (completely untested, no error checking) example on how to
do this:
#include <time.h> static ev_tstamp my_rescheduler (ev_periodic *w, ev_tstamp now) { time_t tnow = (time_t)now; struct tm tm; localtime_r (&tnow, &tm); tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day ++tm.tm_mday; // midnight next day return mktime (&tm); }
Note: this code might run into trouble on days that have more then two midnights (beginning and end).
Simply stops and restarts the periodic watcher again. This is only useful when you changed some parameters or the reschedule callback would return a different time than the last time it was called (e.g. in a crond like program when the crontabs have changed).
When active, returns the absolute time that the watcher is supposed
to trigger next. This is not the same as the offset
argument to
ev_periodic_set
, but indeed works even in interval and manual
rescheduling modes.
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.
The current interval value. Can be modified any time, but changes only
take effect when the periodic timer fires or ev_periodic_again
is being
called.
The current reschedule callback, or 0
, if this functionality is
switched off. Can be changed any time, but changes only take effect when
the periodic timer fires or ev_periodic_again
is being called.
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_periodic *w, int revents) { ... its now a full hour (UTC, or TAI or whatever your clock follows) } ev_periodic hourly_tick; ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0); ev_periodic_start (loop, &hourly_tick);
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 its best to deliver signals synchronously, i.e. as part of the normal event processing, like any other event.
If you want signals to be delivered truly asynchronously, just use
sigaction
as you would do without libev and forget about sharing
the signal. You can even use ev_async
from a signal handler to
synchronously wake up an event loop.
You can configure as many watchers as you like for the same signal, but
only within the same loop, i.e. you can watch for SIGINT
in your
default loop and for SIGIO
in another loop, but you cannot watch for
SIGINT
in both the default loop and another loop at the same time. At
the moment, SIGCHLD
is permanently tied to the default loop.
Only after the first watcher for a signal is started will libev actually register something with the kernel. It thus coexists with your own signal handlers as long as you don't register any with libev for the same signal.
If possible and supported, libev will install its handlers with
SA_RESTART
(or equivalent) behaviour enabled, so system calls should
not be unduly interrupted. If you have a problem with system calls getting
interrupted by signals you can block all signals in an ev_check
watcher
and unblock them in an ev_prepare
watcher.
Both the signal mask (sigprocmask
) and the signal disposition
(sigaction
) are unspecified after starting a signal watcher (and after
stopping it again), that is, libev might or might not block the signal,
and might or might not set or restore the installed signal handler (but
see EVFLAG_NOSIGMASK
).
While this does not matter for the signal disposition (libev never
sets signals to SIG_IGN
, so handlers will be reset to SIG_DFL
on
execve
), this matters for the signal mask: many programs do not expect
certain signals to be blocked.
This means that before calling exec
(from the child) you should reset
the signal mask to whatever "default" you expect (all clear is a good
choice usually).
The simplest way to ensure that the signal mask is reset in the child is
to install a fork handler with pthread_atfork
that resets it. That will
catch fork calls done by libraries (such as the libc) as well.
In current versions of libev, the signal will not be blocked indefinitely
unless you use the signalfd
API (EV_SIGNALFD
). While this reduces
the window of opportunity for problems, it will not go away, as libev
has to modify the signal mask, at least temporarily.
So I can't stress this enough: If you do not reset your signal mask when you expect it to be empty, you have a race condition in your code. This is not a libev-specific thing, this is true for most event libraries.
POSIX threads has problematic signal handling semantics, specifically, a lot of functionality (sigfd, sigwait etc.) only really works if all threads in a process block signals, which is hard to achieve.
When you want to use sigwait (or mix libev signal handling with your own
for the same signals), you can tackle this problem by globally blocking
all signals before creating any threads (or creating them with a fully set
sigprocmask) and also specifying the EVFLAG_NOSIGMASK
when creating
loops. Then designate one thread as "signal receiver thread" which handles
these signals. You can pass on any signals that libev might be interested
in by calling ev_feed_signal
.
Configures the watcher to trigger on the given signal number (usually one
of the SIGxxx
constants).
The signal the watcher watches out for.
Example: Try to exit cleanly on SIGINT.
static void sigint_cb (struct ev_loop *loop, ev_signal *w, int revents) { ev_break (loop, EVBREAK_ALL); } ev_signal signal_watcher; ev_signal_init (&signal_watcher, sigint_cb, SIGINT); ev_signal_start (loop, &signal_watcher);
ev_child
- watch out for process status changesChild 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 or in the next callback invocation 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.
Due to some design glitches inside libev, child watchers will always be
handled at maximum priority (their priority is set to EV_MAXPRI
by
libev)
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 (calling ev_child_stop
twice is not a
problem).
Configures the watcher to wait for status changes of process pid
(or
any process if pid
is specified as 0
). The callback can look
at the rstatus
member of the ev_child
watcher structure to see
the status word (use the macros from sys/wait.h
and see your systems
waitpid
documentation). The rpid
member contains the pid of the
process causing the status change. trace
must be either 0
(only
activate the watcher when the process terminates) or 1
(additionally
activate the watcher when the process is stopped or continued).
The process id this watcher watches out for, or 0
, meaning any process id.
The process id that detected a status change.
The process exit/trace status caused by 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. Starting the watcher stat
's the file, so only changes that
happen after the watcher has been started will be reported.
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 operationLibev 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).
Updates the stat buffer immediately with new values. If you change the watched path in your callback, you could call this function to avoid detecting this change (while introducing a race condition if you are not the only one changing the path). Can also be useful simply to find out the new values.
The most-recently detected attributes of the file. Although the type is
ev_statdata
, this is usually the (or one of the) struct stat
types
suitable for your system, but you can only rely on the POSIX-standardised
members to be present. If the st_nlink
member is 0
, then there was
some error while stat
ing the file.
The previous attributes of the file. The callback gets invoked whenever
prev
!= attr
, or, more precisely, one or more of these members
differ: st_dev
, st_ino
, st_mode
, st_nlink
, st_uid
,
st_gid
, st_rdev
, st_size
, st_atime
, st_mtime
, st_ctime
.
The specified interval.
The file system path that is being watched.
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
watcher for its side-effectAs long as there is at least one active idle watcher, libev will never sleep unnecessarily. Or in other words, it will loop as fast as possible. For this to work, the idle watcher doesn't need to be invoked at all - the lowest priority will do.
This mode of operation can be useful together with an ev_check
watcher,
to do something on each event loop iteration - for example to balance load
between different connections.
See Abusing an ev_check watcher for its side-effect for a longer example.
Initialises and configures the idle watcher - it has no parameters of any
kind. There is a ev_idle_set
macro, but using it is utterly pointless,
believe me.
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) { // stop the watcher ev_idle_stop (loop, w); // now we can free it free (w); // now do something you wanted to do when the program has // no longer anything immediate to do. } ev_idle *idle_watcher = malloc (sizeof (ev_idle)); ev_idle_init (idle_watcher, idle_cb); ev_idle_start (loop, idle_watcher);
ev_prepare
and ev_check
- customise your event loop!Prepare and check watchers are often (but not always) used in pairs: prepare watchers get invoked before the process blocks and check watchers afterwards.
You must not call ev_run
(or similar functions that enter the
current event loop) or ev_loop_fork
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).
When used for this purpose, 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_check
watcher for its side-effectev_check
(and less often also ev_prepare
) watchers can also be
useful because they are called once per event loop iteration. For
example, if you want to handle a large number of connections fairly, you
normally only do a bit of work for each active connection, and if there
is more work to do, you wait for the next event loop iteration, so other
connections have a chance of making progress.
Using an ev_check
watcher is almost enough: it will be called on the
next event loop iteration. However, that isn't as soon as possible -
without external events, your ev_check
watcher will not be invoked.
This is where ev_idle
watchers come in handy - all you need is a
single global idle watcher that is active as long as you have one active
ev_check
watcher. The ev_idle
watcher makes sure the event loop
will not sleep, and the ev_check
watcher makes sure a callback gets
invoked. Neither watcher alone can do that.
Initialises and configures the prepare or check watcher - they have no
parameters of any kind. There are ev_prepare_set
and ev_check_set
macros, but using them is utterly, utterly, utterly and completely
pointless.
There are a number of principal ways to embed other event loops or modules
into libev. Here are some ideas on how to include libadns into libev
(there is a Perl module named EV::ADNS
that does this, which you could
use as a working example. Another Perl module named EV::Glib
embeds a
Glib main context into libev, and finally, Glib::EV
embeds EV into the
Glib event loop).
Method 1: Add IO watchers and a timeout watcher in a prepare handler,
and in a check watcher, destroy them and call into libadns. What follows
is pseudo-code only of course. This requires you to either use a low
priority for the check watcher or use ev_clear_pending
explicitly, as
the callbacks for the IO/timeout watchers might not have been called yet.
static ev_io iow [nfd]; static ev_timer tw; static void io_cb (struct ev_loop *loop, ev_io *w, int revents) { } // create io watchers for each fd and a timer before blocking static void adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents) { int timeout = 3600000; struct pollfd fds [nfd]; // actual code will need to loop here and realloc etc. adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ())); /* the callback is illegal, but won't be called as we stop during check */ ev_timer_init (&tw, 0, timeout * 1e-3, 0.); ev_timer_start (loop, &tw); // create one ev_io per pollfd for (int i = 0; i < nfd; ++i) { ev_io_init (iow + i, io_cb, fds [i].fd, ((fds [i].events & POLLIN ? EV_READ : 0) | (fds [i].events & POLLOUT ? EV_WRITE : 0))); fds [i].revents = 0; ev_io_start (loop, iow + i); } } // stop all watchers after blocking static void adns_check_cb (struct ev_loop *loop, ev_check *w, int revents) { ev_timer_stop (loop, &tw); for (int i = 0; i < nfd; ++i) { // set the relevant poll flags // could also call adns_processreadable etc. here struct pollfd *fd = fds + i; int revents = ev_clear_pending (iow + i); if (revents & EV_READ ) fd->revents |= fd->events & POLLIN; if (revents & EV_WRITE) fd->revents |= fd->events & POLLOUT; // now stop the watcher ev_io_stop (loop, iow + i); } adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop)); }
Method 2: This would be just like method 1, but you run adns_afterpoll
in the prepare watcher and would dispose of the check watcher.
Method 3: If the module to be embedded supports explicit event notification (libadns does), you can also make use of the actual watcher callbacks, and only destroy/create the watchers in the prepare watcher.
static void timer_cb (EV_P_ ev_timer *w, int revents) { adns_state ads = (adns_state)w->data; update_now (EV_A); adns_processtimeouts (ads, &tv_now); } static void io_cb (EV_P_ ev_io *w, int revents) { adns_state ads = (adns_state)w->data; update_now (EV_A); if (revents & EV_READ ) adns_processreadable (ads, w->fd, &tv_now); if (revents & EV_WRITE) adns_processwriteable (ads, w->fd, &tv_now); } // do not ever call adns_afterpoll
Method 4: Do not use a prepare or check watcher because the module you
want to embed is not flexible enough to support it. Instead, you can
override their poll function. The drawback with this solution is that the
main loop is now no longer controllable by EV. The Glib::EV
module uses
this approach, effectively embedding EV as a client into the horrible
libglib event loop.
static gint event_poll_func (GPollFD *fds, guint nfds, gint timeout) { int got_events = 0; for (n = 0; n < nfds; ++n) // create/start io watcher that sets the relevant bits in fds[n] and increment got_events if (timeout >= 0) // create/start timer // poll ev_run (EV_A_ 0); // stop timer again if (timeout >= 0) ev_timer_stop (EV_A_ &to); // stop io watchers again - their callbacks should have set for (n = 0; n < nfds; ++n) ev_io_stop (EV_A_ iow [n]); return got_events; }
ev_embed
- when one backend isn't enough...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 forkWhile 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.
Configures the watcher to embed the given loop, which must be
embeddable. If the callback is 0
, then ev_embed_sweep
will be
invoked automatically, otherwise it is the responsibility of the callback
to invoke it (it will continue to be called until the sweep has been done,
if you do not want that, you need to temporarily stop the embed watcher).
Make a single, non-blocking sweep over the embedded loop. This works
similarly to ev_run (embedded_loop, EVRUN_NOWAIT)
, but in the most
appropriate way for embedded loops.
The embedded event loop.
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 forkFork watchers are called when a fork ()
was detected (usually because
whoever is a good citizen cared to tell libev about it by calling
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 set
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_loop_destroy (EV_DEFAULT)
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.
Initialises and configures the fork watcher - it has no parameters of any
kind. There is a ev_fork_set
macro, but using it is utterly pointless,
really.
ev_cleanup
- even the best things endCleanup watchers are called just before the event loop is being destroyed
by a call to ev_loop_destroy
.
While there is no guarantee that the event loop gets destroyed, cleanup watchers provide a convenient method to install cleanup hooks for your program, worker threads and so on - you just to make sure to destroy the loop when you want them to be invoked.
Cleanup watchers are invoked in the same way as any other watcher. Unlike
all other watchers, they do not keep a reference to the event loop (which
makes a lot of sense if you think about it). Like all other watchers, you
can call libev functions in the callback, except ev_cleanup_start
.
Initialises and configures the cleanup watcher - it has no parameters of
any kind. There is a ev_cleanup_set
macro, but using it is utterly
pointless, I assure you.
Example: Register an atexit handler to destroy the default loop, so any cleanup functions are called.
static void program_exits (void) { ev_loop_destroy (EV_DEFAULT_UC); } ... atexit (program_exits);
ev_async
- how to wake up an event loopIn 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 an 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_send
calls). In fact, you could use signal watchers as a kind
of "global async watchers" by using a watcher on an otherwise unused
signal, and ev_feed_signal
to signal this watcher from another thread,
even without knowing which loop owns the signal.
ev_async
does not support queueing of data in any way. The reason
is that the author does not know of a simple (or any) algorithm for a
multiple-writer-single-reader queue that works in all cases and doesn't
need elaborate support such as pthreads or unportable memory access
semantics.
That means that if you want to queue data, you have to provide your own queue. But at least I can tell you how to implement locking around your queue:
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); }
Initialises and configures the async watcher - it has no parameters of any
kind. There is a ev_async_set
macro, but using it is utterly pointless,
trust me.
Sends/signals/activates the given ev_async
watcher, that is, feeds
an EV_ASYNC
event on the watcher into the event loop, and instantly
returns.
Unlike ev_feed_event
, this call is safe to do from other threads,
signal or similar contexts (see the discussion of EV_ATOMIC_T
in the
embedding section below on what exactly this means).
Note that, as with other watchers in libev, multiple events might get
compressed into a single callback invocation (another way to look at
this is that ev_async
watchers are level-triggered: they are set on
ev_async_send
, reset when the event loop detects that).
This call incurs the overhead of at most one extra system call per event loop iteration, if the event loop is blocked, and no syscall at all if the event loop (or your program) is processing events. That means that repeated calls are basically free (there is no need to avoid calls for performance reasons) and that the overhead becomes smaller (typically zero) under load.
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 is
passed an revents
set like normal event callbacks (a combination of
EV_ERROR
, EV_READ
, EV_WRITE
or EV_TIMER
) and the arg
value passed to ev_once
. Note that it is possible to receive both
a timeout and an io event at the same time - you probably should give io
events precedence.
Example: wait up to ten seconds for data to appear on STDIN_FILENO.
static void stdin_ready (int revents, void *arg) { if (revents & EV_READ) /* stdin might have data for us, joy! */; else if (revents & EV_TIMER) /* doh, nothing entered */; } ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
Feed an event on the given fd, as if a file descriptor backend detected the given events.
Feed an event as if the given signal occurred. See also ev_feed_signal
,
which is async-safe.
This section explains some common idioms that are not immediately obvious. Note that examples are sprinkled over the whole manual, and this section only contains stuff that wouldn't fit anywhere else.
Each watcher has, by default, a void *data
member that you can read
or modify 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 separately 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 function type instead have been omitted.
Another common scenario is to use some data structure with multiple embedded watchers, in effect creating your own watcher that combines multiple libev event sources into one "super-watcher":
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 C++ coders), 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)); }
Often you have structures like this in event-based programs:
callback () { free (request); } request = start_new_request (..., callback);
The intent is to start some "lengthy" operation. The request
could be
used to cancel the operation, or do other things with it.
It's not uncommon to have code paths in start_new_request
that
immediately invoke the callback, for example, to report errors. Or you add
some caching layer that finds that it can skip the lengthy aspects of the
operation and simply invoke the callback with the result.
The problem here is that this will happen before start_new_request
has returned, so request
is not set.
Even if you pass the request by some safer means to the callback, you might want to do something to the request after starting it, such as canceling it, which probably isn't working so well when the callback has already been invoked.
A common way around all these issues is to make sure that
start_new_request
always returns before the callback is invoked. If
start_new_request
immediately knows the result, it can artificially
delay invoking the callback by using a prepare
or idle
watcher for
example, or more sneakily, by reusing an existing (stopped) watcher and
pushing it into the pending queue:
ev_set_cb (watcher, callback); ev_feed_event (EV_A_ watcher, 0);
This way, start_new_request
can safely return before the callback is
invoked, while not delaying callback invocation too much.
Often (especially in GUI toolkits) there are places where you have
modal interaction, which is most easily implemented by recursively
invoking ev_run
.
This brings the problem of exiting - a callback might want to finish the
main ev_run
call, but not the nested one (e.g. user clicked "Quit", but
a modal "Are you sure?" dialog is still waiting), or just the nested one
and not the main one (e.g. user clocked "Ok" in a modal dialog), or some
other combination: In these cases, a simple ev_break
will not work.
The solution is to maintain "break this loop" variable for each ev_run
invocation, and use a loop around ev_run
until the condition is
triggered, using EVRUN_ONCE
:
// main loop int exit_main_loop = 0; while (!exit_main_loop) ev_run (EV_DEFAULT_ EVRUN_ONCE); // in a modal watcher int exit_nested_loop = 0; while (!exit_nested_loop) ev_run (EV_A_ EVRUN_ONCE);
To exit from any of these loops, just set the corresponding exit variable:
// exit modal loop exit_nested_loop = 1; // exit main program, after modal loop is finished exit_main_loop = 1; // exit both exit_main_loop = exit_nested_loop = 1;
Here is a fictitious example of how to run an event loop in a different thread from where callbacks are being invoked and watchers are created/added/removed.
For a real-world example, see the EV::Loop::Async
perl module,
which uses exactly this technique (which is suited for many high-level
languages).
The example uses a pthread mutex to protect the loop data, a condition variable to wait for callback invocations, an async watcher to notify the event loop thread and an unspecified mechanism to wake up the main thread.
First, you need to associate some data with the event loop:
typedef struct { pthread_mutex_t lock; /* global loop lock */ pthread_t tid; pthread_cond_t invoke_cv; ev_async async_w; } userdata; void prepare_loop (EV_P) { // for simplicity, we use a static userdata struct. static userdata u; ev_async_init (&u.async_w, async_cb); ev_async_start (EV_A_ &u.async_w); pthread_mutex_init (&u.lock, 0); pthread_cond_init (&u.invoke_cv, 0); // now associate this with the loop ev_set_userdata (EV_A_ &u); ev_set_invoke_pending_cb (EV_A_ l_invoke); ev_set_loop_release_cb (EV_A_ l_release, l_acquire); // then create the thread running ev_run pthread_create (&u.tid, 0, l_run, EV_A); }
The callback for the ev_async
watcher does nothing: the watcher is used
solely to wake up the event loop so it takes notice of any new watchers
that might have been added:
static void async_cb (EV_P_ ev_async *w, int revents) { // just used for the side effects }
The l_release
and l_acquire
callbacks simply unlock/lock the mutex
protecting the loop data, respectively.
static void l_release (EV_P) { userdata *u = ev_userdata (EV_A); pthread_mutex_unlock (&u->lock); } static void l_acquire (EV_P) { userdata *u = ev_userdata (EV_A); pthread_mutex_lock (&u->lock); }
The event loop thread first acquires the mutex, and then jumps straight
into ev_run
:
void * l_run (void *thr_arg) { struct ev_loop *loop = (struct ev_loop *)thr_arg; l_acquire (EV_A); pthread_setcanceltype (PTHREAD_CANCEL_ASYNCHRONOUS, 0); ev_run (EV_A_ 0); l_release (EV_A); return 0; }
Instead of invoking all pending watchers, the l_invoke
callback will
signal the main thread via some unspecified mechanism (signals? pipe
writes? Async::Interrupt
?) and then waits until all pending watchers
have been called (in a while loop because a) spurious wakeups are possible
and b) skipping inter-thread-communication when there are no pending
watchers is very beneficial):
static void l_invoke (EV_P) { userdata *u = ev_userdata (EV_A); while (ev_pending_count (EV_A)) { wake_up_other_thread_in_some_magic_or_not_so_magic_way (); pthread_cond_wait (&u->invoke_cv, &u->lock); } }
Now, whenever the main thread gets told to invoke pending watchers, it
will grab the lock, call ev_invoke_pending
and then signal the loop
thread to continue:
static void real_invoke_pending (EV_P) { userdata *u = ev_userdata (EV_A); pthread_mutex_lock (&u->lock); ev_invoke_pending (EV_A); pthread_cond_signal (&u->invoke_cv); pthread_mutex_unlock (&u->lock); }
Whenever you want to start/stop a watcher or do other modifications to an event loop, you will now have to lock:
ev_timer timeout_watcher; userdata *u = ev_userdata (EV_A); ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.); pthread_mutex_lock (&u->lock); ev_timer_start (EV_A_ &timeout_watcher); ev_async_send (EV_A_ &u->async_w); pthread_mutex_unlock (&u->lock);
Note that sending the ev_async
watcher is required because otherwise
an event loop currently blocking in the kernel will have no knowledge
about the newly added timer. By waking up the loop it will pick up any new
watchers in the next event loop iteration.
While the overhead of a callback that e.g. schedules a thread is small, it is still an overhead. If you embed libev, and your main usage is with some kind of threads or coroutines, you might want to customise libev so that doesn't need callbacks anymore.
Imagine you have coroutines that you can switch to using a function
switch_to (coro)
, that libev runs in a coroutine called libev_coro
and that due to some magic, the currently active coroutine is stored in a
global called current_coro
. Then you can build your own "wait for libev
event" primitive by changing EV_CB_DECLARE
and EV_CB_INVOKE
(note
the differing ;
conventions):
#define EV_CB_DECLARE(type) struct my_coro *cb; #define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb)
That means instead of having a C callback function, you store the coroutine to switch to in each watcher, and instead of having libev call your callback, you instead have it switch to that coroutine.
A coroutine might now wait for an event with a function called
wait_for_event
. (the watcher needs to be started, as always, but it doesn't
matter when, or whether the watcher is active or not when this function is
called):
void wait_for_event (ev_watcher *w) { ev_set_cb (w, current_coro); switch_to (libev_coro); }
That basically suspends the coroutine inside wait_for_event
and
continues the libev coroutine, which, when appropriate, switches back to
this or any other coroutine.
You can do similar tricks if you have, say, threads with an event queue - instead of storing a coroutine, you store the queue object and instead of switching to a coroutine, you push the watcher onto the queue and notify any waiters.
To embed libev, see EMBEDDING, but in short, it's easiest to create two files, my_ev.h and my_ev.c that include the respective libev files:
// my_ev.h #define EV_CB_DECLARE(type) struct my_coro *cb; #define EV_CB_INVOKE(watcher) switch_to ((watcher)->cb) #include "../libev/ev.h" // my_ev.c #define EV_H "my_ev.h" #include "../libev/ev.c"
And then use my_ev.h when you would normally use ev.h, and compile my_ev.c into your project. When properly specifying include paths, you can even use ev.h as header file name directly.
Libev offers a compatibility emulation layer for libevent. It cannot emulate the internals of libevent, so here are some usage hints:
This was the newest libevent version available when libev was implemented, and is still mostly unchanged in 2010.
The normal C API should work fine when used from C++: both ev.h and the libev sources can be compiled as C++. Therefore, code that uses the C API will work fine.
Proper exception specifications might have to be added to callbacks passed
to libev: exceptions may be thrown only from watcher callbacks, all other
callbacks (allocator, syserr, loop acquire/release and periodic reschedule
callbacks) must not throw exceptions, and might need a noexcept
specification. If you have code that needs to be compiled as both C and
C++ you can use the EV_NOEXCEPT
macro for this:
static void fatal_error (const char *msg) EV_NOEXCEPT { perror (msg); abort (); } ... ev_set_syserr_cb (fatal_error);
The only API functions that can currently throw exceptions are ev_run
,
ev_invoke
, ev_invoke_pending
and ev_loop_destroy
(the latter
because it runs cleanup watchers).
Throwing exceptions in watcher callbacks is only supported if libev itself is compiled with a C++ compiler or your C and C++ environments allow throwing exceptions through C libraries (most do).
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, static and non-static member functions and classes
with operator ()
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).
For all this to work, your C++ compiler either has to use the same calling conventions as your C compiler (for static member functions), or you have to embed libev and compile libev itself as C++.
Here is a list of things available in the ev
namespace:
ev::READ
, ev::WRITE
etc.These are just enum values with the same values as the EV_READ
etc.
macros from ev.h.
ev::tstamp
, ev::now
Aliases to the same types/functions as with the ev_
prefix.
ev::io
, ev::timer
, ev::periodic
, ev::idle
, ev::sig
etc.For each ev_TYPE
watcher in ev.h there is a corresponding class of
the same name in the ev
namespace, with the exception of ev_signal
which is called ev::sig
to avoid clashes with the signal
macro
defined by many implementations.
All of those classes have these methods:
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 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> ();
Associates a different struct ev_loop
with this watcher. You can only
do this when the watcher is inactive (and not pending either).
Basically the same as ev_TYPE_set
(except for ev::embed
watchers>),
with the same arguments. Either this method or a suitable start method
must be called at least once. Unlike the C counterpart, an active watcher
gets automatically stopped and restarted when reconfiguring it with this
method.
For ev::embed
watchers this method is called set_embed
, to avoid
clashing with the set (loop)
method.
For ev::io
watchers there is an additional set
method that acepts a
new event mask only, and internally calls ev_io_modify
.
Starts the watcher. Note that there is no loop
argument, as the
constructor already stores the event loop.
Instead of calling set
and start
methods separately, it is often
convenient to wrap them in one call. Uses the same type of arguments as
the configure set
method of the watcher.
Stops the watcher if it is active. Again, no loop
argument.
ev::timer
, ev::periodic
only)For ev::timer
and ev::periodic
, this invokes the corresponding
ev_TYPE_again
function.
ev::embed
only)Invokes ev_embed_sweep
.
ev::stat
only)Invokes ev_stat_stat
.
Example: Define a class with two I/O and idle watchers, start the I/O watchers in the constructor.
class myclass { ev::io io ; void io_cb (ev::io &w, int revents); ev::io io2 ; void io2_cb (ev::io &w, int revents); ev::idle idle; void idle_cb (ev::idle &w, int revents); myclass (int fd) { io .set <myclass, &myclass::io_cb > (this); io2 .set <myclass, &myclass::io2_cb > (this); idle.set <myclass, &myclass::idle_cb> (this); io.set (fd, ev::WRITE); // configure the watcher io.start (); // start it whenever convenient io2.start (fd, ev::READ); // set + start in one call } };
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.
Python bindings can be found at http://code.google.com/p/pyev/. It seems to be quite complete and well-documented.
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.
A haskell binding to libev is available at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hlibev.
Leandro Lucarella has written a D language binding (ev.d) for libev, to be found at http://www.llucax.com.ar/proj/ev.d/index.html.
Erkki Seppala has written Ocaml bindings for libev, to be found at http://modeemi.cs.tut.fi/~flux/software/ocaml-ev/.
Brian Maher has written a partial interface to libev for lua (at the
time of this writing, only ev_io
and ev_timer
), to be found at
http://github.com/brimworks/lua-ev.
Node.js (http://nodejs.org) uses libev as the underlying event library.
There are others, and I stopped counting.
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_run (EV_A_ 0);
It assumes the variable loop
of type struct ev_loop *
is in scope,
which is often provided by the following macro.
EV_P
, EV_P_
This provides the loop parameter for functions, if one is required ("ev
loop parameter"). The EV_P
form is used when this is the sole parameter,
EV_P_
is used when other parameters are following. Example:
// this is how ev_unref is being declared static void ev_unref (EV_P); // this is how you can declare your typical callback static void cb (EV_P_ ev_timer *w, int revents)
It declares a parameter loop
of type struct ev_loop *
, quite
suitable for use with EV_A
.
EV_DEFAULT
, EV_DEFAULT_
Similar to the other two macros, this gives you the value of the default loop, if multiple loops are supported ("ev loop default"). The default loop will be initialised if it isn't already initialised.
For non-multiplicity builds, these macros do nothing, so you always have to initialise the loop somewhere.
EV_DEFAULT_UC
, EV_DEFAULT_UC_
Usage identical to EV_DEFAULT
and EV_DEFAULT_
, but requires that the
default loop has been initialised (UC
== unchecked). Their behaviour
is undefined when the default loop has not been initialised by a previous
execution of EV_DEFAULT
, EV_DEFAULT_
or ev_default_init (...)
.
It is often prudent to use EV_DEFAULT
when initialising the first
watcher in a function but use EV_DEFAULT_UC
afterwards.
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_run (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 ev_poll.c only when poll backend is enabled ev_epoll.c only when the epoll backend is enabled ev_linuxaio.c only when the linux aio backend is enabled ev_iouring.c only when the linux io_uring backend is enabled ev_kqueue.c only when the kqueue backend is enabled ev_port.c only when the solaris port backend is enabled
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 (or compiling) any of its files. The default in the absence of autoconf is documented for every option.
Symbols marked with "(h)" do not change the ABI, and can have different values when compiling libev vs. including ev.h, so it is permissible to redefine them before including ev.h without breaking compatibility to a compiled library. All other symbols change the ABI, which means all users of libev and the libev code itself must be compiled with compatible settings.
Backwards compatibility is a major concern for libev. This is why this release of libev comes with wrappers for the functions and symbols that have been renamed between libev version 3 and 4.
You can disable these wrappers (to test compatibility with future
versions) by defining EV_COMPAT3
to 0
when compiling your
sources. This has the additional advantage that you can drop the struct
from struct ev_loop
declarations, as libev will provide an ev_loop
typedef in that case.
In some future version, the default for EV_COMPAT3
will become 0
,
and in some even more future version the compatibility code will be
removed completely.
Must always be 1
if you do not use autoconf configuration, which
keeps libev from including config.h, and it also defines dummy
implementations for some libevent functions (such as logging, which is not
supported). It will also not define any of the structs usually found in
event.h that are not directly supported by the libev core alone.
In standalone mode, libev will still try to automatically deduce the configuration, but has to be more conservative.
If defined to be 1
, libev will use the floor ()
function for its
periodic reschedule calculations, otherwise libev will fall back on a
portable (slower) implementation. If you enable this, you usually have to
link against libm or something equivalent. Enabling this when the floor
function is not available will fail, so the safe default is to not enable
this.
If defined to be 1
, libev will try to detect the availability of the
monotonic clock option at both compile time and runtime. Otherwise no
use of the monotonic clock option will be attempted. If you enable this,
you usually have to link against librt or something similar. Enabling it
when the functionality isn't available is safe, though, although you have
to make sure you link against any libraries where the clock_gettime
function is hiding in (often -lrt). See also EV_USE_CLOCK_SYSCALL
.
If defined to be 1
, libev will try to detect the availability of the
real-time clock option at compile time (and assume its availability
at runtime if successful). Otherwise no use of the real-time clock
option will be attempted. This effectively replaces gettimeofday
by clock_get (CLOCK_REALTIME, ...)
and will not normally affect
correctness. See the note about libraries in the description of
EV_USE_MONOTONIC
, though. Defaults to the opposite value of
EV_USE_CLOCK_SYSCALL
.
If defined to be 1
, libev will try to use a direct syscall instead
of calling the system-provided clock_gettime
function. This option
exists because on GNU/Linux, clock_gettime
is in librt
, but librt
unconditionally pulls in libpthread
, slowing down single-threaded
programs needlessly. Using a direct syscall is slightly slower (in
theory), because no optimised vdso implementation can be used, but avoids
the pthread dependency. Defaults to 1
on GNU/Linux with glibc 2.x or
higher, as it simplifies linking (no need for -lrt
).
If defined to be 1
, libev will assume that nanosleep ()
is available
and will use it for delays. Otherwise it will use select ()
.
If defined to be 1
, then libev will assume that eventfd ()
is
available and will probe for kernel support at runtime. This will improve
ev_signal
and ev_async
performance and reduce resource consumption.
If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
2.7 or newer, otherwise disabled.
If defined to be 1
, then libev will assume that signalfd ()
is
available and will probe for kernel support at runtime. This enables
the use of EVFLAG_SIGNALFD for faster and simpler signal handling. If
undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
2.7 or newer, otherwise disabled.
If defined to be 1
, then libev will assume that timerfd ()
is
available and will probe for kernel support at runtime. This allows
libev to detect time jumps accurately. If undefined, it will be enabled
if the headers indicate GNU/Linux + Glibc 2.8 or newer and define
TFD_TIMER_CANCEL_ON_SET
, otherwise disabled.
If defined to be 1
, then libev will assume that eventfd ()
is
available and will probe for kernel support at runtime. This will improve
ev_signal
and ev_async
performance and reduce resource consumption.
If undefined, it will be enabled if the headers indicate GNU/Linux + Glibc
2.7 or newer, otherwise disabled.
If undefined or defined to be 1
, libev will compile in support for the
select
(2) backend. No attempt at auto-detection will be done: if no
other method takes over, select will be it. Otherwise the select backend
will not be compiled in.
If defined to 1
, then the select backend will use the system fd_set
structure. This is useful if libev doesn't compile due to a missing
NFDBITS
or fd_mask
definition or it mis-guesses the bitset layout
on exotic systems. This usually limits the range of file descriptors to
some low limit such as 1024 or might have other limitations (winsocket
only allows 64 sockets). The FD_SETSIZE
macro, set before compilation,
configures the maximum size of the fd_set
.
When defined to 1
, the select backend will assume that
select/socket/connect etc. don't understand file descriptors but
wants osf handles on win32 (this is the case when the select to
be used is the winsock select). This means that it will call
_get_osfhandle
on the fd to convert it to an OS handle. Otherwise,
it is assumed that all these functions actually work on fds, even
on win32. Should not be defined on non-win32 platforms.
If EV_SELECT_IS_WINSOCKET
is enabled, then libev needs a way to map
file descriptors to socket handles. When not defining this symbol (the
default), then libev will call _get_osfhandle
, which is usually
correct. In some cases, programs use their own file descriptor management,
in which case they can provide this function to map fds to socket handles.
If EV_SELECT_IS_WINSOCKET
then libev maps handles to file descriptors
using the standard _open_osfhandle
function. For programs implementing
their own fd to handle mapping, overwriting this function makes it easier
to do so. This can be done by defining this macro to an appropriate value.
If programs implement their own fd to handle mapping on win32, then this
macro can be used to override the close
function, useful to unregister
file descriptors again. Note that the replacement function has to close
the underlying OS handle.
If defined to be 1
, libev will use WSASocket
to create its internal
communication socket, which works better in some environments. Otherwise,
the normal socket
function will be used, which works better in other
environments.
If defined to be 1
, libev will compile in support for the poll
(2)
backend. Otherwise it will be enabled on non-win32 platforms. It
takes precedence over select.
If defined to be 1
, libev will compile in support for the Linux
epoll
(7) backend. Its availability will be detected at runtime,
otherwise another method will be used as fallback. This is the preferred
backend for GNU/Linux systems. If undefined, it will be enabled if the
headers indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
If defined to be 1
, libev will compile in support for the Linux aio
backend (EV_USE_EPOLL
must also be enabled). If undefined, it will be
enabled on linux, otherwise disabled.
If defined to be 1
, libev will compile in support for the Linux
io_uring backend (EV_USE_EPOLL
must also be enabled). Due to it's
current limitations it has to be requested explicitly. If undefined, it
will be enabled on linux, otherwise disabled.
If defined to be 1
, libev will compile in support for the BSD style
kqueue
(2) backend. Its actual availability will be detected at runtime,
otherwise another method will be used as fallback. This is the preferred
backend for BSD and BSD-like systems, although on most BSDs kqueue only
supports some types of fds correctly (the only platform we found that
supports ptys for example was NetBSD), so kqueue might be compiled in, but
not be used unless explicitly requested. The best way to use it is to find
out whether kqueue supports your type of fd properly and use an embedded
kqueue loop.
If defined to be 1
, libev will compile in support for the Solaris
10 port style backend. Its availability will be detected at runtime,
otherwise another method will be used as fallback. This is the preferred
backend for Solaris 10 systems.
Reserved for future expansion, works like the USE symbols above.
If defined to be 1
, libev will compile in support for the Linux inotify
interface to speed up ev_stat
watchers. Its actual availability will
be detected at runtime. If undefined, it will be enabled if the headers
indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
If defined to be 1
, libev will assume that memory is always coherent
between threads, that is, threads can be used, but threads never run on
different cpus (or different cpu cores). This reduces dependencies
and makes libev faster.
If defined to be 1
, libev will assume that it will never be called from
different threads (that includes signal handlers), which is a stronger
assumption than EV_NO_SMP
, above. This reduces dependencies and makes
libev faster.
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.
The name of the ev.h header file used to include it. The default if
undefined is "ev.h"
in event.h, ev.c and ev++.h. This can be
used to virtually rename the ev.h header file in case of conflicts.
If EV_STANDALONE
isn't 1
, this variable can be used to override
ev.c's idea of where to find the config.h file, similarly to
EV_H
, above.
Similarly to EV_H
, this macro can be used to override event.c's idea
of how the event.h header can be found, the default is "event.h"
.
If defined to be 0
, then ev.h will not define any function
prototypes, but still define all the structs and other symbols. This is
occasionally useful if you want to provide your own wrapper functions
around libev functions.
If undefined or defined to 1
, then all event-loop-specific functions
will have the struct ev_loop *
as first argument, and you can create
additional independent event loops. Otherwise there will be no support
for multiple event loops and there is no first event loop pointer
argument. Instead, all functions act on the single default loop.
Note that EV_DEFAULT
and EV_DEFAULT_
will no longer provide a
default loop when multiplicity is switched off - you always have to
initialise the loop manually in this case.
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.
If undefined or defined to be 1
(and the platform supports it), then
the respective watcher type is supported. If defined to be 0
, then it
is not. Disabling watcher types mainly saves code size.
If you need to shave off some kilobytes of code at the expense of some speed (but with the full API), you can define this symbol to request certain subsets of functionality. The default is to enable all features that can be enabled on the platform.
A typical way to use this symbol is to define it to 0
(or to a bitset
with some broad features you want) and then selectively re-enable
additional parts you want, for example if you want everything minimal,
but multiple event loop support, async and child watchers and the poll
backend, use this:
#define EV_FEATURES 0 #define EV_MULTIPLICITY 1 #define EV_USE_POLL 1 #define EV_CHILD_ENABLE 1 #define EV_ASYNC_ENABLE 1
The actual value is a bitset, it can be a combination of the following values (by default, all of these are enabled):
1
- faster/larger codeUse larger code to speed up some operations.
Currently this is used to override some inlining decisions (enlarging the code size by roughly 30% on amd64).
When optimising for size, use of compiler flags such as -Os
with
gcc is recommended, as well as -DNDEBUG
, as libev contains a number of
assertions.
The default is off when __OPTIMIZE_SIZE__
is defined by your compiler
(e.g. gcc with -Os
).
2
- faster/larger data structuresReplaces the small 2-heap for timer management by a faster 4-heap, larger hash table sizes and so on. This will usually further increase code size and can additionally have an effect on the size of data structures at runtime.
The default is off when __OPTIMIZE_SIZE__
is defined by your compiler
(e.g. gcc with -Os
).
4
- full API configurationThis enables priorities (sets EV_MAXPRI
=2 and EV_MINPRI
=-2), and
enables multiplicity (EV_MULTIPLICITY
=1).
8
- full APIThis enables a lot of the "lesser used" API functions. See ev.h
for
details on which parts of the API are still available without this
feature, and do not complain if this subset changes over time.
16
- enable all optional watcher typesEnables all optional watcher types. If you want to selectively enable
only some watcher types other than I/O and timers (e.g. prepare,
embed, async, child...) you can enable them manually by defining
EV_watchertype_ENABLE
to 1
instead.
32
- enable all backendsThis enables all backends - without this feature, you need to enable at
least one backend manually (EV_USE_SELECT
is a good choice).
64
- enable OS-specific "helper" APIsEnable inotify, eventfd, signalfd and similar OS-specific helper APIs by default.
Compiling with gcc -Os -DEV_STANDALONE -DEV_USE_EPOLL=1 -DEV_FEATURES=0
reduces the compiled size of libev from 24.7Kb code/2.8Kb data to 6.5Kb
code/0.3Kb data on my GNU/Linux amd64 system, while still giving you I/O
watchers, timers and monotonic clock support.
With an intelligent-enough linker (gcc+binutils are intelligent enough
when you use -Wl,--gc-sections -ffunction-sections
) functions unused by
your program might be left out as well - a binary starting a timer and an
I/O watcher then might come out at only 5Kb.
If this symbol is defined (by default it is not), then all identifiers will have static linkage. This means that libev will not export any identifiers, and you cannot link against libev anymore. This can be useful when you embed libev, only want to use libev functions in a single file, and do not want its identifiers to be visible.
To use this, define EV_API_STATIC
and include ev.c in the file that
wants to use libev.
This option only works when libev is compiled with a C compiler, as C++ doesn't support the required declaration syntax.
If this is set to 1
at compiletime, then libev will avoid using stdio
functions (printf, scanf, perror etc.). This will increase the code size
somewhat, but if your program doesn't otherwise depend on stdio and your
libc allows it, this avoids linking in the stdio library which is quite
big.
Note that error messages might become less precise when this option is enabled.
The highest supported signal number, +1 (or, the number of
signals): Normally, libev tries to deduce the maximum number of signals
automatically, but sometimes this fails, in which case it can be
specified. Also, using a lower number than detected (32
should be
good for about any system in existence) can save some memory, as libev
statically allocates some 12-24 bytes per signal number.
ev_child
watchers use a small hash table to distribute workload by
pid. The default size is 16
(or 1
with EV_FEATURES
disabled),
usually more than enough. If you need to manage thousands of children you
might want to increase this value (must be a power of two).
ev_stat
watchers use a small hash table to distribute workload by
inotify watch id. The default size is 16
(or 1
with EV_FEATURES
disabled), usually more than enough. If you need to manage thousands of
ev_stat
watchers you might want to increase this value (must be a
power of two).
Heaps are not very cache-efficient. To improve the cache-efficiency of the
timer and periodics heaps, libev uses a 4-heap when this symbol is defined
to 1
. The 4-heap uses more complicated (longer) code but has noticeably
faster performance with many (thousands) of watchers.
The default is 1
, unless EV_FEATURES
overrides it, in which case it
will be 0
.
Heaps are not very cache-efficient. To improve the cache-efficiency of the
timer and periodics heaps, libev can cache the timestamp (at) within
the heap structure (selected by defining EV_HEAP_CACHE_AT
to 1
),
which uses 8-12 bytes more per watcher and a few hundred bytes more code,
but avoids random read accesses on heap changes. This improves performance
noticeably with many (hundreds) of watchers.
The default is 1
, unless EV_FEATURES
overrides it, in which case it
will be 0
.
Controls how much internal verification (see ev_verify ()
) will
be done: If set to 0
, no internal verification code will be compiled
in. If set to 1
, then verification code will be compiled in, but not
called. If set to 2
, then the internal verification code will be
called once per loop, which can slow down libev. If set to 3
, then the
verification code will be called very frequently, which will slow down
libev considerably.
Verification errors are reported via C's assert
mechanism, so if you
disable that (e.g. by defining NDEBUG
) then no errors will be reported.
The default is 1
, unless EV_FEATURES
overrides it, in which case it
will be 0
.
By default, all watchers have a void *data
member. By redefining
this macro to something else you can include more and other types of
members. You have to define it each time you include one of the files,
though, and it must be identical each time.
For example, the perl EV module uses something like this:
#define EV_COMMON \ SV *self; /* contains this struct */ \ SV *cb_sv, *fh /* note no trailing ";" */
Can be used to change the callback member declaration in each watcher,
and the way callbacks are invoked and set. Must expand to a struct member
definition and a statement, respectively. See the ev.h header file for
their default definitions. One possible use for overriding these is to
avoid the struct ev_loop *
as first argument in all cases, or to use
method calls instead of plain function calls in C++.
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_FEATURES 8 #define EV_USE_SELECT 1 #define EV_PREPARE_ENABLE 1 #define EV_IDLE_ENABLE 1 #define EV_SIGNAL_ENABLE 1 #define EV_CHILD_ENABLE 1 #define EV_USE_STDEXCEPT 0 #define EV_CONFIG_H <config.h> #include "ev++.h"
And a ev_cpp.C implementation file that contains libev proper and is compiled:
#include "ev_cpp.h" #include "ev.c"
All libev functions are reentrant and thread-safe unless explicitly
documented otherwise, but libev implements no locking itself. This means
that you can use as many loops as you want in parallel, as long as there
are no concurrent calls into any libev function with the same loop
parameter (ev_default_*
calls have an implicit default loop parameter,
of course): libev guarantees that different event loops share no data
structures that need any locking.
Or to put it differently: calls with different loop parameters can be done concurrently from multiple threads, calls with the same loop parameter must be done serially (but can be done from different threads, as long as only one thread ever is inside a call at any point in time, e.g. by using a mutex per loop).
Specifically to support threads (and signal handlers), libev implements
so-called ev_async
watchers, which allow some limited form of
concurrency on the same event loop, namely waking it up "from the
outside".
If you want to know which design (one loop, locking, or multiple loops without or something else still) is best for your problem, then I cannot help you, but here is some generic advice:
This helps integrating other libraries or software modules that use libev themselves and don't care/know about threading.
Doing this is almost never wrong, sometimes a better-performance model exists, but it is always a good start.
Choosing a model is hard - look around, learn, know that usually you can do better than you currently do :-)
ev_async
watchers can be used to wake them up from other threads safely
(or from signal contexts...).
An example use would be to communicate signals or other events that only
work in the default loop by registering the signal watcher with the
default loop and triggering an ev_async
watcher from the default loop
watcher callback into the event loop interested in the signal.
See also THREAD LOCKING EXAMPLE.
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_run
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_run
, 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 in 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.
GNU/Linux is the only common platform that supports 64 bit file/large file interfaces but disables them by default.
That means that libev compiled in the default environment doesn't support
files larger than 2GiB or so, which mainly affects ev_stat
watchers.
Unfortunately, many programs try to work around this GNU/Linux issue by enabling the large file API, which makes them incompatible with the standard libev compiled for their system.
Likewise, libev cannot enable the large file API itself as this would suddenly make it incompatible to the default compile time environment, i.e. all programs not using special compile switches.
The whole thing is a bug if you ask me - basically any system interface you touch is broken, whether it is locales, poll, kqueue or even the OpenGL drivers.
kqueue
is buggyThe kqueue syscall is broken in all known versions - most versions support only sockets, many support pipes.
Libev tries to work around this by not using kqueue
by default on this
rotten platform, but of course you can still ask for it when creating a
loop - embedding a socket-only kqueue loop into a select-based one is
probably going to work well.
poll
is buggyInstead of fixing kqueue
, Apple replaced their (working) poll
implementation by something calling kqueue
internally around the 10.5.6
release, so now kqueue
and poll
are broken.
Libev tries to work around this by not using poll
by default on
this rotten platform, but of course you can still ask for it when creating
a loop.
select
is buggyAll that's left is select
, and of course Apple found a way to fuck this
one up as well: On OS/X, select
actively limits the number of file
descriptors you can pass in to 1024 - your program suddenly crashes when
you use more.
There is an undocumented "workaround" for this - defining
_DARWIN_UNLIMITED_SELECT
, which libev tries to use, so select should
work on OS/X.
errno
reentrancyThe default compile environment on Solaris is unfortunately so
thread-unsafe that you can't even use components/libraries compiled
without -D_REENTRANT
in a threaded program, which, of course, isn't
defined by default. A valid, if stupid, implementation choice.
If you want to use libev in threaded environments you have to make sure
it's compiled with _REENTRANT
defined.
The scalable event interface for Solaris is called "event ports". Unfortunately, this mechanism is very buggy in all major releases. If you run into high CPU usage, your program freezes or you get a large number of spurious wakeups, make sure you have all the relevant and latest kernel patches applied. No, I don't know which ones, but there are multiple ones to apply, and afterwards, event ports actually work great.
If you can't get it to work, you can try running the program by setting
the environment variable LIBEV_FLAGS=3
to only allow poll
and
select
backends.
AIX unfortunately has a broken poll.h
header. Libev works around
this by trying to avoid the poll backend altogether (i.e. it's not even
compiled in), which normally isn't a big problem as select
works fine
with large bitsets on AIX, and AIX is dead anyway.
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. Actually, it only applies to the microsofts own compilers,
as every compiler comes with a slightly differently broken/incompatible
environment.
Lifting these limitations would basically require the full re-implementation of the I/O system. If you are into this kind of thing, 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"
select
functionThe 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 *
.Libev assumes not only that all watcher pointers have the same internal
structure (guaranteed by POSIX but not by ISO C for example), but it also
assumes that the same (machine) code can be used to call any watcher
callback: The watcher callbacks have different type signatures, but libev
calls them using an ev_watcher *
internally.
Libev uses memset
to initialise structs and arrays to 0
bytes, and
relies on this setting pointers and integers to null.
Accessing a pointer value must be atomic, it must both be readable and writable in one piece - this is the case on all current architectures.
sig_atomic_t volatile
must be thread-atomic as wellThe type sig_atomic_t volatile
(or whatever is defined as
EV_ATOMIC_T
) must be atomic with respect to accesses from different
threads. This is not part of the specification for sig_atomic_t
, but is
believed to be sufficiently portable.
sigprocmask
must work in a threaded environmentLibev 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 signal handling loop in the initial thread as well.
long
must be large enough for common memory allocation sizesTo improve portability and simplify its API, libev uses long
internally
instead of size_t
when allocating its data structures. On non-POSIX
systems (Microsoft...) this might be unexpectedly low, but is still at
least 31 bits everywhere, which is enough for hundreds of millions of
watchers.
double
must hold a time value in seconds with enough accuracyThe type double
is used to represent timestamps. It is required to
have at least 51 bits of mantissa (and 9 bits of exponent), which is
good enough for at least into the year 4000 with millisecond accuracy
(the design goal for libev). This requirement is overfulfilled by
implementations using IEEE 754, which is basically all existing ones.
With IEEE 754 doubles, you get microsecond accuracy until at least the
year 2255 (and millisecond accuracy till the year 287396 - by then, libev
is either obsolete or somebody patched it to use long double
or
something like that, just kidding).
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.
This means that, when you have a watcher that triggers in one hour and
there are 100 watchers that would trigger before that, then inserting will
have to skip roughly seven (ld 100
) of these watchers.
That means that changing a timer costs less than removing/adding them, as only the relative motion in the event queue has to be paid for.
These just add the watcher into an array or at the head of a list.
These watchers are stored in lists, so they need to be walked to find the correct watcher to remove. The lists are usually short (you don't usually have many watchers waiting for the same fd or signal: one is typical, two is rare).
By virtue of using a binary or 4-heap, the next timer is always found at a fixed position in the storage array.
A change means an I/O watcher gets started or stopped, which requires
libev to recalculate its status (and possibly tell the kernel, depending
on backend and whether ev_io_set
was used).
Priorities are implemented by allocating some space for each priority. When doing priority-based operations, libev usually has to linearly search all the priorities, but starting/stopping and activating watchers becomes O(1) with respect to priority handling.
Sending involves a system call iff there were no other ev_async_send
calls in the current loop iteration and the loop is currently
blocked. Checking for async and signal events involves iterating over all
running async watchers or all signal numbers.
The major version 4 introduced some incompatible changes to the API.
At the moment, the ev.h
header file provides compatibility definitions
for all changes, so most programs should still compile. The compatibility
layer might be removed in later versions of libev, so better update to the
new API early than late.
EV_COMPAT3
backwards compatibility mechanismThe backward compatibility mechanism can be controlled by
EV_COMPAT3
. See "PREPROCESSOR SYMBOLS/MACROS" in the EMBEDDING
section.
ev_default_destroy
and ev_default_fork
have been removedThese calls can be replaced easily by their ev_loop_xxx
counterparts:
ev_loop_destroy (EV_DEFAULT_UC); ev_loop_fork (EV_DEFAULT);
A number of functions and symbols have been renamed:
ev_loop => ev_run EVLOOP_NONBLOCK => EVRUN_NOWAIT EVLOOP_ONESHOT => EVRUN_ONCE ev_unloop => ev_break EVUNLOOP_CANCEL => EVBREAK_CANCEL EVUNLOOP_ONE => EVBREAK_ONE EVUNLOOP_ALL => EVBREAK_ALL EV_TIMEOUT => EV_TIMER ev_loop_count => ev_iteration ev_loop_depth => ev_depth ev_loop_verify => ev_verify
Most functions working on struct ev_loop
objects don't have an
ev_loop_
prefix, so it was removed; ev_loop
, ev_unloop
and
associated constants have been renamed to not collide with the struct
ev_loop
anymore and EV_TIMER
now follows the same naming scheme
as all other watcher types. Note that ev_loop_fork
is still called
ev_loop_fork
because it would otherwise clash with the ev_fork
typedef.
EV_MINIMAL
mechanism replaced by EV_FEATURES
The preprocessor symbol EV_MINIMAL
has been replaced by a different
mechanism, EV_FEATURES
. Programs using EV_MINIMAL
usually compile
and work, but the library code will of course be larger.
A watcher is active as long as it has been started and not yet stopped. See WATCHER STATES for details.
In this document, an application is whatever is using libev.
The part of the code dealing with the operating system interfaces.
The address of a function that is called when some event has been detected. Callbacks are being passed the event loop, the watcher that received the event, and the actual event bitset.
The act of calling the callback associated with a watcher.
A change of state of some external event, such as data now being available for reading on a file descriptor, time having passed or simply not having any other events happening anymore.
In libev, events are represented as single bits (such as EV_READ
or
EV_TIMER
).
A software package implementing an event model and loop.
An entity that handles and processes external events and converts them into callback invocations.
The model used to describe how an event loop handles and processes watchers and events.
A watcher is pending as soon as the corresponding event has been detected. See WATCHER STATES for details.
The physical time that is observed. It is apparently strictly monotonic :)
The time and date as shown on clocks. Unlike real time, it can actually be wrong and jump forwards and backwards, e.g. when you adjust your clock.
A data structure that describes interest in certain events. Watchers need to be started (attached to an event loop) before they can receive events.
Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael Magnusson and Emanuele Giaquinta, and minor corrections by many others.