ev_io - is this file descriptor readable or writable?ev_timer - relative and optionally repeating timeoutsev_periodic - to cron or not to cron?ev_signal - signal me when a signal gets signalled!ev_child - watch out for process status changesev_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 forkev_async - how to wake up another event loop
libev - a high performance full-featured event loop written in C
#include <ev.h>
// a single header file is required #include <ev.h>
// 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_ struct ev_io *w, int revents)
{
puts ("stdin ready");
// for one-shot events, one must manually stop the watcher
// with its corresponding stop function.
ev_io_stop (EV_A_ w);
// this causes all nested ev_loop's to stop iterating
ev_unloop (EV_A_ EVUNLOOP_ALL);
}
// another callback, this time for a time-out
static void
timeout_cb (EV_P_ struct ev_timer *w, int revents)
{
puts ("timeout");
// this causes the innermost ev_loop to stop iterating
ev_unloop (EV_A_ EVUNLOOP_ONE);
}
int
main (void)
{
// use the default event loop unless you have special needs
struct ev_loop *loop = ev_default_loop (0);
// initialise an io watcher, then start it
// this one will watch for stdin to become readable
ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
ev_io_start (loop, &stdin_watcher);
// initialise a timer watcher, then start it
// simple non-repeating 5.5 second timeout
ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
ev_timer_start (loop, &timeout_watcher);
// now wait for events to arrive
ev_loop (loop, 0);
// unloop was called, so exit
return 0;
}
The newest version of this document is also available as an html-formatted web page you might find easier to navigate when reading it for the first time: http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod.
Libev is an event loop: you register interest in certain events (such as a file descriptor being readable or a timeout occurring), and it will manage these event sources and provide your program with events.
To do this, it must take more or less complete control over your process (or thread) by executing the event loop handler, and will then communicate events via a callback mechanism.
You register interest in certain events by registering so-called event watchers, which are relatively small C structures you initialise with the details of the event, and then hand it over to libev by starting the watcher.
Libev supports select, poll, the Linux-specific epoll, the
BSD-specific kqueue and the Solaris-specific event port mechanisms
for file descriptor events (ev_io), the Linux inotify interface
(for ev_stat), relative timers (ev_timer), absolute timers
with customised rescheduling (ev_periodic), synchronous signals
(ev_signal), process status change events (ev_child), and event
watchers dealing with the event loop mechanism itself (ev_idle,
ev_embed, ev_prepare and ev_check watchers) as well as
file watchers (ev_stat) and even limited support for fork events
(ev_fork).
It also is quite fast (see this benchmark comparing it to libevent for example).
Libev is very configurable. In this manual the default (and most common)
configuration will be described, which supports multiple event loops. For
more info about various configuration options please have a look at
EMBED section in this manual. If libev was configured without support
for multiple event loops, then all functions taking an initial argument of
name loop (which is always of type 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 (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, and when you need to do any calculations on
it, you should treat it as some floatingpoint value. Unlike the name
component stamp might indicate, it is also used for time differences
throughout libev.
Libev knows three classes of errors: operating system errors, usage errors and internal errors (bugs).
When libev catches an operating system error it cannot handle (for example
a syscall indicating a condition libev cannot fix), it calls the callback
set via ev_set_syserr_cb, which is supposed to fix the problem or
abort. The default is to print a diagnostic message and to call abort
().
When libev detects a usage error such as a negative timer interval, then
it will print a diagnostic message and abort (via the assert mechanism,
so NDEBUG will disable this checking): these are programming errors in
the libev caller and need to be fixed there.
Libev also has a few internal error-checking assertions, and also has
extensive consistency checking code. These do not trigger under normal
circumstances, as they indicate either a bug in libev or worse.
These functions can be called anytime, even before initialising the library in any way.
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.
Sleep for the given interval: The current thread will be blocked until
either it is interrupted or the given time interval has passed. Basically
this is a subsecond-resolution sleep ().
You can find out the major and minor ABI version numbers of the library
you linked against by calling the functions ev_version_major and
ev_version_minor. If you want, you can compare against the global
symbols EV_VERSION_MAJOR and EV_VERSION_MINOR, which specify the
version of the library your program was compiled against.
These version numbers refer to the ABI version of the library, not the release version.
Usually, it's a good idea to terminate if the major versions mismatch, as this indicates an incompatible change. Minor versions are usually compatible to older versions, so a larger minor version alone is usually not a problem.
Example: Make sure we haven't accidentally been linked against the wrong version.
assert (("libev version mismatch",
ev_version_major () == EV_VERSION_MAJOR
&& ev_version_minor () >= EV_VERSION_MINOR));
Return the set of all backends (i.e. their corresponding EV_BACKEND_*
value) compiled into this binary of libev (independent of their
availability on the system you are running on). See ev_default_loop for
a description of the set values.
Example: make sure we have the epoll method, because yeah this is cool and a must have and can we have a torrent of it please!!!11
assert (("sorry, no epoll, no sex",
ev_supported_backends () & EVBACKEND_EPOLL));
Return the set of all backends compiled into this binary of libev and also
recommended for this platform. 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 autodetected unless you explicitly request it
(assuming you know what you are doing). This is the set of backends that
libev will probe for if you specify no backends explicitly.
Returns the set of backends that are embeddable in other event loops. This
is the theoretical, all-platform, value. To find which backends
might be supported on the current system, you would need to look at
ev_embeddable_backends () & ev_supported_backends (), likewise for
recommended ones.
See the description of ev_embed watchers for more info.
Sets the allocation function to use (the prototype is similar - the
semantics are identical to the realloc C89/SuS/POSIX function). It is
used to allocate and free memory (no surprises here). If it returns zero
when memory needs to be allocated (size != 0), the library might abort
or take some potentially destructive action.
Since some systems (at least OpenBSD and Darwin) fail to implement
correct realloc semantics, libev will use a wrapper around the system
realloc and free functions by default.
You could override this function in high-availability programs to, say, free some memory if it cannot allocate memory, to use a special allocator, or even to sleep a while and retry until some memory is available.
Example: Replace the libev allocator with one that waits a bit and then
retries (example requires a standards-compliant realloc).
static void *
persistent_realloc (void *ptr, size_t size)
{
for (;;)
{
void *newptr = realloc (ptr, size);
if (newptr)
return newptr;
sleep (60);
}
}
... ev_set_allocator (persistent_realloc);
Set the callback function to call on a retryable syscall 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 sitution, no matter what, when it returns. That is, libev will generally retry the requested operation, or, if the condition doesn't go away, do bad stuff (such as abort).
Example: This is basically the same thing that libev does internally, too.
static void
fatal_error (const char *msg)
{
perror (msg);
abort ();
}
... ev_set_syserr_cb (fatal_error);
An event loop is described by a struct ev_loop *. The library knows two
types of such loops, the default loop, which supports signals and child
events, and dynamically created loops which do not.
This will initialise the default event loop if it hasn't been initialised
yet and return it. If the default loop could not be initialised, returns
false. If it already was initialised it simply returns it (and ignores the
flags. If that is troubling you, check ev_backend () afterwards).
If you don't know what event loop to use, use the one returned from this function.
Note that this function is not thread-safe, so if you want to use it from multiple threads, you have to lock (note also that this is unlikely, as loops cannot bes hared easily between threads anyway).
The default loop is the only loop that can handle ev_signal and
ev_child watchers, and to do this, it always registers a handler
for SIGCHLD. If this is a problem for your app you can either
create a dynamic loop with ev_loop_new that doesn't do that, or you
can simply overwrite the SIGCHLD signal handler after calling
ev_default_init.
The flags argument can be used to specify special behaviour or specific
backends to use, and is usually specified as 0 (or EVFLAG_AUTO).
The following flags are supported:
EVFLAG_AUTO
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 ored 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, or to work
around bugs.
EVFLAG_FORKCHECK
Instead of calling ev_default_fork or ev_loop_fork manually after
a fork, you can also make libev check for a fork in each iteration by
enabling this flag.
This works by calling getpid () on every iteration of the loop,
and thus this might slow down your event loop if you do a lot of loop
iterations and little real work, but is usually not noticeable (on my
GNU/Linux system for example, getpid is actually a simple 5-insn sequence
without a syscall and thus very fast, but my GNU/Linux system also has
pthread_atfork which is even faster).
The big advantage of this flag is that you can forget about fork (and forget about forgetting to tell libev about forking) when you use this flag.
This flag setting cannot be overriden or specified in the LIBEV_FLAGS
environment variable.
EVBACKEND_SELECT (value 1, portable select backend)
This is your standard select(2) backend. Not completely standard, as
libev tries to roll its own fd_set with no limits on the number of fds,
but if that fails, expect a fairly low limit on the number of fds when
using this backend. It doesn't scale too well (O(highest_fd)), but its
usually the fastest backend for a low number of (low-numbered :) fds.
To get good performance out of this backend you need a high amount of
parallelity (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.
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.
EVBACKEND_EPOLL (value 4, Linux)
For few fds, this backend is a bit little slower than poll and select,
but it scales phenomenally better. While poll and select usually scale
like O(total_fds) where n is the total number of fds (or the highest fd),
epoll scales either O(1) or O(active_fds). The epoll design has a number
of shortcomings, such as silently dropping events in some hard-to-detect
cases and requiring a syscall per fd change, no fork support and bad
support for dup.
While stopping, setting and starting an I/O watcher in the same iteration
will result in some caching, there is still a syscall per such incident
(because the fd 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 fds.
Please note that epoll sometimes generates spurious notifications, so you need to use non-blocking I/O or other means to avoid blocking when no data (or space) is available.
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.
While nominally embeddeble in other event loops, this feature is broken in all kernel versions tested so far.
EVBACKEND_KQUEUE (value 8, most BSD clones)
Kqueue deserves special mention, as at the time of this writing, it
was broken on all BSDs except NetBSD (usually it doesn't work reliably
with anything but sockets and pipes, except on Darwin, where of course
it's completely useless). For this reason it's not being ``autodetected''
unless you explicitly specify it explicitly 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 syscall as with EVBACKEND_EPOLL, it still adds up to
two event changes per incident, support for fork () is very bad 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) and using it only for
sockets.
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)).
Please note that solaris event ports can deliver a lot of spurious notifications, so you need to use non-blocking I/O or other means to avoid blocking when no data (or space) is available.
While this backend scales well, it requires one system call per active
file descriptor per loop iteration. For small and medium numbers of file
descriptors a ``slow'' EVBACKEND_SELECT or EVBACKEND_POLL backend
might perform better.
On the positive side, ignoring the spurious readiness notifications, this backend actually performed to specification in all tests and is fully embeddable, which is a rare feat among the OS-specific backends.
EVBACKEND_ALL
Try all backends (even potentially broken ones that wouldn't be tried
with EVFLAG_AUTO). Since this is a mask, you can do stuff such as
EVBACKEND_ALL & ~EVBACKEND_KQUEUE.
It is definitely not recommended to use this flag.
If one or more of these are ored 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.
The most typical usage is like this:
if (!ev_default_loop (0))
fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");
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);
Use whatever libev has to offer, but make sure that kqueue is used if available (warning, breaks stuff, best use only with your own private event loop and only if you know the OS supports your types of fds):
ev_default_loop (ev_recommended_backends () | EVBACKEND_KQUEUE);
Similar to ev_default_loop, but always creates a new event loop that is
always distinct from the default loop. Unlike the default loop, it cannot
handle signal and child watchers, and attempts to do so will be greeted by
undefined behaviour (or a failed assertion if assertions are enabled).
Note that this function is thread-safe, and the recommended way to use libev with threads is indeed to create one loop per thread, and using the default loop in the ``main'' or ``initial'' thread.
Example: Try to create a event loop that uses epoll and nothing else.
struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
if (!epoller)
fatal ("no epoll found here, maybe it hides under your chair");
Destroys the default loop again (frees all memory and kernel state
etc.). None of the active event watchers will be stopped in the normal
sense, so e.g. ev_is_active might still return true. It is your
responsibility to either stop all watchers cleanly yoursef 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, will not be freed by this function, and related watchers (such as signal and child watchers) would need to be stopped manually.
In general it is not advisable to call this function except in the
rare occasion where you really need to free e.g. the signal handling
pipe fds. If you need dynamically allocated loops it is better to use
ev_loop_new and ev_loop_destroy).
Like ev_default_destroy, but destroys an event loop created by an
earlier call to ev_loop_new.
This function sets a flag that causes subsequent ev_loop iterations
to reinitialise the kernel state for backends that have one. Despite the
name, you can call it anytime, but it makes most sense after forking, in
the child process (or both child and parent, but that again makes little
sense). You must call it in the child before using any of the libev
functions, and it will only take effect at the next ev_loop iteration.
On the other hand, you only need to call this function in the child process if and only if you want to use the event library in the child. If you just fork+exec, you don't have to call it at all.
The function itself is quite fast and it's usually not a problem to call
it just in case after a fork. To make this easy, the function will fit in
quite nicely into a call to pthread_atfork:
pthread_atfork (0, 0, ev_default_fork);
Like ev_default_fork, but acts on an event loop created by
ev_loop_new. Yes, you have to call this on every allocated event loop
after fork, and how you do this is entirely your own problem.
Returns true when the given loop actually is the default loop, false otherwise.
Returns the count of loop iterations for the loop, which is identical to
the number of times libev did poll for new events. It starts at 0 and
happily wraps around with enough iterations.
This value can sometimes be useful as a generation counter of sorts (it
``ticks'' the number of loop iterations), as it roughly corresponds with
ev_prepare and ev_check calls.
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).
Finally, this is it, the event handler. This function usually is called after you initialised all your watchers and you want to start handling events.
If the flags argument is specified as 0, it will not return until
either no event watchers are active anymore or ev_unloop was called.
Please note that an explicit ev_unloop is usually better than
relying on all watchers to be stopped when deciding when a program has
finished (especially in interactive programs), but having a program that
automatically loops as long as it has to and no longer by virtue of
relying on its watchers stopping correctly is a thing of beauty.
A flags value of EVLOOP_NONBLOCK will look for new events, will handle
those events and any outstanding ones, but will not block your process in
case there are no events and will return after one iteration of the loop.
A flags value of EVLOOP_ONESHOT will look for new events (waiting if
neccessary) and will handle those and any outstanding ones. It will block
your process until at least one new event arrives, 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. However, a pair of ev_prepare/ev_check watchers is
usually a better approach for this kind of thing.
Here are the gory details of what ev_loop does:
- Before the first iteration, call any pending watchers.
* If EVFLAG_FORKCHECK was used, check for a fork.
- If a fork was detected, queue and call all fork watchers.
- Queue and call all prepare watchers.
- If we have been forked, recreate the kernel state.
- Update the kernel state with all outstanding changes.
- Update the "event loop time".
- Calculate for how long to sleep or block, if at all
(active idle watchers, EVLOOP_NONBLOCK or not having
any active watchers at all will result in not sleeping).
- Sleep if the I/O and timer collect interval say so.
- Block the process, waiting for any events.
- Queue all outstanding I/O (fd) events.
- Update the "event loop time" and do time jump handling.
- Queue all outstanding timers.
- Queue all outstanding periodics.
- If no events are pending now, queue all idle watchers.
- Queue all check watchers.
- Call all queued watchers in reverse order (i.e. check watchers first).
Signals and child watchers are implemented as I/O watchers, and will
be handled here by queueing them when their watcher gets executed.
- If ev_unloop has been called, or EVLOOP_ONESHOT or EVLOOP_NONBLOCK
were used, or there are no active watchers, return, otherwise
continue with step *.
Example: Queue some jobs and then loop until no events are outstanding anymore.
... queue jobs here, make sure they register event watchers as long ... as they still have work to do (even an idle watcher will do..) ev_loop (my_loop, 0); ... jobs done. yeah!
Can be used to make a call to ev_loop return early (but only after it
has processed all outstanding events). The how argument must be either
EVUNLOOP_ONE, which will make the innermost ev_loop call return, or
EVUNLOOP_ALL, which will make all nested ev_loop calls return.
This ``unloop state'' will be cleared when entering ev_loop again.
Ref/unref can be used to add or remove a reference count on the event
loop: Every watcher keeps one reference, and as long as the reference
count is nonzero, ev_loop will not return on its own. If you have
a watcher you never unregister that should not keep ev_loop from
returning, ev_unref() after starting, and ev_ref() before stopping it. For
example, libev itself uses this for its internal signal pipe: It is not
visible to the libev user and should not keep ev_loop from exiting if
no event watchers registered by it are active. It is also an excellent
way to do this for generic recurring timers or from within third-party
libraries. Just remember to unref after start and ref before stop
(but only if the watcher wasn't active before, or was active before,
respectively).
Example: Create a signal watcher, but keep it from keeping ev_loop
running when nothing else is active.
struct ev_signal exitsig; ev_signal_init (&exitsig, sig_cb, SIGINT); ev_signal_start (loop, &exitsig); evf_unref (loop);
Example: For some weird reason, unregister the above signal handler again.
ev_ref (loop); ev_signal_stop (loop, &exitsig);
These advanced functions influence the time that libev will spend waiting
for events. Both 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.
The background is that sometimes your program runs just fast enough to
handle one (or very few) event(s) per loop iteration. While this makes
the program responsive, it also wastes a lot of CPU time to poll for new
events, especially with backends like select () which have a high
overhead for the actual polling but can deliver many events at once.
By setting a higher io collect interval you allow libev to spend more
time collecting I/O events, so you can handle more events per iteration,
at the cost of increasing latency. Timeouts (both ev_periodic and
ev_timer) will be not affected. Setting this to a non-null value will
introduce an additional ev_sleep () call into most loop iterations.
Likewise, by setting a higher timeout collect interval you allow libev
to spend more time collecting timeouts, at the expense of increased
latency (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 io 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 approsaches the timing granularity of most systems.
This function only does something when EV_VERIFY support has been
compiled in. 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.
A watcher is a structure that you create and register to record your
interest in some event. For instance, if you want to wait for STDIN to
become readable, you would create an ev_io watcher for that:
static void my_cb (struct ev_loop *loop, struct ev_io *w, int revents)
{
ev_io_stop (w);
ev_unloop (loop, EVUNLOOP_ALL);
}
struct ev_loop *loop = ev_default_loop (0); struct ev_io stdin_watcher; ev_init (&stdin_watcher, my_cb); ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ); ev_io_start (loop, &stdin_watcher); ev_loop (loop, 0);
As you can see, you are responsible for allocating the memory for your watcher structures (and it is usually a bad idea to do this on the stack, although this can sometimes be quite valid).
Each watcher structure must be initialised by a call to ev_init
(watcher *, callback), which expects a callback to be provided. This
callback gets invoked each time the event occurs (or, in the case of io
watchers, each time the event loop detects that the file descriptor given
is readable and/or writable).
Each watcher type has its own ev_<type>_set (watcher *, ...) macro
with arguments specific to this watcher type. There is also a macro
to combine initialisation and setting in one call: ev_<type>_init
(watcher *, callback, ...).
To make the watcher actually watch out for events, you have to start it
with a watcher-specific start function (ev_<type>_start (loop, watcher
*)), and you can stop watching for events at any time by calling the
corresponding stop function (ev_<type>_stop (loop, watcher *).
As long as your watcher is active (has been started but not stopped) you
must not touch the values stored in it. Most specifically you must never
reinitialise it or call its 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_TIMEOUT
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_loop starts
to gather new events, and all ev_check watchers are invoked just after
ev_loop has gathered them, but before it invokes any callbacks for any
received events. Callbacks of both watcher types can start and stop as
many watchers as they want, and all of them will be taken into account
(for example, a ev_prepare watcher might start an idle watcher to keep
ev_loop from blocking).
EV_EMBED
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_ASYNC
The given async watcher has been asynchronously notified (see ev_async).
EV_ERROR
An unspecified error has occured, 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. You best act on it by reporting the problem and somehow coping with the watcher being stopped.
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 multithreaded
programs, though, so beware.
In the following description, TYPE stands for the watcher type,
e.g. timer for ev_timer watchers and io for ev_io watchers.
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 (*)(ev_loop *loop, ev_TYPE *watcher,
int revents).
ev_TYPE_set (ev_TYPE *, [args])
This macro initialises the type-specific parts of a watcher. You need to
call ev_init at least once before you call this macro, but you can
call ev_TYPE_set any number of times. You must not, however, call this
macro on a watcher that is active (it can be pending, however, which is a
difference to the ev_init macro).
Although some watcher types do not have type-specific arguments
(e.g. ev_prepare) you still need to call its set macro.
ev_TYPE_init (ev_TYPE *watcher, callback, [args])
This convinience macro rolls both ev_init and ev_TYPE_set macro
calls into a single call. This is the most convinient method to initialise
a watcher. The same limitations apply, of course.
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.
ev_TYPE_stop (loop *, ev_TYPE *watcher)
Stops the given watcher again (if active) and clears the pending
status. It is possible that stopped watchers are pending (for example,
non-repeating timers are being stopped when they become pending), but
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.
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).
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).
This means that priorities are only used for ordering callback invocation after new events have been received. This is useful, for example, to reduce latency after idling, or more often, to bind two watchers on the same event and make sure one is called first.
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.
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 :).
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 adjusted to be within valid range.
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.
If the watcher is pending, this function returns 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.
Each watcher has, by default, a member void *data that you can change
and read at any time, libev will completely ignore it. This can be used
to associate arbitrary data with your watcher. If you need more data and
don't want to allocate memory and store a pointer to it in that data
member, you can also ``subclass'' the watcher type and provide your own
data:
struct my_io
{
struct ev_io io;
int otherfd;
void *somedata;
struct whatever *mostinteresting;
}
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, struct ev_io *w_, int revents)
{
struct my_io *w = (struct my_io *)w_;
...
}
More interesting and less C-conformant ways of casting your callback type instead have been omitted.
Another common scenario is having some data structure with multiple watchers:
struct my_biggy
{
int some_data;
ev_timer t1;
ev_timer t2;
}
In this case getting the pointer to my_biggy is a bit more complicated,
you need to use offsetof:
#include <stddef.h>
static void
t1_cb (EV_P_ struct 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_ struct ev_timer *w, int revents)
{
struct my_biggy big = (struct my_biggy *
(((char *)w) - offsetof (struct my_biggy, t2));
}
This section describes each watcher in detail, but will not repeat information given in the last section. Any initialisation/set macros, functions and members specific to the watcher type are explained.
Members are additionally marked with either [read-only], meaning that, while the watcher is active, you can look at the member and expect some sensible content, but you must not modify it (you can modify it while the watcher is stopped to your hearts content), or [read-write], which means you can expect it to have some sensible content while the watcher is active, but you can also modify it. Modifying it may not do something sensible or take immediate effect (or do anything at all), but libev will not crash or malfunction in any way.
ev_io - is this file descriptor readable or writable?I/O watchers check whether a file descriptor is readable or writable in each iteration of the event loop, or, more precisely, when reading would not block the process and writing would at least be able to write some data. This behaviour is called level-triggering because you keep receiving events as long as the condition persists. Remember you can stop the watcher if you don't want to act on the event and neither want to receive future events.
In general you can register as many read and/or write event watchers per fd as you want (as long as you don't confuse yourself). Setting all file descriptors to non-blocking mode is also usually a good idea (but not required if you know what you are doing).
If you must do this, then force the use of a known-to-be-good backend
(at the time of this writing, this includes only EVBACKEND_SELECT and
EVBACKEND_POLL).
Another thing you have to watch out for is that it is quite easy to
receive ``spurious'' readiness notifications, that is your callback might
be called with EV_READ but a subsequent read(2) will actually block
because there is no data. Not only are some backends known to create a
lot of those (for example solaris ports), it is very easy to get into
this situation even with a relatively standard program structure. Thus
it is best to always use non-blocking I/O: An extra read(2) returning
EAGAIN is far preferable to a program hanging until some data arrives.
If you cannot run the fd in non-blocking mode (for example you should not play around with an Xlib connection), then you have to seperately re-test whether a file descriptor is really ready with a known-to-be good interface such as poll (fortunately in our Xlib example, Xlib already does this on its own, so its quite safe to use).
Some backends (e.g. kqueue, epoll) need to be told about closing a file
descriptor (either by calling close explicitly or by any other means,
such as dup). The reason is that you register interest in some file
descriptor, but when it goes away, the operating system will silently drop
this interest. If another file descriptor with the same number then is
registered with libev, there is no efficient way to see that this is, in
fact, a different file descriptor.
To avoid having to explicitly tell libev about such cases, libev follows
the following policy: Each time ev_io_set is being called, libev
will assume that this is potentially a new file descriptor, otherwise
it is assumed that the file descriptor stays the same. That means that
you have to call ev_io_set (or ev_io_init) when you change the
descriptor even if the file descriptor number itself did not change.
This is how one would do it normally anyway, the important point is that the libev application should not optimise around libev but should leave optimisations to libev.
Some backends (e.g. epoll), cannot register events for file descriptors,
but only events for the underlying file descriptions. That means when you
have dup ()'ed file descriptors or weirder constellations, and register
events for them, only one file descriptor might actually receive events.
There is no workaround possible except not registering events
for potentially dup ()'ed file descriptors, or to resort to
EVBACKEND_SELECT or EVBACKEND_POLL.
Some backends (epoll, kqueue) do not support fork () at all or exhibit
useless behaviour. Libev fully supports fork, but needs to be told about
it in the child.
To support fork in your programs, you either have to call
ev_default_fork () or ev_loop_fork () after a fork in the child,
enable EVFLAG_FORKCHECK, or resort to EVBACKEND_SELECT or
EVBACKEND_POLL.
While not really specific to libev, it is easy to forget about SIGPIPE: when reading from a pipe whose other end has been closed, your program gets send 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).
Configures an ev_io watcher. The fd is the file descriptor to
rceeive events for and events is either EV_READ, EV_WRITE or
EV_READ | EV_WRITE to receive the given events.
The file descriptor being watched.
The events being watched.
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, struct ev_io *w, int revents)
{
ev_io_stop (loop, w);
.. read from stdin here (or from w->fd) and haqndle any I/O errors
}
... struct ev_loop *loop = ev_default_init (0); struct ev_io stdin_readable; ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ); ev_io_start (loop, &stdin_readable); ev_loop (loop, 0);
ev_timer - relative and optionally repeating 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) and hour. ``Roughly'' because detecting time jumps is hard, and some inaccuracies are unavoidable (the monotonic clock option helps a lot here).
The relative timeouts are calculated relative to the ev_now ()
time. This is usually the right thing as this timestamp refers to the time
of the event triggering whatever timeout you are modifying/starting. If
you suspect event processing to be delayed and you need to base the timeout
on the current time, use something like this to adjust for this:
ev_timer_set (&timer, after + ev_now () - ev_time (), 0.);
The callback is guarenteed to be invoked only after its timeout has passed, but if multiple timers become ready during the same loop iteration then order of execution is undefined.
Configure the timer to trigger after after seconds. If repeat
is 0., then it will automatically be stopped once the timeout is
reached. If it is positive, then the timer will automatically be
configured to trigger again repeat seconds later, again, and again,
until stopped manually.
The timer itself will do a best-effort at avoiding drift, that is, if you configure a timer to trigger every 10 seconds, then it will normally trigger at exactly 10 second intervals. If, however, your program cannot keep up with the timer (because it takes longer than those 10 seconds to do stuff) the timer will not fire more than once per event loop iteration.
This will act as if the timer timed out and restart it again if it is repeating. The exact semantics are:
If the timer is pending, its pending status is cleared.
If the timer is started but nonrepeating, stop it (as if it timed out).
If the timer is repeating, either start it if necessary (with the
repeat value), or reset the running timer to the repeat value.
This sounds a bit complicated, but here is a useful and typical
example: Imagine you have a tcp connection and you want a so-called idle
timeout, that is, you want to be called when there have been, say, 60
seconds of inactivity on the socket. The easiest way to do this is to
configure an ev_timer with a repeat value of 60 and then call
ev_timer_again 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 the after value and ev_timer_start
altogether and only ever use the repeat value and ev_timer_again:
ev_timer_init (timer, callback, 0., 5.); ev_timer_again (loop, timer); ... timer->again = 17.; ev_timer_again (loop, timer); ... timer->again = 10.; ev_timer_again (loop, timer);
This is more slightly efficient then stopping/starting the timer each time you want to modify its timeout value.
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, struct ev_timer *w, int revents)
{
.. one minute over, w is actually stopped right here
}
struct 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, struct ev_timer *w, int revents)
{
.. ten seconds without any activity
}
struct ev_timer mytimer; ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */ ev_timer_again (&mytimer); /* start timer */ ev_loop (loop, 0);
// and in some piece of code that gets executed on any "activity": // reset the timeout to start ticking again at 10 seconds ev_timer_again (&mytimer);
ev_periodic - to cron or not to cron?Periodic watchers are also timers of a kind, but they are very versatile (and unfortunately a bit complex).
Unlike ev_timer's, they are not based on real time (or relative time)
but on wallclock time (absolute time). 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 specifiying 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 more than year
to trigger the event (unlike an ev_timer, which would still trigger
roughly 10 seconds later as it uses a relative timeout).
ev_periodics can also be used to implement vastly more complex timers,
such as triggering an event on each ``midnight, local time'', or other
complicated, rules.
As with timers, the callback is guarenteed to be invoked only when the
time (at) has passed, but if multiple periodic timers become ready
during the same loop iteration then order of execution is undefined.
Lots of arguments, lets sort it out... There are basically three modes of operation, and we will explain them from simplest to complex:
In this configuration the watcher triggers an event after the wallclock
time at has passed and doesn't repeat. It will not adjust when a time
jump occurs, that is, if it is to be run at January 1st 2011 then it will
run when the system time reaches or surpasses this time.
In this mode the watcher will always be scheduled to time out at the next
at + N * interval time (for some integer N, which can also be negative)
and then repeat, regardless of any time jumps.
This can be used to create timers that do not drift with respect to system
time, for example, here is a ev_periodic that triggers each hour, on
the hour:
ev_periodic_set (&periodic, 0., 3600., 0);
This doesn't mean there will always be 3600 seconds in between triggers, but only that the 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 = at (mod interval), regardless of any time jumps.
For numerical stability it is preferable that the at value is near
ev_now () (the current time), but there is no range requirement for
this value, and in fact is often specified as zero.
Note also that there is an upper limit to how often a timer can fire (cpu
speed for example), so if interval is very small then timing stability
will of course detoriate. 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 at 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 event loop modifications whatsoever.
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)(struct ev_periodic
*w, ev_tstamp now), e.g.:
static ev_tstamp my_rescheduler (struct ev_periodic *w, ev_tstamp now)
{
return now + 60.;
}
It must return the next time to trigger, based on the passed time value (that is, the lowest time value larger than to the second argument). It will usually be called just before the callback will be triggered, but might be called at other times, too.
NOTE: This callback must always return a time that is higher than or
equal to the passed now value >.
This can be used to create very complex timers, such as a timer that
triggers on ``next midnight, local time''. To do this, you would calculate the
next midnight after now and return the timestamp value for this. How
you do this is, again, up to you (but it is not trivial, which is the main
reason I omitted it as an example).
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.
When repeating, this contains the offset value, otherwise this is the
absolute point in time (the at value passed to ev_periodic_set).
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 clock is divisible by 3600. The callback invocation times have potentially a lot of jittering, but good long-term stability.
static void
clock_cb (struct ev_loop *loop, struct ev_io *w, int revents)
{
... its now a full hour (UTC, or TAI or whatever your clock follows)
}
struct 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 (struct ev_periodic *w, ev_tstamp now)
{
return fmod (now, 3600.) + 3600.;
}
ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
Example: Call a callback every hour, starting now:
struct ev_periodic hourly_tick;
ev_periodic_init (&hourly_tick, clock_cb,
fmod (ev_now (loop), 3600.), 3600., 0);
ev_periodic_start (loop, &hourly_tick);
ev_signal - signal me when a signal gets signalled!Signal watchers will trigger an event when the process receives a specific signal one or more times. Even though signals are very asynchronous, libev will try it's best to deliver signals synchronously, i.e. as part of the normal event processing, like any other event.
You can configure as many watchers as you like per signal. Only when the first watcher gets started will libev actually register a signal watcher with the kernel (thus it coexists with your own signal handlers as long as you don't register any with libev). Similarly, when the last signal watcher for a signal is stopped libev will reset the signal handler to SIG_DFL (regardless of what it was set to before).
If possible and supported, libev will install its handlers with
SA_RESTART behaviour enabled, so syscalls should not be unduly
interrupted. If you have a problem with syscalls getting interrupted by
signals you can block all signals in an ev_check watcher and unblock
them in an ev_prepare watcher.
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 and SIGTERM.
static void
sigint_cb (struct ev_loop *loop, struct ev_signal *w, int revents)
{
ev_unloop (loop, EVUNLOOP_ALL);
}
struct ev_signal signal_watcher; ev_signal_init (&signal_watcher, sigint_cb, SIGINT); ev_signal_start (loop, &sigint_cb);
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). 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).
Only the default event loop is capable of handling signals, and therefore you can only rgeister child watchers in the default event loop.
Libev grabs SIGCHLD as soon as the default event loop is
initialised. This is necessary to guarantee proper behaviour even if
the first child watcher is started after the child exits. The occurance
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.
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_ struct 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 filesystem path for attribute changes. That is, it calls
stat regularly (or when the OS says it changed) and sees if it changed
compared to the last time, invoking the callback if it did.
The path does not need to exist: changing from ``path exists'' to ``path does
not exist'' is a status change like any other. The condition ``path does
not exist'' 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 should be absolute and must not end in a slash. If it is relative and your working directory changes, the behaviour is undefined.
Since there is no standard to do this, 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 thats
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, only the Linux inotify interface is
implemented (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). Inotify will be used to give hints only and should
not change the semantics of ev_stat watchers, which means that libev
sometimes needs to fall back to regular polling again even with inotify,
but changes are usually detected immediately, and if the file exists there
will be no polling.
Libev by default (unless the user overrides this) uses the default compilation environment, which means that on systems with optionally disabled large file support, 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 noticably with ev_stat and largefile support.
When inotify (7) support has been compiled into libev (generally only
available on Linux) 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.
(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).
The stat () syscall only supports full-second resolution portably, and
even on systems where the resolution is higher, many filesystems 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 it as the stat
data does not change.
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 EV_STAT 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 stating 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 filesystem 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).
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.
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, struct ev_idle *w, int revents)
{
free (w);
// now do something you wanted to do when the program has
// no longer anything immediate to do.
}
struct ev_idle *idle_watcher = malloc (sizeof (struct ev_idle)); ev_idle_init (idle_watcher, idle_cb); ev_idle_start (loop, idle_cb);
ev_prepare and ev_check - customise your event loop!Prepare and check watchers are usually (but not always) used in tandem: prepare watchers get invoked before the process blocks and check watchers afterwards.
You must not call ev_loop or similar functions that enter
the current event loop from either ev_prepare or ev_check
watchers. Other loops than the current one are fine, however. The
rationale behind this is that you do not need to check for recursion in
those watchers, i.e. the sequence will always be ev_prepare, blocking,
ev_check so if you have one watcher of each kind they will always be
called in pairs bracketing the blocking call.
Their main purpose is to integrate other event mechanisms into libev and
their use is somewhat advanced. This 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 just this functionality). Then, in the check watcher you check for
any events that occured (by checking the pending status of all watchers
and stopping them) and call back into the library. The I/O and timer
callbacks will never actually be called (but must be valid nevertheless,
because you never know, you know?).
As another example, the Perl Coro module uses these hooks to integrate coroutines into libev programs, by yielding to other active coroutines during each prepare and only letting the process block if no coroutines are ready to run (it's actually more complicated: it only runs coroutines with priority higher than or equal to the event loop and one coroutine of lower priority, but only once, using idle watchers to keep the event loop from blocking if lower-priority coroutines are active, thus mapping low-priority coroutines to idle/background tasks).
It is recommended to give ev_check watchers highest (EV_MAXPRI)
priority, to ensure that they are being run before any other watchers
after the poll. 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).
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 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 (ev_loop *loop, ev_io *w, int revents)
{
}
// create io watchers for each fd and a timer before blocking
static void
adns_prepare_cb (ev_loop *loop, ev_prepare *w, int revents)
{
int timeout = 3600000;
struct pollfd fds [nfd];
// actual code will need to loop here and realloc etc.
adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));
/* the callback is illegal, but won't be called as we stop during check */
ev_timer_init (&tw, 0, timeout * 1e-3);
ev_timer_start (loop, &tw);
// create one ev_io per pollfd
for (int i = 0; i < nfd; ++i)
{
ev_io_init (iow + i, io_cb, fds [i].fd,
((fds [i].events & POLLIN ? EV_READ : 0)
| (fds [i].events & POLLOUT ? EV_WRITE : 0)));
fds [i].revents = 0;
ev_io_start (loop, iow + i);
}
}
// stop all watchers after blocking
static void
adns_check_cb (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 (adns 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 too inflexible to support it. Instead, youc na 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 does
this.
static gint
event_poll_func (GPollFD *fds, guint nfds, gint timeout)
{
int got_events = 0;
for (n = 0; n < nfds; ++n)
// create/start io watcher that sets the relevant bits in fds[n] and increment got_events
if (timeout >= 0)
// create/start timer
// poll
ev_loop (EV_A_ 0);
// stop timer again
if (timeout >= 0)
ev_timer_stop (EV_A_ &to);
// stop io watchers again - their callbacks should have set
for (n = 0; n < nfds; ++n)
ev_io_stop (EV_A_ iow [n]);
return got_events;
}
ev_embed - when one backend isn't enough...This is a rather advanced watcher type that lets you embed one event loop
into another (currently only ev_io events are supported in the embedded
loop, other types of watchers might be handled in a delayed or incorrect
fashion and must not be used).
There are primarily two reasons you would want that: work around bugs and prioritise I/O.
As an example for a bug workaround, the kqueue backend might only support sockets on some platform, so it is unusable as generic backend, but you still want to make use of it because you have many sockets and it scales so nicely. In this case, you would create a kqueue-based loop and embed it into your default loop (which might use e.g. poll). Overall operation will be a bit slower because first libev has to poll and then call kevent, but at least you can use both at what they are best.
As for prioritising I/O: rarely 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 (you 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.
As long as the watcher is started it will automatically handle events. The
callback will be invoked whenever some events have been handled. You can
set the callback to 0 to avoid having to specify one if you are not
interested in that.
Also, there have not currently been made special provisions for forking:
when you fork, you not only have to call ev_loop_fork on both loops,
but you will also have to stop and restart any ev_embed watchers
yourself.
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.
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 thta, you need to temporarily stop the embed watcher).
Make a single, non-blocking sweep over the embedded loop. This works
similarly to ev_loop (embedded_loop, EVLOOP_NONBLOCK), but in the most
apropriate 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 mebeddable loop is stored in
loop_lo (which is loop_hi in the acse no embeddable loop can be
used).
struct ev_loop *loop_hi = ev_default_init (0);
struct ev_loop *loop_lo = 0;
struct 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;
struct 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_default_fork or ev_loop_fork). The invocation is done before the
event loop blocks next and before ev_check watchers are being called,
and only in the child after the fork. If whoever good citizen calling
ev_default_fork cheats and calls it in the wrong process, the fork
handlers will be invoked, too, of course.
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,
believe me.
ev_async - how to wake up another 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 another event loop you do not
control, for example because it belongs to another thread. This is what
ev_async watchers do: as long as the ev_async watcher is active, you
can signal it by calling ev_async_send, which is thread- and signal
safe.
This functionality is very similar to ev_signal watchers, as signals,
too, are asynchronous in nature, and signals, too, will be compressed
(i.e. the number of callback invocations may be less than the number of
ev_async_sent calls).
Unlike ev_signal watchers, ev_async works with any event loop, not
just the default loop.
ev_async does not support queueing of data in any way. The reason
is that the author does not know of a simple (or any) algorithm for a
multiple-writer-single-reader queue that works in all cases and doesn't
need elaborate support such as pthreads.
That means that if you want to queue data, you have to provide your own queue. But at least I can tell you would 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 fictitiuous 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_asynd_set macro, but using it is utterly pointless,
believe me.
Sends/signals/activates the given ev_async watcher, that is, feeds
an EV_ASYNC event on the watcher into the event loop. Unlike
ev_feed_event, this call is safe to do in other threads, signal or
similar contexts (see the dicusssion of EV_ATOMIC_T in the embedding
section below on what exactly this means).
This call incurs the overhead of a syscall only once per loop iteration,
so while the overhead might be noticable, it doesn't apply to repeated
calls to ev_async_send.
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 wether invoking the loop might be a good idea.
Not that this does not check wether the watcher itself is pending, only wether it has been requested to make this watcher pending.
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 stop 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 events
is being ignored. Otherwise, an ev_io watcher for the given fd and
events set will be craeted 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. While 0 is a valid timeout, it is of
dubious value.
The callback has the type void (*cb)(int revents, void *arg) and gets
passed an revents set like normal event callbacks (a combination of
EV_ERROR, EV_READ, EV_WRITE or EV_TIMEOUT) and the arg
value passed to ev_once:
static void stdin_ready (int revents, void *arg)
{
if (revents & EV_TIMEOUT)
/* doh, nothing entered */;
else if (revents & EV_READ)
/* stdin might have data for us, joy! */;
}
ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
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).
Feed an event on the given fd, as if a file descriptor backend detected the given events it.
Feed an event as if the given signal occured (loop must be the default
loop!).
Libev offers a compatibility emulation layer for libevent. It cannot emulate the internals of libevent, so here are some usage hints:
Libev comes with some simplistic wrapper classes for C++ that mainly allow you to use some convinience methods to start/stop watchers and also change the callback model to a model using method callbacks on objects.
To use it,
#include <ev++.h>
This automatically includes ev.h and puts all of its definitions (many
of them macros) into the global namespace. All C++ specific things are
put into the ev namespace. It should support all the same embedding
options as ev.h, most notably EV_MULTIPLICITY.
Care has been taken to keep the overhead low. The only data member the C++
classes add (compared to plain C-style watchers) is the event loop pointer
that the watcher is associated with (or no additional members at all if
you disable EV_MULTIPLICITY when embedding libev).
Currently, functions, and static and non-static member functions can be used as callbacks. Other types should be easy to add as long as they only need one additional pointer for context. If you need support for other types of functors please contact the author (preferably after implementing it).
Here is a list of things available in the ev namespace:
ev::READ, ev::WRITE etc.
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
defines by many implementations.
All of those classes have these methods:
The constructor (optionally) takes an event loop to associate the watcher
with. If it is omitted, it will use EV_DEFAULT.
The constructor calls ev_init for you, which means you have to call the
set method before starting it.
It will not set a callback, however: You have to call the templated set
method to set a callback before you can start the watcher.
(The reason why you have to use a method is a limitation in C++ which does not allow explicit template arguments for constructors).
The destructor automatically stops the watcher if it is active.
This method sets the callback method to call. The method has to have a
signature of void (*)(ev_TYPE &, int), it receives the watcher as
first argument and the revents as second. The object must be given as
parameter and is stored in the data member of the watcher.
This method synthesizes efficient thunking code to call your method from
the C callback that libev requires. If your compiler can inline your
callback (i.e. it is visible to it at the place of the set call and
your compiler is good :), then the method will be fully inlined into the
thunking function, making it as fast as a direct C callback.
Example: simple class declaration and watcher initialisation
struct myclass
{
void io_cb (ev::io &w, int revents) { }
}
myclass obj; ev::io iow; iow.set <myclass, &myclass::io_cb> (&obj);
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:
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, with the same args. Must be
called at least once. Unlike the C counterpart, an active watcher gets
automatically stopped and restarted when reconfiguring it with this
method.
Starts the watcher. Note that there is no loop argument, as the
constructor already stores the event loop.
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 an IO and idle watcher, start one of them in the constructor.
class myclass
{
ev::io io; void io_cb (ev::io &w, int revents);
ev:idle idle void idle_cb (ev::idle &w, int revents);
myclass (int fd)
{
io .set <myclass, &myclass::io_cb > (this);
idle.set <myclass, &myclass::idle_cb> (this);
io.start (fd, ev::READ);
}
};
Libev does not offer other language bindings itself, but bindings for a numbe rof 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), 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 found at http://software.schmorp.de/pkg/EV.
Tony Arcieri has written a ruby extension that offers access to a subset of the libev API and adds filehandle abstractions, asynchronous DNS and more on top of it. It can be found via gem servers. Its homepage is at http://rev.rubyforge.org/.
Leandro Lucarella has written a D language binding (ev.d) for libev, to be found at http://git.llucax.com.ar/.
Libev can be compiled with a variety of options, the most fundamantal
of which is EV_MULTIPLICITY. This option determines whether (most)
functions and callbacks have an initial struct ev_loop * argument.
To make it easier to write programs that cope with either variant, the following macros are defined:
EV_A, EV_A_
This provides the loop argument for functions, if one is required (``ev
loop argument''). The EV_A form is used when this is the sole argument,
EV_A_ is used when other arguments are following. Example:
ev_unref (EV_A); ev_timer_add (EV_A_ watcher); ev_loop (EV_A_ 0);
It assumes the variable loop of type struct ev_loop * is in scope,
which is often provided by the following macro.
EV_P, EV_P_
This provides the loop parameter for functions, if one is required (``ev
loop parameter''). The EV_P form is used when this is the sole parameter,
EV_P_ is used when other parameters are following. Example:
// this is how ev_unref is being declared static void ev_unref (EV_P);
// this is how you can declare your typical callback static void cb (EV_P_ ev_timer *w, int revents)
It declares a parameter loop of type struct ev_loop *, quite
suitable for use with EV_A.
EV_DEFAULT, EV_DEFAULT_
Similar to the other two macros, this gives you the value of the default loop, if multiple loops are supported (``ev loop default'').
EV_DEFAULT_UC, EV_DEFAULT_UC_
Usage identical to EV_DEFAULT and EV_DEFAULT_, but requires that the
default loop has been initialised (UC == unchecked). Their behaviour
is undefined when the default loop has not been initialised by a previous
execution of EV_DEFAULT, EV_DEFAULT_ or ev_default_init (...).
It is often prudent to use EV_DEFAULT when initialising the first
watcher in a function but use EV_DEFAULT_UC afterwards.
Example: Declare and initialise a check watcher, utilising the above macros so it will work regardless of whether multiple loops are supported or not.
static void
check_cb (EV_P_ ev_timer *w, int revents)
{
ev_check_stop (EV_A_ w);
}
ev_check check; ev_check_init (&check, check_cb); ev_check_start (EV_DEFAULT_ &check); ev_loop (EV_DEFAULT_ 0);
Libev can (and often is) directly embedded into host applications. Examples of applications that embed it include the Deliantra Game Server, the EV perl module, the GNU Virtual Private Ethernet (gvpe) and rxvt-unicode.
The goal is to enable you to just copy the necessary files into your source directory without having to change even a single line in them, so you can easily upgrade by simply copying (or having a checked-out copy of libev somewhere in your source tree).
Depending on what features you need you need to include one or more sets of files in your app.
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, thats a stated goal, and breakage will be treated as a bug).
You need the following files in your source tree, or in a directory in your include path (e.g. in libev/ when using -Ilibev):
ev.h ev.c ev_vars.h ev_wrap.h
ev_win32.c required on win32 platforms only
ev_select.c only when select backend is enabled (which is enabled by default) ev_poll.c only when poll backend is enabled (disabled by default) ev_epoll.c only when the epoll backend is enabled (disabled by default) ev_kqueue.c only when the kqueue backend is enabled (disabled by default) ev_port.c only when the solaris port backend is enabled (disabled by default)
ev.c includes the backend files directly when enabled, so you only need to compile this single file.
To include the libevent compatibility API, also include:
#include "event.c"
in the file including ev.c, and:
#include "event.h"
in the files that want to use the libevent API. This also includes ev.h.
You need the following additional files for this:
event.h event.c
Instead of using EV_STANDALONE=1 and providing your config in
whatever way you want, you can also m4_include([libev.m4]) in your
configure.ac and leave EV_STANDALONE undefined. ev.c will then
include config.h and configure itself accordingly.
For this of course you need the m4 file:
libev.m4
Libev can be configured via a variety of preprocessor symbols you have to define before including any of its files. The default in the absense of autoconf is noted for every option.
Must always be 1 if you do not use autoconf configuration, which
keeps libev from including config.h, and it also defines dummy
implementations for some libevent functions (such as logging, which is not
supported). It will also not define any of the structs usually found in
event.h that are not directly supported by the libev core alone.
If defined to be 1, libev will try to detect the availability of the
monotonic clock option at both compiletime 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).
If defined to be 1, libev will try to detect the availability of the
realtime clock option at compiletime (and assume its availability at
runtime if successful). Otherwise no use of the realtime 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.
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 undefined or defined to be 1, libev will compile in support for the
select(2) backend. No attempt at autodetection 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 misguesses 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, might
influence the size of the fd_set used.
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 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 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.
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 absense 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.
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 app does not need any priorities, defining these both to
0 will save some memory and cpu.
If undefined or defined to be 1, then periodic timers are supported. If
defined to be 0, then they are not. Disabling them saves a few kB of
code.
If undefined or defined to be 1, then idle watchers are supported. If
defined to be 0, then they are not. Disabling them saves a few kB of
code.
If undefined or defined to be 1, then embed watchers are supported. If
defined to be 0, then they are not.
If undefined or defined to be 1, then stat watchers are supported. If
defined to be 0, then they are not.
If undefined or defined to be 1, then fork watchers are supported. If
defined to be 0, then they are not.
If undefined or defined to be 1, then async watchers are supported. If
defined to be 0, then they are not.
If you need to shave off some kilobytes of code at the expense of some
speed, define this symbol to 1. Currently this is used to override some
inlining decisions, saves roughly 30% codesize of amd64. It also selects a
much smaller 2-heap for timer management over the default 4-heap.
ev_child watchers use a small hash table to distribute workload by
pid. The default size is 16 (or 1 with EV_MINIMAL), usually more
than enough. If you need to manage thousands of children you might want to
increase this value (must be a power of two).
ev_stat watchers use a small hash table to distribute workload by
inotify watch id. The default size is 16 (or 1 with EV_MINIMAL),
usually more than enough. If you need to manage thousands of ev_stat
watchers you might want to increase this value (must be a power of
two).
Heaps are not very cache-efficient. To improve the cache-efficiency of the
timer and periodics heap, libev uses a 4-heap when this symbol is defined
to 1. The 4-heap uses more complicated (longer) code but has
noticably faster performance with many (thousands) of watchers.
The default is 1 unless EV_MINIMAL is set in which case it is 0
(disabled).
Heaps are not very cache-efficient. To improve the cache-efficiency of the
timer and periodics heap, 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
noticably with with many (hundreds) of watchers.
The default is 1 unless EV_MINIMAL is set in which case it is 0
(disabled).
Controls how much internal verification (see ev_loop_verify ()) will
be done: If set to 0, no internal verification code will be compiled
in. If set to 1, then verification code will be compiled in, but not
called. If set to 2, then the internal verification code will be
called once per loop, which can slow down libev. If set to 3, then the
verification code will be called very frequently, which will slow down
libev considerably.
The default is 1, unless EV_MINIMAL is set, in which case it will be
0.
By default, all watchers have a void *data member. By redefining
this macro to a something else you can include more and other types of
members. You have to define it each time you include one of the files,
though, and it must be identical each time.
For example, the perl EV module uses something like this:
#define EV_COMMON \
SV *self; /* contains this struct */ \
SV *cb_sv, *fh /* note no trailing ";" */
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 inconvinient to avoid this).
A sed command like this will create wrapper #define's that you need to
include before including ev.h:
<Symbols.ev sed -e "s/.*/#define & myprefix_&/" >wrap.h
This would create a file wrap.h which essentially looks like this:
#define ev_backend myprefix_ev_backend #define ev_check_start myprefix_ev_check_start #define ev_check_stop myprefix_ev_check_stop ...
For a real-world example of a program the includes libev verbatim, you can have a look at the EV perl module (http://software.schmorp.de/pkg/EV.html). It has the libev files in the libev/ subdirectory and includes them in the EV/EVAPI.h (public interface) and EV.xs (implementation) files. Only the EV.xs file will be compiled. It is pretty complex because it provides its own header file.
The usage in rxvt-unicode is simpler. It has a ev_cpp.h header file that everybody includes and which overrides some configure choices:
#define EV_MINIMAL 1 #define EV_USE_POLL 0 #define EV_MULTIPLICITY 0 #define EV_PERIODIC_ENABLE 0 #define EV_STAT_ENABLE 0 #define EV_FORK_ENABLE 0 #define EV_CONFIG_H <config.h> #define EV_MINPRI 0 #define EV_MAXPRI 0
#include "ev++.h"
And a ev_cpp.C implementation file that contains libev proper and is compiled:
#include "ev_cpp.h" #include "ev.c"
Libev itself is completely threadsafe, but it uses no locking. This means that you can use as many loops as you want in parallel, as long as only one thread ever calls into one libev function with the same loop parameter.
Or put differently: calls with different loop parameters can be done in parallel 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).
If you want to know which design is best for your problem, then I cannot help you but by giving 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.
Chosing a model is hard - look around, learn, know that usually you cna do better than you currently do :-)
ev_async watchers can be used to wake them up from other
threads safely (or from signal contexts...).
Libev is much more accomodating to coroutines (``cooperative threads''):
libev fully supports nesting calls to it's functions from different
coroutines (e.g. you can call ev_loop on the same loop from two
different coroutines and switch freely between both coroutines running the
loop, as long as you don't confuse yourself). The only exception is that
you must not do this from ev_periodic reschedule callbacks.
Care has been invested into making sure that libev does not keep local
state inside ev_loop, and other calls do not usually allow coroutine
switches.
In this section the complexities of (many of) the algorithms used inside
libev will be explained. 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 never with higher number of elements, so O(1) might
mean it might do 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.
O(1)
These just add the watcher into an array or at the head of a list.
O(1)
These watchers are stored in lists then 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).
O(1)
By virtue of using a binary or 4-heap, the next timer is always found at a fixed position in the storage array.
O(number_of_watchers_for_this_fd)
A change means an I/O watcher gets started or stopped, which requires
libev to recalculate its status (and possibly tell the kernel, depending
on backend and wether ev_io_set was used).
O(1)
O(number_of_priorities)
Priorities are implemented by allocating some space for each
priority. When doing priority-based operations, libev usually has to
linearly search all the priorities, but starting/stopping and activating
watchers becomes O(1) w.r.t. priority handling.
O(1)
O(number_of_async_watchers)
O(max_signal_number)
Sending involves a syscall iff there were no other ev_async_send
calls in the current loop iteration. Checking for async and signal events
involves iterating over all running async watchers or all signal numbers.
Win32 doesn't support any of the standards (e.g. POSIX) that libev
requires, and its I/O model is fundamentally incompatible with the POSIX
model. Libev still offers limited functionality on this platform in
the form of the EVBACKEND_SELECT backend, and only supports socket
descriptors. This only applies when using Win32 natively, not when using
e.g. cygwin.
Lifting these limitations would basically require the full re-implementation of the I/O system. If you are into these kinds of things, then note that glib does exactly that for you in a very portable way (note also that glib is the slowest event library known to man).
There is no supported compilation method available on windows except embedding it into other applications.
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 (microsoft monopoly games).
The winsocket select function doesn't follow POSIX in that it
requires socket handles and not socket file descriptors (it is
also extremely buggy). This makes select very inefficient, and also
requires a mapping from file descriptors to socket handles. 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. 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 does its 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, libev relies on a few additional extensions:
sig_atomic_t volatile must be thread-atomic as well
The type sig_atomic_t volatile (or whatever is defined as
EV_ATOMIC_T) must be atomic w.r.t. accesses from different
threads. This is not part of the specification for sig_atomic_t, but is
believed to be sufficiently portable.
sigprocmask must work in a threaded environment
Libev uses sigprocmask to temporarily block signals. This is not
allowed in a threaded program (pthread_sigmask has to be used). Typical
pthread implementations will either allow sigprocmask in the ``main
thread'' or will block signals process-wide, both behaviours would
be compatible with libev. Interaction between sigprocmask and
pthread_sigmask could complicate things, however.
The most portable way to handle signals is to block signals in all threads except the initial one, and run the default loop in the initial thread as well.
long must be large enough for common memory allocation sizes
To improve portability and simplify using libev, libev uses long
internally instead of size_t when allocating its data structures. On
non-POSIX systems (Microsoft...) this might be unexpectedly low, but
is still at least 31 bits everywhere, which is enough for hundreds of
millions of watchers.
double must hold a time value in seconds with enough accuracy
The type double is used to represent timestamps. It is required to
have at least 51 bits of mantissa (and 9 bits of exponent), which is good
enough for at least into the year 4000. This requirement is fulfilled by
implementations implementing IEEE 754 (basically all existing ones).
If you know of other additional requirements drop me a note.
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 targetting 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 cindition their message seems to warn about).
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, but 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. Similarly, under some circumstances, valgrind might report kernel bugs as if it were a bug in libev, or it might be confused (it is a very good tool, but only a tool).
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. 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.
Marc Lehmann <libev@schmorp.de>.