libev - 用C写的高性能多功能事件循环库

[code type="c"]
#include
[/code]

[code type="c"]
// a single header file is required
#include
// every watcher type has its own typedef'd struct
// with the name ev_;

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;
}

[/code]

本手册最新版本为html格式的web页面，你在首次读它的时候更容易浏览：http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod.

Libev 是一个事件循环：你注册感兴趣的事件（如文件描述符可读或者发生超时），它还管理这些事件源并为你的程序提供这些事件。

要实现这些，至少要在你的进程（或者线程）中执行event loop句柄控制，然后就能通过回调机制进行事件通信。

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 [code type="c"]select[/code], [code type="c"]poll[/code], the Linux-specific [code type="c"]epoll[/code], the

BSD-specific [code type="c"]kqueue[/code] and the Solaris-specific event port mechanisms

for file descriptor events ([code type="c"]ev_io[/code]), the Linux [code type="c"]inotify[/code] interface

(for [code type="c"]ev_stat[/code]), relative timers ([code type="c"]ev_timer[/code]), absolute timers

with customised rescheduling ([code type="c"]ev_periodic[/code]), synchronous signals

([code type="c"]ev_signal[/code]), process status change events ([code type="c"]ev_child[/code]), and event

watchers dealing with the event loop mechanism itself ([code type="c"]ev_idle[/code],

[code type="c"]ev_embed[/code], [code type="c"]ev_prepare[/code] and [code type="c"]ev_check[/code] watchers) as well as

file watchers ([code type="c"]ev_stat[/code]) and even limited support for fork events

([code type="c"]ev_fork[/code]).

It also is quite fast (see this

benchmark comparing it to libevent

for example).

Libev 非常容量配置。在本手册中描述的都是默认配置（一般都如此），默认配置支持多事件循环。要了解更多关于可变配置的信息，请阅读本手册
EMBED 一节。如果libev配置为不支持多事件循环，那么所有带初始参数 [code type="c"]loop[/code] (它总是 [code type="c"]struct ev_loop *[/code] 类型的)的函数不再有这个参数。

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 [code type="c"]ev_tstamp[/code], which is what you should use too. It usually aliases

to the [code type="c"]double[/code] type in C, and when you need to do any calculations on

it, you should treat it as some floating point value. Unlike the name

component [code type="c"]stamp[/code] might indicate, it is also used for time differences

throughout libev.

Libev 知道三类错误：操作系统错误、用法错误和内部错误（bugs）

当 libev 捕捉无法处理的到操作系统错误时（如一个系统调用返回了libev不能修正的返回值），它调用通过 [code type="c"]ev_set_syserr_cb[/code]设定的回调函数,它假设这个回调函数可修正这个问题或者是退出。默认行为是打印一个摘要信息并调用 [code type="c"]abort()[/code]。

当 libev 检测到用法错误（如负的时间），它会打印一条摘要信息并退出（使用 [code type="c"]assert[/code] 机制,因此宏 [code type="c"]NDEBUG[/code] 会关闭这种检测）; 其实这里有libev调用端的编程错误需要修正。

Libev also has a few internal error-checking [code type="c"]assert[/code]ions, and also has

extensive consistency checking code. These do not trigger under normal

circumstances, as they indicate either a bug in libev or worse.

可以在任何时间调用这些函数，甚至可在初始化库之前。

ev_tstamp ev_time ()

Returns the current time as libev would use it. Please note that the

[code type="c"]ev_now[/code] function is usually faster and also often returns the timestamp

you actually want to know.

ev_sleep (ev_tstamp interval)

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 sub-second-resolution [code type="c"]sleep ()[/code].

int ev_version_major ()

int ev_version_minor ()

You can find out the major and minor ABI version numbers of the library

you linked against by calling the functions [code type="c"]ev_version_major[/code] and

[code type="c"]ev_version_minor[/code]. If you want, you can compare against the global

symbols [code type="c"]EV_VERSION_MAJOR[/code] and [code type="c"]EV_VERSION_MINOR[/code], 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.

[code type="c"] assert (("libev version mismatch";,

ev_version_major () == EV_VERSION_MAJOR

&& ev_version_minor () >= EV_VERSION_MINOR));



[/code]

unsigned int ev_supported_backends ()

Return the set of all backends (i.e. their corresponding [code type="c"]EV_BACKEND_*[/code]

value) compiled into this binary of libev (independent of their

availability on the system you are running on). See [code type="c"]ev_default_loop[/code] 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

[code type="c"] assert (("sorry, no epoll, no sex",

ev_supported_backends () & EVBACKEND_EPOLL));



[/code]

unsigned int ev_recommended_backends ()

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 [code type="c"]ev_supported_backends[/code], as for example kqueue is broken on

most BSDs and will not be auto-detected unless you explicitly request it

(assuming you know what you are doing). This is the set of backends that

libev will probe for if you specify no backends explicitly.

unsigned int ev_embeddable_backends ()

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

[code type="c"]ev_embeddable_backends () & ev_supported_backends ()[/code], likewise for

recommended ones.

See the description of [code type="c"]ev_embed[/code] watchers for more info.

ev_set_allocator (void *(*cb)(void *ptr, long size))

Sets the allocation function to use (the prototype is similar - the

semantics are identical to the [code type="c"]realloc[/code] 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 ([code type="c"]size != 0[/code]), the library might abort

or take some potentially destructive action.

Since some systems (at least OpenBSD and Darwin) fail to implement

correct [code type="c"]realloc[/code] semantics, libev will use a wrapper around the system

[code type="c"]realloc[/code] and [code type="c"]free[/code] 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 [code type="c"]realloc[/code]).

[code type="c"] 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);

[/code]

ev_set_syserr_cb (void (*cb)(const char *msg));

Set the callback function to call on a retryable system call error (such

as failed select, poll, epoll_wait). The message is a printable string

indicating the system call or subsystem causing the problem. If this

callback is set, then libev will expect it to remedy the situation, no

matter what, when it returns. That is, libev will generally retry the

requested operation, or, if the condition doesn't go away, do bad stuff

(such as abort).

Example: This is basically the same thing that libev does internally, too.

[code type="c"] static void
fatal_error (const char *msg)
{
perror (msg);
abort ();
}
...

ev_set_syserr_cb (fatal_error);

[/code]

An event loop is described by a [code type="c"]struct ev_loop *[/code]. The library knows two

types of such loops, the default loop, which supports signals and child

events, and dynamically created loops which do not.

struct ev_loop *ev_default_loop (unsigned int flags)

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 [code type="c"]ev_backend ()[/code] 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 [code type="c"]ev_signal[/code] and

[code type="c"]ev_child[/code] watchers, and to do this, it always registers a handler

for [code type="c"]SIGCHLD[/code]. If this is a problem for your application you can either

create a dynamic loop with [code type="c"]ev_loop_new[/code] that doesn't do that, or you

can simply overwrite the [code type="c"]SIGCHLD[/code] signal handler after calling

[code type="c"]ev_default_init[/code].

The flags argument can be used to specify special behaviour or specific

backends to use, and is usually specified as [code type="c"]0[/code] (or [code type="c"]EVFLAG_AUTO[/code]).

The following flags are supported:

[code type="c"]EVFLAG_AUTO[/code]

The default flags value. Use this if you have no clue (it's the right

thing, believe me).

[code type="c"]EVFLAG_NOENV[/code]

If this flag bit is or'ed into the flag value (or the program runs setuid

or setgid) then libev will not look at the environment variable

[code type="c"]LIBEV_FLAGS[/code]. 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.

[code type="c"]EVFLAG_FORKCHECK[/code]

Instead of calling [code type="c"]ev_default_fork[/code] or [code type="c"]ev_loop_fork[/code] manually after

a fork, you can also make libev check for a fork in each iteration by

enabling this flag.

This works by calling [code type="c"]getpid ()[/code] 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, [code type="c"]getpid[/code] is actually a simple 5-insn sequence

without a system call and thus very fast, but my GNU/Linux system also has

[code type="c"]pthread_atfork[/code] which is even faster).

The big advantage of this flag is that you can forget about fork (and

forget about forgetting to tell libev about forking) when you use this

flag.

This flag setting cannot be overridden or specified in the [code type="c"]LIBEV_FLAGS[/code]

environment variable.

[code type="c"]EVBACKEND_SELECT[/code] (value 1, portable select backend)

This is your standard select(2) backend. Not completely standard, as

libev tries to roll its own fd_set with no limits on the number of fds,

but if that fails, expect a fairly low limit on the number of fds when

using this backend. It doesn't scale too well (O(highest_fd)), but its

usually the fastest backend for a low number of (low-numbered :) fds.

To get good performance out of this backend you need a high amount of

parallelism (most of the file descriptors should be busy). If you are

writing a server, you should [code type="c"]accept ()[/code] in a loop to accept as many

connections as possible during one iteration. You might also want to have

a look at [code type="c"]ev_set_io_collect_interval ()[/code] to increase the amount of

readiness notifications you get per iteration.

[code type="c"]EVBACKEND_POLL[/code] (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 [code type="c"]EVBACKEND_SELECT[/code], above, for

performance tips.

[code type="c"]EVBACKEND_EPOLL[/code] (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 system call 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 system call per such incident

(because the fd could point to a different file description now), so its

best to avoid that. Also, [code type="c"]dup ()[/code]'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 embeddable in other event loops, this feature is broken in

all kernel versions tested so far.

[code type="c"]EVBACKEND_KQUEUE[/code] (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 "auto-detected"

unless you explicitly specify it explicitly in the flags (i.e. using

[code type="c"]EVBACKEND_KQUEUE[/code]) 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 [code type="c"]ev_embed[/code] watchers for more info.

It scales in the same way as the epoll backend, but the interface to the

kernel is more efficient (which says nothing about its actual speed, of

course). While stopping, setting and starting an I/O watcher does never

cause an extra system call as with [code type="c"]EVBACKEND_EPOLL[/code], it still adds up to

two event changes per incident, support for [code type="c"]fork ()[/code] 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. [code type="c"]EVBACKEND_SELECT[/code] or [code type="c"]EVBACKEND_POLL[/code]) and using it only for

sockets.

[code type="c"]EVBACKEND_DEVPOLL[/code] (value 16, Solaris 8)

This is not implemented yet (and might never be, unless you send me an

implementation). According to reports, [code type="c"]/dev/poll[/code] only supports sockets

and is not embeddable, which would limit the usefulness of this backend

immensely.

[code type="c"]EVBACKEND_PORT[/code] (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" [code type="c"]EVBACKEND_SELECT[/code] or [code type="c"]EVBACKEND_POLL[/code] 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.

[code type="c"]EVBACKEND_ALL[/code]

Try all backends (even potentially broken ones that wouldn't be tried

with [code type="c"]EVFLAG_AUTO[/code]). Since this is a mask, you can do stuff such as

[code type="c"]EVBACKEND_ALL & ~EVBACKEND_KQUEUE[/code].

It is definitely not recommended to use this flag.

If one or more of these are or'ed into the flags value, then only these

backends will be tried (in the reverse order as listed here). If none are

specified, all backends in [code type="c"]ev_recommended_backends ()[/code] will be tried.

The most typical usage is like this:

[code type="c"] if (!ev_default_loop (0))

fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");



[/code]

Restrict libev to the select and poll backends, and do not allow

environment settings to be taken into account:

[code type="c"] ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);



[/code]

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):

[code type="c"] ev_default_loop (ev_recommended_backends () | EVBACKEND_KQUEUE);



[/code]

struct ev_loop *ev_loop_new (unsigned int flags)

Similar to [code type="c"]ev_default_loop[/code], 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.

[code type="c"] struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);

if (!epoller)

fatal ("no epoll found here, maybe it hides under your chair");



[/code]

ev_default_destroy ()

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. [code type="c"]ev_is_active[/code] might still return true. It is your

responsibility to either stop all watchers cleanly yourself before

calling this function, or cope with the fact afterwards (which is usually

the easiest thing, you can just ignore the watchers and/or [code type="c"]free ()[/code] 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

[code type="c"]ev_loop_new[/code] and [code type="c"]ev_loop_destroy[/code]).

ev_loop_destroy (loop)

Like [code type="c"]ev_default_destroy[/code], but destroys an event loop created by an

earlier call to [code type="c"]ev_loop_new[/code].

ev_default_fork ()

This function sets a flag that causes subsequent [code type="c"]ev_loop[/code] 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 [code type="c"]ev_loop[/code] 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 [code type="c"]pthread_atfork[/code]:

[code type="c"] pthread_atfork (0, 0, ev_default_fork);



[/code]

ev_loop_fork (loop)

Like [code type="c"]ev_default_fork[/code], but acts on an event loop created by

[code type="c"]ev_loop_new[/code]. Yes, you have to call this on every allocated event loop

after fork, and how you do this is entirely your own problem.

int ev_is_default_loop (loop)

Returns true when the given loop actually is the default loop, false otherwise.

unsigned int ev_loop_count (loop)

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 [code type="c"]0[/code] 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

[code type="c"]ev_prepare[/code] and [code type="c"]ev_check[/code] calls.

unsigned int ev_backend (loop)

Returns one of the [code type="c"]EVBACKEND_*[/code] flags indicating the event backend in

use.

ev_tstamp ev_now (loop)

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).

ev_loop (loop, int flags)

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 [code type="c"]0[/code], it will not return until

either no event watchers are active anymore or [code type="c"]ev_unloop[/code] was called.

Please note that an explicit [code type="c"]ev_unloop[/code] 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 [code type="c"]EVLOOP_NONBLOCK[/code] 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 [code type="c"]EVLOOP_ONESHOT[/code] will look for new events (waiting if

necessary) 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 [code type="c"]ev_prepare[/code]/[code type="c"]ev_check[/code] watchers is

usually a better approach for this kind of thing.

Here are the gory details of what [code type="c"]ev_loop[/code] 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!

ev_unloop (loop, how)

Can be used to make a call to [code type="c"]ev_loop[/code] return early (but only after it

has processed all outstanding events). The [code type="c"]how[/code] argument must be either

[code type="c"]EVUNLOOP_ONE[/code], which will make the innermost [code type="c"]ev_loop[/code] call return, or

[code type="c"]EVUNLOOP_ALL[/code], which will make all nested [code type="c"]ev_loop[/code] calls return.

This "unloop state" will be cleared when entering [code type="c"]ev_loop[/code] again.

ev_ref (loop)

ev_unref (loop)

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, [code type="c"]ev_loop[/code] will not return on its own. If you have

a watcher you never unregister that should not keep [code type="c"]ev_loop[/code] 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 [code type="c"]ev_loop[/code] 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 [code type="c"]ev_loop[/code]

running when nothing else is active.

[code type="c"] struct ev_signal exitsig;

ev_signal_init (&exitsig, sig_cb, SIGINT);

ev_signal_start (loop, &exitsig);

evf_unref (loop);



[/code]

Example: For some weird reason, unregister the above signal handler again.

[code type="c"] ev_ref (loop);

ev_signal_stop (loop, &exitsig);



[/code]

ev_set_io_collect_interval (loop, ev_tstamp interval)

ev_set_timeout_collect_interval (loop, ev_tstamp interval)

These advanced functions influence the time that libev will spend waiting

for events. Both are by default [code type="c"]0[/code], meaning that libev will try to

invoke timer/periodic callbacks and I/O callbacks with minimum latency.

Setting these to a higher value (the [code type="c"]interval[/code] must be >= [code type="c"]0[/code])

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 [code type="c"]select ()[/code] 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 [code type="c"]ev_periodic[/code] and

[code type="c"]ev_timer[/code]) will be not affected. Setting this to a non-null value will

introduce an additional [code type="c"]ev_sleep ()[/code] 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). [code type="c"]ev_io[/code] watchers

will not be affected. Setting this to a non-null value will not introduce

any overhead in libev.

Many (busy) programs can usually benefit by setting the I/O collect

interval to a value near [code type="c"]0.1[/code] 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 [code type="c"]0.01[/code],

as this approaches the timing granularity of most systems.

ev_loop_verify (loop)

This function only does something when [code type="c"]EV_VERIFY[/code] 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 [code type="c"]abort ()[/code].

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 [code type="c"]ev_io[/code] watcher for that:

[code type="c"] 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);



[/code]

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 [code type="c"]ev_init

(watcher *, callback)[/code], which expects a callback to be provided. This

callback gets invoked each time the event occurs (or, in the case of I/O

watchers, each time the event loop detects that the file descriptor given

is readable and/or writable).

Each watcher type has its own [code type="c"]ev__set (watcher *, ...)[/code] macro

with arguments specific to this watcher type. There is also a macro

to combine initialisation and setting in one call: [code type="c"]ev__init

(watcher *, callback, ...)[/code].

To make the watcher actually watch out for events, you have to start it

with a watcher-specific start function ([code type="c"]ev__start (loop, watcher

*)[/code]), and you can stop watching for events at any time by calling the

corresponding stop function ([code type="c"]ev__stop (loop, watcher *)[/code].

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 [code type="c"]set[/code] 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:

[code type="c"]EV_READ[/code]

[code type="c"]EV_WRITE[/code]

The file descriptor in the [code type="c"]ev_io[/code] watcher has become readable and/or

writable.

[code type="c"]EV_TIMEOUT[/code]

The [code type="c"]ev_timer[/code] watcher has timed out.

[code type="c"]EV_PERIODIC[/code]

The [code type="c"]ev_periodic[/code] watcher has timed out.

[code type="c"]EV_SIGNAL[/code]

The signal specified in the [code type="c"]ev_signal[/code] watcher has been received by a thread.

[code type="c"]EV_CHILD[/code]

The pid specified in the [code type="c"]ev_child[/code] watcher has received a status change.

[code type="c"]EV_STAT[/code]

The path specified in the [code type="c"]ev_stat[/code] watcher changed its attributes somehow.

[code type="c"]EV_IDLE[/code]

The [code type="c"]ev_idle[/code] watcher has determined that you have nothing better to do.

[code type="c"]EV_PREPARE[/code]

[code type="c"]EV_CHECK[/code]

All [code type="c"]ev_prepare[/code] watchers are invoked just before [code type="c"]ev_loop[/code] starts

to gather new events, and all [code type="c"]ev_check[/code] watchers are invoked just after

[code type="c"]ev_loop[/code] 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 [code type="c"]ev_prepare[/code] watcher might start an idle watcher to keep

[code type="c"]ev_loop[/code] from blocking).

[code type="c"]EV_EMBED[/code]

The embedded event loop specified in the [code type="c"]ev_embed[/code] watcher needs attention.

[code type="c"]EV_FORK[/code]

The event loop has been resumed in the child process after fork (see

[code type="c"]ev_fork[/code]).

[code type="c"]EV_ASYNC[/code]

The given async watcher has been asynchronously notified (see [code type="c"]ev_async[/code]).

[code type="c"]EV_ERROR[/code]

An unspecified error has occurred, the watcher has been stopped. This might

happen because the watcher could not be properly started because libev

ran out of memory, a file descriptor was found to be closed or any other

problem. 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 multi-threaded

programs, though, so beware.

In the following description, [code type="c"]TYPE[/code] stands for the watcher type,

e.g. [code type="c"]timer[/code] for [code type="c"]ev_timer[/code] watchers and [code type="c"]io[/code] for [code type="c"]ev_io[/code] watchers.

[code type="c"]ev_init[/code] (ev_TYPE *watcher, callback)

This macro initialises the generic portion of a watcher. The contents

of the watcher object can be arbitrary (so [code type="c"]malloc[/code] will do). Only

the generic parts of the watcher are initialised, you need to call

the type-specific [code type="c"]ev_TYPE_set[/code] macro afterwards to initialise the

type-specific parts. For each type there is also a [code type="c"]ev_TYPE_init[/code] 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 [code type="c"]void (*)(ev_loop *loop, ev_TYPE *watcher,

int revents)[/code].

[code type="c"]ev_TYPE_set[/code] (ev_TYPE *, [args])

This macro initialises the type-specific parts of a watcher. You need to

call [code type="c"]ev_init[/code] at least once before you call this macro, but you can

call [code type="c"]ev_TYPE_set[/code] 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 [code type="c"]ev_init[/code] macro).

Although some watcher types do not have type-specific arguments

(e.g. [code type="c"]ev_prepare[/code]) you still need to call its [code type="c"]set[/code] macro.

[code type="c"]ev_TYPE_init[/code] (ev_TYPE *watcher, callback, [args])

This convenience macro rolls both [code type="c"]ev_init[/code] and [code type="c"]ev_TYPE_set[/code] macro

calls into a single call. This is the most convenient method to initialise

a watcher. The same limitations apply, of course.

[code type="c"]ev_TYPE_start[/code] (loop *, ev_TYPE *watcher)

Starts (activates) the given watcher. Only active watchers will receive

events. If the watcher is already active nothing will happen.

[code type="c"]ev_TYPE_stop[/code] (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

[code type="c"]ev_TYPE_stop[/code] 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 [code type="c"]ev_TYPE_stop[/code] function.

bool ev_is_active (ev_TYPE *watcher)

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.

bool ev_is_pending (ev_TYPE *watcher)

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

[code type="c"]ev_TYPE_set[/code] is safe), you must not change its priority, and you must

make sure the watcher is available to libev (e.g. you cannot [code type="c"]free ()[/code]

it).

callback ev_cb (ev_TYPE *watcher)

Returns the callback currently set on the watcher.

ev_cb_set (ev_TYPE *watcher, callback)

Change the callback. You can change the callback at virtually any time

(modulo threads).

ev_set_priority (ev_TYPE *watcher, priority)

int ev_priority (ev_TYPE *watcher)

Set and query the priority of the watcher. The priority is a small

integer between [code type="c"]EV_MAXPRI[/code] (default: [code type="c"]2[/code]) and [code type="c"]EV_MINPRI[/code]

(default: [code type="c"]-2[/code]). Pending watchers with higher priority will be invoked

before watchers with lower priority, but priority will not keep watchers

from being executed (except for [code type="c"]ev_idle[/code] 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 [code type="c"]ev_idle[/code] 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 [code type="c"]0[/code], which is supposed to not be too high and not be too low :).

Setting a priority outside the range of [code type="c"]EV_MINPRI[/code] to [code type="c"]EV_MAXPRI[/code] 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.

ev_invoke (loop, ev_TYPE *watcher, int revents)

Invoke the [code type="c"]watcher[/code] with the given [code type="c"]loop[/code] and [code type="c"]revents[/code]. Neither

[code type="c"]loop[/code] nor [code type="c"]revents[/code] need to be valid as long as the watcher callback

can deal with that fact.

int ev_clear_pending (loop, ev_TYPE *watcher)

If the watcher is pending, this function returns clears its pending status

and returns its [code type="c"]revents[/code] bitset (as if its callback was invoked). If the

watcher isn't pending it does nothing and returns [code type="c"]0[/code].

Each watcher has, by default, a member [code type="c"]void *data[/code] 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:

[code type="c"] struct my_io

{

struct ev_io io;

int otherfd;

void *somedata;

struct whatever *mostinteresting;

}



[/code]

And since your callback will be called with a pointer to the watcher, you

can cast it back to your own type:

[code type="c"] static void my_cb (struct ev_loop *loop, struct ev_io *w_, int revents)

{

struct my_io *w = (struct my_io *)w_;

...

}



[/code]

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:

[code type="c"] struct my_biggy

{

int some_data;

ev_timer t1;

ev_timer t2;

}



[/code]

In this case getting the pointer to [code type="c"]my_biggy[/code] is a bit more complicated,

you need to use [code type="c"]offsetof[/code]:

[code type="c"]
#include

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));

}

[/code]

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