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libev is a high-performance event loop/event model with lots of features.
(see benchmark at
It is modelled (very losely) after libevent
( and the Event perl module, but aims
to be faster and more correct, and also more featureful.
If you downloaded a distribution of libev, you will find it looks
very much like libevent. In fact, the distributed libev tarballs are
indeed libevent tarballs patched up with the libev event core, taking
the evbuffer, evtag, evdns and evhttpd parts from libevent (they use
the libevent emulation inside libev). Configure and Makefile stuff is
also a more or less direct copy of libevent, and are maintained by the
libevent authors.
If you are looking for an easily embeddable version, I recommend using
the CVS repository (linked from the homepage, above), which contains
only the libev core parts.
Examples of programs that embed libev: the EV perl module,
rxvt-unicode, gvpe (GNU Virtual Private Ethernet) and deliantra
The comparisons below are relative to libevent-1.3e.
- multiple watchers can wait for the same event without deregistering others,
both for file descriptors as well as signals.
(registering two read events on fd 10 and unregistering one will not
break the other).
- fork() is supported and can be handled
(there is no way to recover from a fork with libevent).
- timers are handled as a priority queue (important operations are O(1))
(libevent uses a much less efficient but more complex red-black tree).
- supports absolute (wallclock-based) timers in addition to relative ones,
i.e. can schedule timers to occur after n seconds, or at a specific time.
- timers can be repeating (both absolute and relative ones).
- absolute timers can have customised rescheduling hooks (suitable for cron-like
- detects time jumps and adjusts timers
(works for both forward and backward time jumps and also for absolute timers).
- race-free signal processing
(libevent may delay processing signals till after the next event).
- more efficient epoll backend
(stopping and starting an io watcher between two loop iterations will not
result in spurious epoll_ctl calls).
- usually less calls to gettimeofday and clock_gettime
(libevent calls it on every timer event change, libev twice per iteration).
- watchers use less memory
(libevent watcher on amd64: 152 bytes, libev native: <= 56 bytes, libevent emulation: 144 bytes).
- library uses less memory
(libevent allocates large data structures wether used or not, libev
scales all its data structures dynamically).
- no hardcoded arbitrary limits
(libevent contains an off-by-one bug and sometimes hardcodes limits).
- libev separates timer, signal and io watchers from each other
(libevent combines them, but with libev you can combine them yourself
by reusing the same callback and still save memory).
- simpler design, backends are potentially much simpler
(in libevent, backends have to deal with watchers, thus the problems with
wildly different semantics between diferent backends)
(epoll backend in libevent: 366 lines no caching, libev: 90 lines full caching).
- libev handles EBADF gracefully by removing the offending fds.
- libev communicates errors to the callback, libevent to the
event adder or not at all.
- doesn't rely on nonportable BSD header files.
- an event.h compatibility header exists, and can be used to run a wide
range of libevent programs unchanged (such as evdns.c).
- win32 compatibility for the core parts.
(the backend is fd-based as documented and on other platforms,
not handle-based like libevent, and can be used for both winscoket environments
and unix-like ones).
- libev can be embedded easily with or without autoconf support into
other programs, with no changes to the source code necessary.
- the event core library (ev and event layer) compiles and works both as
C and C++.
- a simple C++ wrapper that supports methods as callbacks exists.
- a full featured and widely used perl module is available.
whats missing?
- no event-like priority support at the moment (the ev priorities work
differently, but you can use idle watchers to get a similar effect).
libev was written and designed by Marc Lehmann and Emanuele Giaquinta.
The following people sent in patches or made other noteworthy
contributions to the design (if I forgot to include you, please shout
at me, it was an accident):
W.C.A. Wijngaards
Christopher Layne
Chris Brody