libev - a high performance full-featured event loop written in C
/* this is the only header you need */ - #include <ev.h> +#include <ev.h> + ++ +
#include <ev.h> - /* what follows is a fully working example program */ ev_io stdin_watcher; ev_timer timeout_watcher; @@ -113,8 +119,11 @@
The newest version of this document is also available as a html-formatted +web page you might find easier to navigate when reading it for the first +time: http://cvs.schmorp.de/libev/ev.html.
Libev is an event loop: you register interest in certain events (such as a file descriptor being readable or a timeout occuring), and it will manage these event sources and provide your program with events.
@@ -127,29 +136,34 @@ details of the event, and then hand it over to libev by starting the watcher.Libev supports select, poll, the linux-specific epoll and the bsd-specific -kqueue mechanisms for file descriptor events, relative timers, absolute -timers with customised rescheduling, signal events, process status change -events (related to SIGCHLD), and event watchers dealing with the event -loop mechanism itself (idle, prepare and check watchers). It also is quite -fast (see this benchmark comparing -it to libevent for example).
+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 configuration
-will be described, which supports multiple event loops. For more info
-about various configuration options please have a look at the file
-README.embed in the libev distribution. 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 is very configurable. In this manual the default 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
@@ -159,7 +173,7 @@ to the double type in C, and when you need to do any calculations o
it, you should treat it as such.
These functions can be called anytime, even before initialising the library in any way.
@@ -182,8 +196,8 @@ version of the library your program was compiled against. 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:
+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));
@@ -221,18 +235,19 @@ might be supported on the current system, you would need to look at
recommended ones.
See the description of ev_embed watchers for more info.
- Sets the allocation function to use (the prototype and semantics are -identical to the realloc C function). It is used to allocate and free -memory (no surprises here). If it returns zero when memory needs to be -allocated, the library might abort or take some potentially destructive -action. The default is your system realloc function.
+Sets the allocation function to use (the prototype is similar - the +semantics is identical - to the realloc C function). It is used to +allocate and free memory (no surprises here). If it returns zero when +memory needs to be allocated, the library might abort or take some +potentially destructive action. The default is your system realloc +function.
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: better than mine).
+Example: Replace the libev allocator with one that waits a bit and then +retries).
static void *
persistent_realloc (void *ptr, size_t size)
{
@@ -261,7 +276,7 @@ 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: do the same thing as libev does internally:
+ Example: This is basically the same thing that libev does internally, too.
static void
fatal_error (const char *msg)
{
@@ -277,7 +292,7 @@ requested operation, or, if the condition doesn't go away, do bad stuff
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
@@ -315,6 +330,23 @@ or setgid) then libev will not look at the environment variable
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_FORKCHECKInstead 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
+Linux system for example, getpid is actually a simple 5-insn sequence
+without a syscall and thus very fast, but my 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)Example: try to create a event loop that uses epoll and nothing else.
+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");
@@ -453,6 +485,15 @@ do not need to care.
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 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.
Example: queue some jobs and then loop until no events are outsanding +
Example: Queue some jobs and then loop until no events are outsanding 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..)
@@ -539,17 +580,17 @@ visible to the libev user and should not keep ev_loop from exiting
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.
- Example: create a signal watcher, but keep it from keeping ev_loop
+
Example: Create a signal watcher, but keep it from keeping ev_loop
running when nothing else is active.
- struct dv_signal exitsig;
+ struct ev_signal exitsig;
ev_signal_init (&exitsig, sig_cb, SIGINT);
- ev_signal_start (myloop, &exitsig);
- evf_unref (myloop);
+ ev_signal_start (loop, &exitsig);
+ evf_unref (loop);
- Example: for some weird reason, unregister the above signal handler again.
- ev_ref (myloop);
- ev_signal_stop (myloop, &exitsig);
+ Example: For some weird reason, unregister the above signal handler again.
+ ev_ref (loop);
+ ev_signal_stop (loop, &exitsig);
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
@@ -733,7 +774,7 @@ is pending (but not active) you must not call an init function on it (but
ev_TYPE_set is safe) and you must make sure the watcher is available to
libev (e.g. you cnanot free () it).
Returns the callback currently set on the watcher.
free () it).
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.
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.
More interesting and less C-conformant ways of catsing your callback type -have been omitted....
+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,
@@ -833,7 +922,7 @@ it is best to always use non-blocking I/O: An extra read(2) returni
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 -wether a file descriptor is really ready with a known-to-be good interface +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).
EV_READ, EV_WRITEThe events being watched.
Example: call stdin_readable_cb when STDIN_FILENO has become, well
+
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)
{
@@ -914,20 +1003,21 @@ 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 started but nonrepeating, stop it.
-If the timer is repeating, either start it if necessary (with the repeat -value), or reset the running timer to the repeat value.
+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 after=repeat=60 and calling
+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 stop the timer, and again will automatically restart it if
-need be.
You can also ignore the after value and ev_timer_start altogether
-and only ever use the repeat value:
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);
...
@@ -938,8 +1028,8 @@ and only ever use the repeat value:
ev_timer_again (loop, timer);
- This is more efficient then stopping/starting the timer eahc time you want -to modify its timeout value.
+This is more slightly efficient then stopping/starting the timer each time +you want to modify its timeout value.
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.
+Example: Create a timer that fires after 60 seconds.
static void
one_minute_cb (struct ev_loop *loop, struct ev_timer *w, int revents)
{
@@ -960,7 +1050,7 @@ which is also when any modifications are taken into account.
ev_timer_start (loop, &mytimer);
-Example: create a timeout timer that times out after 10 seconds of +
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) @@ -1086,7 +1176,7 @@ switched off. Can be changed any time, but changes only take effect when the periodic timer fires orev_periodic_againis being called. -Example: call a callback every hour, or, more precisely, whenever the +
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 @@ -1100,7 +1190,7 @@ potentially a lot of jittering, but good long-term stability. ev_periodic_start (loop, &hourly_tick);-Example: the same as above, but use a reschedule callback to do it:
+Example: The same as above, but use a reschedule callback to do it:
#include <math.h> static ev_tstamp @@ -1112,7 +1202,7 @@ potentially a lot of jittering, but good long-term stability. ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);-Example: call a callback every hour, starting now:
+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); @@ -1183,7 +1273,7 @@ process causing the status change.waitpidandsys/wait.hdocumentation for details). -Example: try to exit cleanly on SIGINT and SIGTERM.
+Example: Try to exit cleanly on SIGINT and SIGTERM.
static void sigint_cb (struct ev_loop *loop, struct ev_signal *w, int revents) { @@ -1210,8 +1300,10 @@ not exist" is a status change like any other. The condition "path does not exist" is signified by thest_nlinkfield 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)regulalry on the path to see if it changed somehow. You +callsstat (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 of0(highly recommended!) then a suitable, unspecified default value will be used (which you can expect to be around @@ -1221,8 +1313,13 @@ 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, no specific OS backends are implemented, but -if demand increases, at least a kqueue and inotify backend will be added.
+At the time of this writing, only the Linux inotify interface is +implemented (implementing kqueue support is left as an exercise for the +reader). Inotify will be used to give hints only and should not change the +semantics of
ev_statwatchers, 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.
stating the file.
ev_idle - when you've got nothing better to do...Idle watchers trigger events when there are no other events 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) it will not be triggered. But when your process is idle all idle -watchers are being called again and again, once per event loop iteration - -until stopped, that is, or your process receives more events and becomes -busy.
+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
@@ -1316,8 +1415,8 @@ kind. There is a ev_idle_set macro, but using it is utterly pointle
believe me.
Example: dynamically allocate an ev_idle, start it, and in the
-callback, free it. Alos, use no error checking, as usual.
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)
{
@@ -1401,7 +1500,8 @@ pseudo-code only of course:
static void
adns_prepare_cb (ev_loop *loop, ev_prepare *w, int revents)
{
- int timeout = 3600000;truct pollfd fds [nfd];
+ 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 ()));
@@ -1552,7 +1652,7 @@ believe me.
There are some other functions of possible interest. Described. Here. Now.
Libev offers a compatibility emulation layer for libevent. It cannot emulate the internals of libevent, so here are some usage hints:
@@ -1629,7 +1729,7 @@ to use the libev header file and library.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 @@ -1734,10 +1834,10 @@ the constructor.
Libev can be compiled with a variety of options, the most fundemantal is
-EV_MULTIPLICITY. This option determines wether (most) functions and
+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:
@@ -1776,8 +1876,9 @@ suitable for use withEV_A.
loop, if multiple loops are supported ("ev loop default").
-Example: Declare and initialise a check watcher, working regardless of -wether multiple loops are supported or not.
+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)
{
@@ -1789,13 +1890,10 @@ wether multiple loops are supported or not.
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 @@ -1842,7 +1940,7 @@ in your include path (e.g. in libev/ when using -Ilibev):
ev_win32.c required on win32 platforms only - ev_select.c only when select backend is enabled (which is by default) + 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) @@ -1979,6 +2077,12 @@ 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.
The name of the ev.h header file used to include it. The default if
@@ -2010,11 +2114,31 @@ will have the struct ev_loop * as first argument, and you can creat
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.
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.
ev_staz 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).
The usage in rxvt-unicode is simpler. It has a ev_cpp.h header file -that everybody includes and which overrides some autoconf choices:
-#define EV_USE_POLL 0 +that everybody includes and which overrides some configure choices: +#define EV_MINIMAL 1 + #define EV_USE_POLL 0 #define EV_MULTIPLICITY 0 - #define EV_PERIODICS 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" @@ -2099,7 +2236,7 @@ that everybody includes and which overrides some autoconf choices:
In this section the complexities of (many of) the algorithms used inside
libev will be explained. For complexity discussions about backends see the
@@ -2107,13 +2244,43 @@ documentation for ev_default_init.
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 those 100 watchers.
+That means that for changing a timer costs less than removing/adding them +as only the relative motion in the event queue has to be paid for.
+These just add the watcher into an array or at the head of a list. If +the array needs to be extended libev needs to realloc and move the whole +array, but this happen asymptotically less and less with more watchers, +thus amortised 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).
+A change means an I/O watcher gets started or stopped, which requires +libev to recalculate its status (and possibly tell the kernel).
+Priorities are implemented by allocating some space for each +priority. When doing priority-based operations, libev usually has to +linearly search all the priorities.
+ev_default_init.
Marc Lehmann <libev@schmorp.de>.