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5746 lines
263 KiB
Groff
5746 lines
263 KiB
Groff
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.\" ========================================================================
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.\"
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.IX Title "LIBEV 3"
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.TH LIBEV 3 "2019-06-24" "libev-4.25" "libev - high performance full featured event loop"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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libev \- a high performance full\-featured event loop written in C
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <ev.h>
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.Ve
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.SS "\s-1EXAMPLE PROGRAM\s0"
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.IX Subsection "EXAMPLE PROGRAM"
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.Vb 2
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\& // a single header file is required
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\& #include <ev.h>
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\&
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\& #include <stdio.h> // for puts
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\&
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\& // every watcher type has its own typedef\*(Aqd struct
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\& // with the name ev_TYPE
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\& ev_io stdin_watcher;
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\& ev_timer timeout_watcher;
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\&
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\& // all watcher callbacks have a similar signature
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\& // this callback is called when data is readable on stdin
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|
\& static void
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\& stdin_cb (EV_P_ ev_io *w, int revents)
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|
\& {
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|
\& puts ("stdin ready");
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\& // for one\-shot events, one must manually stop the watcher
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\& // with its corresponding stop function.
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|
\& ev_io_stop (EV_A_ w);
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\&
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\& // this causes all nested ev_run\*(Aqs to stop iterating
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|
\& ev_break (EV_A_ EVBREAK_ALL);
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\& }
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\&
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\& // another callback, this time for a time\-out
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\& static void
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|
\& timeout_cb (EV_P_ ev_timer *w, int revents)
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|
\& {
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|
\& puts ("timeout");
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|
\& // this causes the innermost ev_run to stop iterating
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|
\& ev_break (EV_A_ EVBREAK_ONE);
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|
\& }
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\&
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|
\& int
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|
\& main (void)
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|
\& {
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|
\& // use the default event loop unless you have special needs
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|
\& struct ev_loop *loop = EV_DEFAULT;
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\&
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\& // initialise an io watcher, then start it
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\& // this one will watch for stdin to become readable
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\& ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
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\& ev_io_start (loop, &stdin_watcher);
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\&
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\& // initialise a timer watcher, then start it
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\& // simple non\-repeating 5.5 second timeout
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\& ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
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\& ev_timer_start (loop, &timeout_watcher);
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\&
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\& // now wait for events to arrive
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\& ev_run (loop, 0);
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\&
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|
\& // break was called, so exit
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\& return 0;
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|
\& }
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.Ve
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.SH "ABOUT THIS DOCUMENT"
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|
.IX Header "ABOUT THIS DOCUMENT"
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|
This document documents the libev software package.
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.PP
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The newest version of this document is also available as an html-formatted
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web page you might find easier to navigate when reading it for the first
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|
time: <http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod>.
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.PP
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|
While this document tries to be as complete as possible in documenting
|
|
libev, its usage and the rationale behind its design, it is not a tutorial
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|
on event-based programming, nor will it introduce event-based programming
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|
with libev.
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.PP
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|
Familiarity with event based programming techniques in general is assumed
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|
throughout this document.
|
|
.SH "WHAT TO READ WHEN IN A HURRY"
|
|
.IX Header "WHAT TO READ WHEN IN A HURRY"
|
|
This manual tries to be very detailed, but unfortunately, this also makes
|
|
it very long. If you just want to know the basics of libev, I suggest
|
|
reading \*(L"\s-1ANATOMY OF A WATCHER\*(R"\s0, then the \*(L"\s-1EXAMPLE PROGRAM\*(R"\s0 above and
|
|
look up the missing functions in \*(L"\s-1GLOBAL FUNCTIONS\*(R"\s0 and the \f(CW\*(C`ev_io\*(C'\fR and
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\&\f(CW\*(C`ev_timer\*(C'\fR sections in \*(L"\s-1WATCHER TYPES\*(R"\s0.
|
|
.SH "ABOUT LIBEV"
|
|
.IX Header "ABOUT LIBEV"
|
|
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
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|
these event sources and provide your program with events.
|
|
.PP
|
|
To do this, it must take more or less complete control over your process
|
|
(or thread) by executing the \fIevent loop\fR handler, and will then
|
|
communicate events via a callback mechanism.
|
|
.PP
|
|
You register interest in certain events by registering so-called \fIevent
|
|
watchers\fR, which are relatively small C structures you initialise with the
|
|
details of the event, and then hand it over to libev by \fIstarting\fR the
|
|
watcher.
|
|
.SS "\s-1FEATURES\s0"
|
|
.IX Subsection "FEATURES"
|
|
Libev supports \f(CW\*(C`select\*(C'\fR, \f(CW\*(C`poll\*(C'\fR, the Linux-specific aio and \f(CW\*(C`epoll\*(C'\fR
|
|
interfaces, the BSD-specific \f(CW\*(C`kqueue\*(C'\fR and the Solaris-specific event port
|
|
mechanisms for file descriptor events (\f(CW\*(C`ev_io\*(C'\fR), the Linux \f(CW\*(C`inotify\*(C'\fR
|
|
interface (for \f(CW\*(C`ev_stat\*(C'\fR), Linux eventfd/signalfd (for faster and cleaner
|
|
inter-thread wakeup (\f(CW\*(C`ev_async\*(C'\fR)/signal handling (\f(CW\*(C`ev_signal\*(C'\fR)) relative
|
|
timers (\f(CW\*(C`ev_timer\*(C'\fR), absolute timers with customised rescheduling
|
|
(\f(CW\*(C`ev_periodic\*(C'\fR), synchronous signals (\f(CW\*(C`ev_signal\*(C'\fR), process status
|
|
change events (\f(CW\*(C`ev_child\*(C'\fR), and event watchers dealing with the event
|
|
loop mechanism itself (\f(CW\*(C`ev_idle\*(C'\fR, \f(CW\*(C`ev_embed\*(C'\fR, \f(CW\*(C`ev_prepare\*(C'\fR and
|
|
\&\f(CW\*(C`ev_check\*(C'\fR watchers) as well as file watchers (\f(CW\*(C`ev_stat\*(C'\fR) and even
|
|
limited support for fork events (\f(CW\*(C`ev_fork\*(C'\fR).
|
|
.PP
|
|
It also is quite fast (see this
|
|
benchmark <http://libev.schmorp.de/bench.html> comparing it to libevent
|
|
for example).
|
|
.SS "\s-1CONVENTIONS\s0"
|
|
.IX Subsection "CONVENTIONS"
|
|
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
|
|
\&\fB\s-1EMBED\s0\fR section in this manual. If libev was configured without support
|
|
for multiple event loops, then all functions taking an initial argument of
|
|
name \f(CW\*(C`loop\*(C'\fR (which is always of type \f(CW\*(C`struct ev_loop *\*(C'\fR) will not have
|
|
this argument.
|
|
.SS "\s-1TIME REPRESENTATION\s0"
|
|
.IX Subsection "TIME REPRESENTATION"
|
|
Libev represents time as a single floating point number, representing
|
|
the (fractional) number of seconds since the (\s-1POSIX\s0) epoch (in practice
|
|
somewhere near the beginning of 1970, details are complicated, don't
|
|
ask). This type is called \f(CW\*(C`ev_tstamp\*(C'\fR, which is what you should use
|
|
too. It usually aliases to the \f(CW\*(C`double\*(C'\fR type in C. When you need to do
|
|
any calculations on it, you should treat it as some floating point value.
|
|
.PP
|
|
Unlike the name component \f(CW\*(C`stamp\*(C'\fR might indicate, it is also used for
|
|
time differences (e.g. delays) throughout libev.
|
|
.SH "ERROR HANDLING"
|
|
.IX Header "ERROR HANDLING"
|
|
Libev knows three classes of errors: operating system errors, usage errors
|
|
and internal errors (bugs).
|
|
.PP
|
|
When libev catches an operating system error it cannot handle (for example
|
|
a system call indicating a condition libev cannot fix), it calls the callback
|
|
set via \f(CW\*(C`ev_set_syserr_cb\*(C'\fR, which is supposed to fix the problem or
|
|
abort. The default is to print a diagnostic message and to call \f(CW\*(C`abort
|
|
()\*(C'\fR.
|
|
.PP
|
|
When libev detects a usage error such as a negative timer interval, then
|
|
it will print a diagnostic message and abort (via the \f(CW\*(C`assert\*(C'\fR mechanism,
|
|
so \f(CW\*(C`NDEBUG\*(C'\fR will disable this checking): these are programming errors in
|
|
the libev caller and need to be fixed there.
|
|
.PP
|
|
Libev also has a few internal error-checking \f(CW\*(C`assert\*(C'\fRions, and also has
|
|
extensive consistency checking code. These do not trigger under normal
|
|
circumstances, as they indicate either a bug in libev or worse.
|
|
.SH "GLOBAL FUNCTIONS"
|
|
.IX Header "GLOBAL FUNCTIONS"
|
|
These functions can be called anytime, even before initialising the
|
|
library in any way.
|
|
.IP "ev_tstamp ev_time ()" 4
|
|
.IX Item "ev_tstamp ev_time ()"
|
|
Returns the current time as libev would use it. Please note that the
|
|
\&\f(CW\*(C`ev_now\*(C'\fR function is usually faster and also often returns the timestamp
|
|
you actually want to know. Also interesting is the combination of
|
|
\&\f(CW\*(C`ev_now_update\*(C'\fR and \f(CW\*(C`ev_now\*(C'\fR.
|
|
.IP "ev_sleep (ev_tstamp interval)" 4
|
|
.IX Item "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 (approximately \- it might return a bit earlier even if not
|
|
interrupted). Returns immediately if \f(CW\*(C`interval <= 0\*(C'\fR.
|
|
.Sp
|
|
Basically this is a sub-second-resolution \f(CW\*(C`sleep ()\*(C'\fR.
|
|
.Sp
|
|
The range of the \f(CW\*(C`interval\*(C'\fR is limited \- libev only guarantees to work
|
|
with sleep times of up to one day (\f(CW\*(C`interval <= 86400\*(C'\fR).
|
|
.IP "int ev_version_major ()" 4
|
|
.IX Item "int ev_version_major ()"
|
|
.PD 0
|
|
.IP "int ev_version_minor ()" 4
|
|
.IX Item "int ev_version_minor ()"
|
|
.PD
|
|
You can find out the major and minor \s-1ABI\s0 version numbers of the library
|
|
you linked against by calling the functions \f(CW\*(C`ev_version_major\*(C'\fR and
|
|
\&\f(CW\*(C`ev_version_minor\*(C'\fR. If you want, you can compare against the global
|
|
symbols \f(CW\*(C`EV_VERSION_MAJOR\*(C'\fR and \f(CW\*(C`EV_VERSION_MINOR\*(C'\fR, which specify the
|
|
version of the library your program was compiled against.
|
|
.Sp
|
|
These version numbers refer to the \s-1ABI\s0 version of the library, not the
|
|
release version.
|
|
.Sp
|
|
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.
|
|
.Sp
|
|
Example: Make sure we haven't accidentally been linked against the wrong
|
|
version (note, however, that this will not detect other \s-1ABI\s0 mismatches,
|
|
such as \s-1LFS\s0 or reentrancy).
|
|
.Sp
|
|
.Vb 3
|
|
\& assert (("libev version mismatch",
|
|
\& ev_version_major () == EV_VERSION_MAJOR
|
|
\& && ev_version_minor () >= EV_VERSION_MINOR));
|
|
.Ve
|
|
.IP "unsigned int ev_supported_backends ()" 4
|
|
.IX Item "unsigned int ev_supported_backends ()"
|
|
Return the set of all backends (i.e. their corresponding \f(CW\*(C`EV_BACKEND_*\*(C'\fR
|
|
value) compiled into this binary of libev (independent of their
|
|
availability on the system you are running on). See \f(CW\*(C`ev_default_loop\*(C'\fR for
|
|
a description of the set values.
|
|
.Sp
|
|
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
|
|
.Sp
|
|
.Vb 2
|
|
\& assert (("sorry, no epoll, no sex",
|
|
\& ev_supported_backends () & EVBACKEND_EPOLL));
|
|
.Ve
|
|
.IP "unsigned int ev_recommended_backends ()" 4
|
|
.IX Item "unsigned int ev_recommended_backends ()"
|
|
Return the set of all backends compiled into this binary of libev and
|
|
also recommended for this platform, meaning it will work for most file
|
|
descriptor types. This set is often smaller than the one returned by
|
|
\&\f(CW\*(C`ev_supported_backends\*(C'\fR, 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.
|
|
.IP "unsigned int ev_embeddable_backends ()" 4
|
|
.IX Item "unsigned int ev_embeddable_backends ()"
|
|
Returns the set of backends that are embeddable in other event loops. This
|
|
value is platform-specific but can include backends not available on the
|
|
current system. To find which embeddable backends might be supported on
|
|
the current system, you would need to look at \f(CW\*(C`ev_embeddable_backends ()
|
|
& ev_supported_backends ()\*(C'\fR, likewise for recommended ones.
|
|
.Sp
|
|
See the description of \f(CW\*(C`ev_embed\*(C'\fR watchers for more info.
|
|
.IP "ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())" 4
|
|
.IX Item "ev_set_allocator (void *(*cb)(void *ptr, long size) throw ())"
|
|
Sets the allocation function to use (the prototype is similar \- the
|
|
semantics are identical to the \f(CW\*(C`realloc\*(C'\fR 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 (\f(CW\*(C`size != 0\*(C'\fR), the library might abort
|
|
or take some potentially destructive action.
|
|
.Sp
|
|
Since some systems (at least OpenBSD and Darwin) fail to implement
|
|
correct \f(CW\*(C`realloc\*(C'\fR semantics, libev will use a wrapper around the system
|
|
\&\f(CW\*(C`realloc\*(C'\fR and \f(CW\*(C`free\*(C'\fR functions by default.
|
|
.Sp
|
|
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.
|
|
.Sp
|
|
Example: The following is the \f(CW\*(C`realloc\*(C'\fR function that libev itself uses
|
|
which should work with \f(CW\*(C`realloc\*(C'\fR and \f(CW\*(C`free\*(C'\fR functions of all kinds and
|
|
is probably a good basis for your own implementation.
|
|
.Sp
|
|
.Vb 5
|
|
\& static void *
|
|
\& ev_realloc_emul (void *ptr, long size) EV_NOEXCEPT
|
|
\& {
|
|
\& if (size)
|
|
\& return realloc (ptr, size);
|
|
\&
|
|
\& free (ptr);
|
|
\& return 0;
|
|
\& }
|
|
.Ve
|
|
.Sp
|
|
Example: Replace the libev allocator with one that waits a bit and then
|
|
retries.
|
|
.Sp
|
|
.Vb 8
|
|
\& static void *
|
|
\& persistent_realloc (void *ptr, size_t size)
|
|
\& {
|
|
\& if (!size)
|
|
\& {
|
|
\& free (ptr);
|
|
\& return 0;
|
|
\& }
|
|
\&
|
|
\& for (;;)
|
|
\& {
|
|
\& void *newptr = realloc (ptr, size);
|
|
\&
|
|
\& if (newptr)
|
|
\& return newptr;
|
|
\&
|
|
\& sleep (60);
|
|
\& }
|
|
\& }
|
|
\&
|
|
\& ...
|
|
\& ev_set_allocator (persistent_realloc);
|
|
.Ve
|
|
.IP "ev_set_syserr_cb (void (*cb)(const char *msg) throw ())" 4
|
|
.IX Item "ev_set_syserr_cb (void (*cb)(const char *msg) throw ())"
|
|
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).
|
|
.Sp
|
|
Example: This is basically the same thing that libev does internally, too.
|
|
.Sp
|
|
.Vb 6
|
|
\& static void
|
|
\& fatal_error (const char *msg)
|
|
\& {
|
|
\& perror (msg);
|
|
\& abort ();
|
|
\& }
|
|
\&
|
|
\& ...
|
|
\& ev_set_syserr_cb (fatal_error);
|
|
.Ve
|
|
.IP "ev_feed_signal (int signum)" 4
|
|
.IX Item "ev_feed_signal (int signum)"
|
|
This function can be used to \*(L"simulate\*(R" a signal receive. It is completely
|
|
safe to call this function at any time, from any context, including signal
|
|
handlers or random threads.
|
|
.Sp
|
|
Its main use is to customise signal handling in your process, especially
|
|
in the presence of threads. For example, you could block signals
|
|
by default in all threads (and specifying \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR when
|
|
creating any loops), and in one thread, use \f(CW\*(C`sigwait\*(C'\fR or any other
|
|
mechanism to wait for signals, then \*(L"deliver\*(R" them to libev by calling
|
|
\&\f(CW\*(C`ev_feed_signal\*(C'\fR.
|
|
.SH "FUNCTIONS CONTROLLING EVENT LOOPS"
|
|
.IX Header "FUNCTIONS CONTROLLING EVENT LOOPS"
|
|
An event loop is described by a \f(CW\*(C`struct ev_loop *\*(C'\fR (the \f(CW\*(C`struct\*(C'\fR is
|
|
\&\fInot\fR optional in this case unless libev 3 compatibility is disabled, as
|
|
libev 3 had an \f(CW\*(C`ev_loop\*(C'\fR function colliding with the struct name).
|
|
.PP
|
|
The library knows two types of such loops, the \fIdefault\fR loop, which
|
|
supports child process events, and dynamically created event loops which
|
|
do not.
|
|
.IP "struct ev_loop *ev_default_loop (unsigned int flags)" 4
|
|
.IX Item "struct ev_loop *ev_default_loop (unsigned int flags)"
|
|
This returns the \*(L"default\*(R" event loop object, which is what you should
|
|
normally use when you just need \*(L"the event loop\*(R". Event loop objects and
|
|
the \f(CW\*(C`flags\*(C'\fR parameter are described in more detail in the entry for
|
|
\&\f(CW\*(C`ev_loop_new\*(C'\fR.
|
|
.Sp
|
|
If the default loop is already initialised then this function simply
|
|
returns it (and ignores the flags. If that is troubling you, check
|
|
\&\f(CW\*(C`ev_backend ()\*(C'\fR afterwards). Otherwise it will create it with the given
|
|
flags, which should almost always be \f(CW0\fR, unless the caller is also the
|
|
one calling \f(CW\*(C`ev_run\*(C'\fR or otherwise qualifies as \*(L"the main program\*(R".
|
|
.Sp
|
|
If you don't know what event loop to use, use the one returned from this
|
|
function (or via the \f(CW\*(C`EV_DEFAULT\*(C'\fR macro).
|
|
.Sp
|
|
Note that this function is \fInot\fR thread-safe, so if you want to use it
|
|
from multiple threads, you have to employ some kind of mutex (note also
|
|
that this case is unlikely, as loops cannot be shared easily between
|
|
threads anyway).
|
|
.Sp
|
|
The default loop is the only loop that can handle \f(CW\*(C`ev_child\*(C'\fR watchers,
|
|
and to do this, it always registers a handler for \f(CW\*(C`SIGCHLD\*(C'\fR. If this is
|
|
a problem for your application you can either create a dynamic loop with
|
|
\&\f(CW\*(C`ev_loop_new\*(C'\fR which doesn't do that, or you can simply overwrite the
|
|
\&\f(CW\*(C`SIGCHLD\*(C'\fR signal handler \fIafter\fR calling \f(CW\*(C`ev_default_init\*(C'\fR.
|
|
.Sp
|
|
Example: This is the most typical usage.
|
|
.Sp
|
|
.Vb 2
|
|
\& if (!ev_default_loop (0))
|
|
\& fatal ("could not initialise libev, bad $LIBEV_FLAGS in environment?");
|
|
.Ve
|
|
.Sp
|
|
Example: Restrict libev to the select and poll backends, and do not allow
|
|
environment settings to be taken into account:
|
|
.Sp
|
|
.Vb 1
|
|
\& ev_default_loop (EVBACKEND_POLL | EVBACKEND_SELECT | EVFLAG_NOENV);
|
|
.Ve
|
|
.IP "struct ev_loop *ev_loop_new (unsigned int flags)" 4
|
|
.IX Item "struct ev_loop *ev_loop_new (unsigned int flags)"
|
|
This will create and initialise a new event loop object. If the loop
|
|
could not be initialised, returns false.
|
|
.Sp
|
|
This function is thread-safe, and one common way to use libev with
|
|
threads is indeed to create one loop per thread, and using the default
|
|
loop in the \*(L"main\*(R" or \*(L"initial\*(R" thread.
|
|
.Sp
|
|
The flags argument can be used to specify special behaviour or specific
|
|
backends to use, and is usually specified as \f(CW0\fR (or \f(CW\*(C`EVFLAG_AUTO\*(C'\fR).
|
|
.Sp
|
|
The following flags are supported:
|
|
.RS 4
|
|
.ie n .IP """EVFLAG_AUTO""" 4
|
|
.el .IP "\f(CWEVFLAG_AUTO\fR" 4
|
|
.IX Item "EVFLAG_AUTO"
|
|
The default flags value. Use this if you have no clue (it's the right
|
|
thing, believe me).
|
|
.ie n .IP """EVFLAG_NOENV""" 4
|
|
.el .IP "\f(CWEVFLAG_NOENV\fR" 4
|
|
.IX Item "EVFLAG_NOENV"
|
|
If this flag bit is or'ed into the flag value (or the program runs setuid
|
|
or setgid) then libev will \fInot\fR look at the environment variable
|
|
\&\f(CW\*(C`LIBEV_FLAGS\*(C'\fR. Otherwise (the default), this environment variable will
|
|
override the flags completely if it is found in the environment. This is
|
|
useful to try out specific backends to test their performance, to work
|
|
around bugs, or to make libev threadsafe (accessing environment variables
|
|
cannot be done in a threadsafe way, but usually it works if no other
|
|
thread modifies them).
|
|
.ie n .IP """EVFLAG_FORKCHECK""" 4
|
|
.el .IP "\f(CWEVFLAG_FORKCHECK\fR" 4
|
|
.IX Item "EVFLAG_FORKCHECK"
|
|
Instead of calling \f(CW\*(C`ev_loop_fork\*(C'\fR manually after a fork, you can also
|
|
make libev check for a fork in each iteration by enabling this flag.
|
|
.Sp
|
|
This works by calling \f(CW\*(C`getpid ()\*(C'\fR 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, \f(CW\*(C`getpid\*(C'\fR is actually a simple 5\-insn
|
|
sequence without a system call and thus \fIvery\fR fast, but my GNU/Linux
|
|
system also has \f(CW\*(C`pthread_atfork\*(C'\fR which is even faster). (Update: glibc
|
|
versions 2.25 apparently removed the \f(CW\*(C`getpid\*(C'\fR optimisation again).
|
|
.Sp
|
|
The big advantage of this flag is that you can forget about fork (and
|
|
forget about forgetting to tell libev about forking, although you still
|
|
have to ignore \f(CW\*(C`SIGPIPE\*(C'\fR) when you use this flag.
|
|
.Sp
|
|
This flag setting cannot be overridden or specified in the \f(CW\*(C`LIBEV_FLAGS\*(C'\fR
|
|
environment variable.
|
|
.ie n .IP """EVFLAG_NOINOTIFY""" 4
|
|
.el .IP "\f(CWEVFLAG_NOINOTIFY\fR" 4
|
|
.IX Item "EVFLAG_NOINOTIFY"
|
|
When this flag is specified, then libev will not attempt to use the
|
|
\&\fIinotify\fR \s-1API\s0 for its \f(CW\*(C`ev_stat\*(C'\fR watchers. Apart from debugging and
|
|
testing, this flag can be useful to conserve inotify file descriptors, as
|
|
otherwise each loop using \f(CW\*(C`ev_stat\*(C'\fR watchers consumes one inotify handle.
|
|
.ie n .IP """EVFLAG_SIGNALFD""" 4
|
|
.el .IP "\f(CWEVFLAG_SIGNALFD\fR" 4
|
|
.IX Item "EVFLAG_SIGNALFD"
|
|
When this flag is specified, then libev will attempt to use the
|
|
\&\fIsignalfd\fR \s-1API\s0 for its \f(CW\*(C`ev_signal\*(C'\fR (and \f(CW\*(C`ev_child\*(C'\fR) watchers. This \s-1API\s0
|
|
delivers signals synchronously, which makes it both faster and might make
|
|
it possible to get the queued signal data. It can also simplify signal
|
|
handling with threads, as long as you properly block signals in your
|
|
threads that are not interested in handling them.
|
|
.Sp
|
|
Signalfd will not be used by default as this changes your signal mask, and
|
|
there are a lot of shoddy libraries and programs (glib's threadpool for
|
|
example) that can't properly initialise their signal masks.
|
|
.ie n .IP """EVFLAG_NOSIGMASK""" 4
|
|
.el .IP "\f(CWEVFLAG_NOSIGMASK\fR" 4
|
|
.IX Item "EVFLAG_NOSIGMASK"
|
|
When this flag is specified, then libev will avoid to modify the signal
|
|
mask. Specifically, this means you have to make sure signals are unblocked
|
|
when you want to receive them.
|
|
.Sp
|
|
This behaviour is useful when you want to do your own signal handling, or
|
|
want to handle signals only in specific threads and want to avoid libev
|
|
unblocking the signals.
|
|
.Sp
|
|
It's also required by \s-1POSIX\s0 in a threaded program, as libev calls
|
|
\&\f(CW\*(C`sigprocmask\*(C'\fR, whose behaviour is officially unspecified.
|
|
.Sp
|
|
This flag's behaviour will become the default in future versions of libev.
|
|
.ie n .IP """EVBACKEND_SELECT"" (value 1, portable select backend)" 4
|
|
.el .IP "\f(CWEVBACKEND_SELECT\fR (value 1, portable select backend)" 4
|
|
.IX Item "EVBACKEND_SELECT (value 1, portable select backend)"
|
|
This is your standard \fBselect\fR\|(2) backend. Not \fIcompletely\fR 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.
|
|
.Sp
|
|
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 \f(CW\*(C`accept ()\*(C'\fR in a loop to accept as many
|
|
connections as possible during one iteration. You might also want to have
|
|
a look at \f(CW\*(C`ev_set_io_collect_interval ()\*(C'\fR to increase the amount of
|
|
readiness notifications you get per iteration.
|
|
.Sp
|
|
This backend maps \f(CW\*(C`EV_READ\*(C'\fR to the \f(CW\*(C`readfds\*(C'\fR set and \f(CW\*(C`EV_WRITE\*(C'\fR to the
|
|
\&\f(CW\*(C`writefds\*(C'\fR set (and to work around Microsoft Windows bugs, also onto the
|
|
\&\f(CW\*(C`exceptfds\*(C'\fR set on that platform).
|
|
.ie n .IP """EVBACKEND_POLL"" (value 2, poll backend, available everywhere except on windows)" 4
|
|
.el .IP "\f(CWEVBACKEND_POLL\fR (value 2, poll backend, available everywhere except on windows)" 4
|
|
.IX Item "EVBACKEND_POLL (value 2, poll backend, available everywhere except on windows)"
|
|
And this is your standard \fBpoll\fR\|(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 \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR, above, for
|
|
performance tips.
|
|
.Sp
|
|
This backend maps \f(CW\*(C`EV_READ\*(C'\fR to \f(CW\*(C`POLLIN | POLLERR | POLLHUP\*(C'\fR, and
|
|
\&\f(CW\*(C`EV_WRITE\*(C'\fR to \f(CW\*(C`POLLOUT | POLLERR | POLLHUP\*(C'\fR.
|
|
.ie n .IP """EVBACKEND_EPOLL"" (value 4, Linux)" 4
|
|
.el .IP "\f(CWEVBACKEND_EPOLL\fR (value 4, Linux)" 4
|
|
.IX Item "EVBACKEND_EPOLL (value 4, Linux)"
|
|
Use the linux-specific \fBepoll\fR\|(7) interface (for both pre\- and post\-2.6.9
|
|
kernels).
|
|
.Sp
|
|
For few fds, this backend is a bit little slower than poll and select, but
|
|
it scales phenomenally better. While poll and select usually scale like
|
|
O(total_fds) where total_fds is the total number of fds (or the highest
|
|
fd), epoll scales either O(1) or O(active_fds).
|
|
.Sp
|
|
The epoll mechanism deserves honorable mention as the most misdesigned
|
|
of the more advanced event mechanisms: mere annoyances include silently
|
|
dropping file descriptors, requiring a system call per change per file
|
|
descriptor (and unnecessary guessing of parameters), problems with dup,
|
|
returning before the timeout value, resulting in additional iterations
|
|
(and only giving 5ms accuracy while select on the same platform gives
|
|
0.1ms) and so on. The biggest issue is fork races, however \- if a program
|
|
forks then \fIboth\fR parent and child process have to recreate the epoll
|
|
set, which can take considerable time (one syscall per file descriptor)
|
|
and is of course hard to detect.
|
|
.Sp
|
|
Epoll is also notoriously buggy \- embedding epoll fds \fIshould\fR work,
|
|
but of course \fIdoesn't\fR, and epoll just loves to report events for
|
|
totally \fIdifferent\fR file descriptors (even already closed ones, so
|
|
one cannot even remove them from the set) than registered in the set
|
|
(especially on \s-1SMP\s0 systems). Libev tries to counter these spurious
|
|
notifications by employing an additional generation counter and comparing
|
|
that against the events to filter out spurious ones, recreating the set
|
|
when required. Epoll also erroneously rounds down timeouts, but gives you
|
|
no way to know when and by how much, so sometimes you have to busy-wait
|
|
because epoll returns immediately despite a nonzero timeout. And last
|
|
not least, it also refuses to work with some file descriptors which work
|
|
perfectly fine with \f(CW\*(C`select\*(C'\fR (files, many character devices...).
|
|
.Sp
|
|
Epoll is truly the train wreck among event poll mechanisms, a frankenpoll,
|
|
cobbled together in a hurry, no thought to design or interaction with
|
|
others. Oh, the pain, will it ever stop...
|
|
.Sp
|
|
While stopping, setting and starting an I/O watcher in the same iteration
|
|
will result in some caching, there is still a system call per such
|
|
incident (because the same \fIfile descriptor\fR could point to a different
|
|
\&\fIfile description\fR now), so its best to avoid that. Also, \f(CW\*(C`dup ()\*(C'\fR'ed
|
|
file descriptors might not work very well if you register events for both
|
|
file descriptors.
|
|
.Sp
|
|
Best performance from this backend is achieved by not unregistering all
|
|
watchers for a file descriptor until it has been closed, if possible,
|
|
i.e. keep at least one watcher active per fd at all times. Stopping and
|
|
starting a watcher (without re-setting it) also usually doesn't cause
|
|
extra overhead. A fork can both result in spurious notifications as well
|
|
as in libev having to destroy and recreate the epoll object, which can
|
|
take considerable time and thus should be avoided.
|
|
.Sp
|
|
All this means that, in practice, \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR can be as fast or
|
|
faster than epoll for maybe up to a hundred file descriptors, depending on
|
|
the usage. So sad.
|
|
.Sp
|
|
While nominally embeddable in other event loops, this feature is broken in
|
|
a lot of kernel revisions, but probably(!) works in current versions.
|
|
.Sp
|
|
This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
|
|
\&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
|
|
.ie n .IP """EVBACKEND_LINUXAIO"" (value 64, Linux)" 4
|
|
.el .IP "\f(CWEVBACKEND_LINUXAIO\fR (value 64, Linux)" 4
|
|
.IX Item "EVBACKEND_LINUXAIO (value 64, Linux)"
|
|
Use the linux-specific linux aio (\fInot\fR \f(CWaio(7)\fR but \f(CWio_submit(2)\fR) event interface available in post\-4.18 kernels.
|
|
.Sp
|
|
If this backend works for you (as of this writing, it was very
|
|
experimental), it is the best event interface available on linux and might
|
|
be well worth enabling it \- if it isn't available in your kernel this will
|
|
be detected and this backend will be skipped.
|
|
.Sp
|
|
This backend can batch oneshot requests and supports a user-space ring
|
|
buffer to receive events. It also doesn't suffer from most of the design
|
|
problems of epoll (such as not being able to remove event sources from the
|
|
epoll set), and generally sounds too good to be true. Because, this being
|
|
the linux kernel, of course it suffers from a whole new set of limitations.
|
|
.Sp
|
|
For one, it is not easily embeddable (but probably could be done using
|
|
an event fd at some extra overhead). It also is subject to a system wide
|
|
limit that can be configured in \fI/proc/sys/fs/aio\-max\-nr\fR \- each loop
|
|
currently requires \f(CW61\fR of this number. If no aio requests are left, this
|
|
backend will be skipped during initialisation.
|
|
.Sp
|
|
Most problematic in practise, however, is that not all file descriptors
|
|
work with it. For example, in linux 5.1, tcp sockets, pipes, event fds,
|
|
files, \fI/dev/null\fR and a few others are supported, but ttys do not work
|
|
properly (a known bug that the kernel developers don't care about, see
|
|
<https://lore.kernel.org/patchwork/patch/1047453/>), so this is not
|
|
(yet?) a generic event polling interface.
|
|
.Sp
|
|
Overall, it seems the linux developers just don't want it to have a
|
|
generic event handling mechanism other than \f(CW\*(C`select\*(C'\fR or \f(CW\*(C`poll\*(C'\fR.
|
|
.Sp
|
|
To work around the fd type problem, the current version of libev uses
|
|
epoll as a fallback for file deescriptor types that do not work. Epoll
|
|
is used in, kind of, slow mode that hopefully avoids most of its design
|
|
problems and requires 1\-3 extra syscalls per active fd every iteration.
|
|
.Sp
|
|
This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
|
|
\&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
|
|
.ie n .IP """EVBACKEND_KQUEUE"" (value 8, most \s-1BSD\s0 clones)" 4
|
|
.el .IP "\f(CWEVBACKEND_KQUEUE\fR (value 8, most \s-1BSD\s0 clones)" 4
|
|
.IX Item "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). Unlike epoll, however, whose brokenness
|
|
is by design, these kqueue bugs can (and eventually will) be fixed
|
|
without \s-1API\s0 changes to existing programs. For this reason it's not being
|
|
\&\*(L"auto-detected\*(R" unless you explicitly specify it in the flags (i.e. using
|
|
\&\f(CW\*(C`EVBACKEND_KQUEUE\*(C'\fR) or libev was compiled on a known-to-be-good (\-enough)
|
|
system like NetBSD.
|
|
.Sp
|
|
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 \f(CW\*(C`ev_embed\*(C'\fR watchers for more info.
|
|
.Sp
|
|
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 \f(CW\*(C`EVBACKEND_EPOLL\*(C'\fR, it still adds up to
|
|
two event changes per incident. Support for \f(CW\*(C`fork ()\*(C'\fR is very bad (you
|
|
might have to leak fd's on fork, but it's more sane than epoll) and it
|
|
drops fds silently in similarly hard-to-detect cases.
|
|
.Sp
|
|
This backend usually performs well under most conditions.
|
|
.Sp
|
|
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. \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR (but \f(CW\*(C`poll\*(C'\fR is of course
|
|
also broken on \s-1OS X\s0)) and, did I mention it, using it only for sockets.
|
|
.Sp
|
|
This backend maps \f(CW\*(C`EV_READ\*(C'\fR into an \f(CW\*(C`EVFILT_READ\*(C'\fR kevent with
|
|
\&\f(CW\*(C`NOTE_EOF\*(C'\fR, and \f(CW\*(C`EV_WRITE\*(C'\fR into an \f(CW\*(C`EVFILT_WRITE\*(C'\fR kevent with
|
|
\&\f(CW\*(C`NOTE_EOF\*(C'\fR.
|
|
.ie n .IP """EVBACKEND_DEVPOLL"" (value 16, Solaris 8)" 4
|
|
.el .IP "\f(CWEVBACKEND_DEVPOLL\fR (value 16, Solaris 8)" 4
|
|
.IX Item "EVBACKEND_DEVPOLL (value 16, Solaris 8)"
|
|
This is not implemented yet (and might never be, unless you send me an
|
|
implementation). According to reports, \f(CW\*(C`/dev/poll\*(C'\fR only supports sockets
|
|
and is not embeddable, which would limit the usefulness of this backend
|
|
immensely.
|
|
.ie n .IP """EVBACKEND_PORT"" (value 32, Solaris 10)" 4
|
|
.el .IP "\f(CWEVBACKEND_PORT\fR (value 32, Solaris 10)" 4
|
|
.IX Item "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)).
|
|
.Sp
|
|
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 \*(L"slow\*(R" \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR backend
|
|
might perform better.
|
|
.Sp
|
|
On the positive side, this backend actually performed fully to
|
|
specification in all tests and is fully embeddable, which is a rare feat
|
|
among the OS-specific backends (I vastly prefer correctness over speed
|
|
hacks).
|
|
.Sp
|
|
On the negative side, the interface is \fIbizarre\fR \- so bizarre that
|
|
even sun itself gets it wrong in their code examples: The event polling
|
|
function sometimes returns events to the caller even though an error
|
|
occurred, but with no indication whether it has done so or not (yes, it's
|
|
even documented that way) \- deadly for edge-triggered interfaces where you
|
|
absolutely have to know whether an event occurred or not because you have
|
|
to re-arm the watcher.
|
|
.Sp
|
|
Fortunately libev seems to be able to work around these idiocies.
|
|
.Sp
|
|
This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
|
|
\&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
|
|
.ie n .IP """EVBACKEND_ALL""" 4
|
|
.el .IP "\f(CWEVBACKEND_ALL\fR" 4
|
|
.IX Item "EVBACKEND_ALL"
|
|
Try all backends (even potentially broken ones that wouldn't be tried
|
|
with \f(CW\*(C`EVFLAG_AUTO\*(C'\fR). Since this is a mask, you can do stuff such as
|
|
\&\f(CW\*(C`EVBACKEND_ALL & ~EVBACKEND_KQUEUE\*(C'\fR.
|
|
.Sp
|
|
It is definitely not recommended to use this flag, use whatever
|
|
\&\f(CW\*(C`ev_recommended_backends ()\*(C'\fR returns, or simply do not specify a backend
|
|
at all.
|
|
.ie n .IP """EVBACKEND_MASK""" 4
|
|
.el .IP "\f(CWEVBACKEND_MASK\fR" 4
|
|
.IX Item "EVBACKEND_MASK"
|
|
Not a backend at all, but a mask to select all backend bits from a
|
|
\&\f(CW\*(C`flags\*(C'\fR value, in case you want to mask out any backends from a flags
|
|
value (e.g. when modifying the \f(CW\*(C`LIBEV_FLAGS\*(C'\fR environment variable).
|
|
.RE
|
|
.RS 4
|
|
.Sp
|
|
If one or more of the backend flags are or'ed into the flags value,
|
|
then only these backends will be tried (in the reverse order as listed
|
|
here). If none are specified, all backends in \f(CW\*(C`ev_recommended_backends
|
|
()\*(C'\fR will be tried.
|
|
.Sp
|
|
Example: Try to create a event loop that uses epoll and nothing else.
|
|
.Sp
|
|
.Vb 3
|
|
\& struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
|
|
\& if (!epoller)
|
|
\& fatal ("no epoll found here, maybe it hides under your chair");
|
|
.Ve
|
|
.Sp
|
|
Example: Use whatever libev has to offer, but make sure that kqueue is
|
|
used if available.
|
|
.Sp
|
|
.Vb 1
|
|
\& struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);
|
|
.Ve
|
|
.Sp
|
|
Example: Similarly, on linux, you mgiht want to take advantage of the
|
|
linux aio backend if possible, but fall back to something else if that
|
|
isn't available.
|
|
.Sp
|
|
.Vb 1
|
|
\& struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_LINUXAIO);
|
|
.Ve
|
|
.RE
|
|
.IP "ev_loop_destroy (loop)" 4
|
|
.IX Item "ev_loop_destroy (loop)"
|
|
Destroys an event loop object (frees all memory and kernel state
|
|
etc.). None of the active event watchers will be stopped in the normal
|
|
sense, so e.g. \f(CW\*(C`ev_is_active\*(C'\fR might still return true. It is your
|
|
responsibility to either stop all watchers cleanly yourself \fIbefore\fR
|
|
calling this function, or cope with the fact afterwards (which is usually
|
|
the easiest thing, you can just ignore the watchers and/or \f(CW\*(C`free ()\*(C'\fR them
|
|
for example).
|
|
.Sp
|
|
Note that certain global state, such as signal state (and installed signal
|
|
handlers), will not be freed by this function, and related watchers (such
|
|
as signal and child watchers) would need to be stopped manually.
|
|
.Sp
|
|
This function is normally used on loop objects allocated by
|
|
\&\f(CW\*(C`ev_loop_new\*(C'\fR, but it can also be used on the default loop returned by
|
|
\&\f(CW\*(C`ev_default_loop\*(C'\fR, in which case it is not thread-safe.
|
|
.Sp
|
|
Note that it is not advisable to call this function on the default loop
|
|
except in the rare occasion where you really need to free its resources.
|
|
If you need dynamically allocated loops it is better to use \f(CW\*(C`ev_loop_new\*(C'\fR
|
|
and \f(CW\*(C`ev_loop_destroy\*(C'\fR.
|
|
.IP "ev_loop_fork (loop)" 4
|
|
.IX Item "ev_loop_fork (loop)"
|
|
This function sets a flag that causes subsequent \f(CW\*(C`ev_run\*(C'\fR iterations
|
|
to reinitialise the kernel state for backends that have one. Despite
|
|
the name, you can call it anytime you are allowed to start or stop
|
|
watchers (except inside an \f(CW\*(C`ev_prepare\*(C'\fR callback), but it makes most
|
|
sense after forking, in the child process. You \fImust\fR call it (or use
|
|
\&\f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR) in the child before resuming or calling \f(CW\*(C`ev_run\*(C'\fR.
|
|
.Sp
|
|
In addition, if you want to reuse a loop (via this function or
|
|
\&\f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR), you \fIalso\fR have to ignore \f(CW\*(C`SIGPIPE\*(C'\fR.
|
|
.Sp
|
|
Again, you \fIhave\fR to call it on \fIany\fR loop that you want to re-use after
|
|
a fork, \fIeven if you do not plan to use the loop in the parent\fR. This is
|
|
because some kernel interfaces *cough* \fIkqueue\fR *cough* do funny things
|
|
during fork.
|
|
.Sp
|
|
On the other hand, you only need to call this function in the child
|
|
process if and only if you want to use the event loop in the child. If
|
|
you just fork+exec or create a new loop in the child, you don't have to
|
|
call it at all (in fact, \f(CW\*(C`epoll\*(C'\fR is so badly broken that it makes a
|
|
difference, but libev will usually detect this case on its own and do a
|
|
costly reset of the backend).
|
|
.Sp
|
|
The function itself is quite fast and it's usually not a problem to call
|
|
it just in case after a fork.
|
|
.Sp
|
|
Example: Automate calling \f(CW\*(C`ev_loop_fork\*(C'\fR on the default loop when
|
|
using pthreads.
|
|
.Sp
|
|
.Vb 5
|
|
\& static void
|
|
\& post_fork_child (void)
|
|
\& {
|
|
\& ev_loop_fork (EV_DEFAULT);
|
|
\& }
|
|
\&
|
|
\& ...
|
|
\& pthread_atfork (0, 0, post_fork_child);
|
|
.Ve
|
|
.IP "int ev_is_default_loop (loop)" 4
|
|
.IX Item "int ev_is_default_loop (loop)"
|
|
Returns true when the given loop is, in fact, the default loop, and false
|
|
otherwise.
|
|
.IP "unsigned int ev_iteration (loop)" 4
|
|
.IX Item "unsigned int ev_iteration (loop)"
|
|
Returns the current iteration count for the event loop, which is identical
|
|
to the number of times libev did poll for new events. It starts at \f(CW0\fR
|
|
and happily wraps around with enough iterations.
|
|
.Sp
|
|
This value can sometimes be useful as a generation counter of sorts (it
|
|
\&\*(L"ticks\*(R" the number of loop iterations), as it roughly corresponds with
|
|
\&\f(CW\*(C`ev_prepare\*(C'\fR and \f(CW\*(C`ev_check\*(C'\fR calls \- and is incremented between the
|
|
prepare and check phases.
|
|
.IP "unsigned int ev_depth (loop)" 4
|
|
.IX Item "unsigned int ev_depth (loop)"
|
|
Returns the number of times \f(CW\*(C`ev_run\*(C'\fR was entered minus the number of
|
|
times \f(CW\*(C`ev_run\*(C'\fR was exited normally, in other words, the recursion depth.
|
|
.Sp
|
|
Outside \f(CW\*(C`ev_run\*(C'\fR, this number is zero. In a callback, this number is
|
|
\&\f(CW1\fR, unless \f(CW\*(C`ev_run\*(C'\fR was invoked recursively (or from another thread),
|
|
in which case it is higher.
|
|
.Sp
|
|
Leaving \f(CW\*(C`ev_run\*(C'\fR abnormally (setjmp/longjmp, cancelling the thread,
|
|
throwing an exception etc.), doesn't count as \*(L"exit\*(R" \- consider this
|
|
as a hint to avoid such ungentleman-like behaviour unless it's really
|
|
convenient, in which case it is fully supported.
|
|
.IP "unsigned int ev_backend (loop)" 4
|
|
.IX Item "unsigned int ev_backend (loop)"
|
|
Returns one of the \f(CW\*(C`EVBACKEND_*\*(C'\fR flags indicating the event backend in
|
|
use.
|
|
.IP "ev_tstamp ev_now (loop)" 4
|
|
.IX Item "ev_tstamp ev_now (loop)"
|
|
Returns the current \*(L"event loop time\*(R", 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).
|
|
.IP "ev_now_update (loop)" 4
|
|
.IX Item "ev_now_update (loop)"
|
|
Establishes the current time by querying the kernel, updating the time
|
|
returned by \f(CW\*(C`ev_now ()\*(C'\fR in the progress. This is a costly operation and
|
|
is usually done automatically within \f(CW\*(C`ev_run ()\*(C'\fR.
|
|
.Sp
|
|
This function is rarely useful, but when some event callback runs for a
|
|
very long time without entering the event loop, updating libev's idea of
|
|
the current time is a good idea.
|
|
.Sp
|
|
See also \*(L"The special problem of time updates\*(R" in the \f(CW\*(C`ev_timer\*(C'\fR section.
|
|
.IP "ev_suspend (loop)" 4
|
|
.IX Item "ev_suspend (loop)"
|
|
.PD 0
|
|
.IP "ev_resume (loop)" 4
|
|
.IX Item "ev_resume (loop)"
|
|
.PD
|
|
These two functions suspend and resume an event loop, for use when the
|
|
loop is not used for a while and timeouts should not be processed.
|
|
.Sp
|
|
A typical use case would be an interactive program such as a game: When
|
|
the user presses \f(CW\*(C`^Z\*(C'\fR to suspend the game and resumes it an hour later it
|
|
would be best to handle timeouts as if no time had actually passed while
|
|
the program was suspended. This can be achieved by calling \f(CW\*(C`ev_suspend\*(C'\fR
|
|
in your \f(CW\*(C`SIGTSTP\*(C'\fR handler, sending yourself a \f(CW\*(C`SIGSTOP\*(C'\fR and calling
|
|
\&\f(CW\*(C`ev_resume\*(C'\fR directly afterwards to resume timer processing.
|
|
.Sp
|
|
Effectively, all \f(CW\*(C`ev_timer\*(C'\fR watchers will be delayed by the time spend
|
|
between \f(CW\*(C`ev_suspend\*(C'\fR and \f(CW\*(C`ev_resume\*(C'\fR, and all \f(CW\*(C`ev_periodic\*(C'\fR watchers
|
|
will be rescheduled (that is, they will lose any events that would have
|
|
occurred while suspended).
|
|
.Sp
|
|
After calling \f(CW\*(C`ev_suspend\*(C'\fR you \fBmust not\fR call \fIany\fR function on the
|
|
given loop other than \f(CW\*(C`ev_resume\*(C'\fR, and you \fBmust not\fR call \f(CW\*(C`ev_resume\*(C'\fR
|
|
without a previous call to \f(CW\*(C`ev_suspend\*(C'\fR.
|
|
.Sp
|
|
Calling \f(CW\*(C`ev_suspend\*(C'\fR/\f(CW\*(C`ev_resume\*(C'\fR has the side effect of updating the
|
|
event loop time (see \f(CW\*(C`ev_now_update\*(C'\fR).
|
|
.IP "bool ev_run (loop, int flags)" 4
|
|
.IX Item "bool ev_run (loop, int flags)"
|
|
Finally, this is it, the event handler. This function usually is called
|
|
after you have initialised all your watchers and you want to start
|
|
handling events. It will ask the operating system for any new events, call
|
|
the watcher callbacks, and then repeat the whole process indefinitely: This
|
|
is why event loops are called \fIloops\fR.
|
|
.Sp
|
|
If the flags argument is specified as \f(CW0\fR, it will keep handling events
|
|
until either no event watchers are active anymore or \f(CW\*(C`ev_break\*(C'\fR was
|
|
called.
|
|
.Sp
|
|
The return value is false if there are no more active watchers (which
|
|
usually means \*(L"all jobs done\*(R" or \*(L"deadlock\*(R"), and true in all other cases
|
|
(which usually means " you should call \f(CW\*(C`ev_run\*(C'\fR again").
|
|
.Sp
|
|
Please note that an explicit \f(CW\*(C`ev_break\*(C'\fR is usually better than
|
|
relying on all watchers to be stopped when deciding when a program has
|
|
finished (especially in interactive programs), but having a program
|
|
that automatically loops as long as it has to and no longer by virtue
|
|
of relying on its watchers stopping correctly, that is truly a thing of
|
|
beauty.
|
|
.Sp
|
|
This function is \fImostly\fR exception-safe \- you can break out of a
|
|
\&\f(CW\*(C`ev_run\*(C'\fR call by calling \f(CW\*(C`longjmp\*(C'\fR in a callback, throwing a \*(C+
|
|
exception and so on. This does not decrement the \f(CW\*(C`ev_depth\*(C'\fR value, nor
|
|
will it clear any outstanding \f(CW\*(C`EVBREAK_ONE\*(C'\fR breaks.
|
|
.Sp
|
|
A flags value of \f(CW\*(C`EVRUN_NOWAIT\*(C'\fR will look for new events, will handle
|
|
those events and any already outstanding ones, but will not wait and
|
|
block your process in case there are no events and will return after one
|
|
iteration of the loop. This is sometimes useful to poll and handle new
|
|
events while doing lengthy calculations, to keep the program responsive.
|
|
.Sp
|
|
A flags value of \f(CW\*(C`EVRUN_ONCE\*(C'\fR will look for new events (waiting if
|
|
necessary) and will handle those and any already outstanding ones. It
|
|
will block your process until at least one new event arrives (which could
|
|
be an event internal to libev itself, so there is no guarantee that a
|
|
user-registered callback will be called), and will return after one
|
|
iteration of the loop.
|
|
.Sp
|
|
This is useful if you are waiting for some external event in conjunction
|
|
with something not expressible using other libev watchers (i.e. "roll your
|
|
own \f(CW\*(C`ev_run\*(C'\fR"). However, a pair of \f(CW\*(C`ev_prepare\*(C'\fR/\f(CW\*(C`ev_check\*(C'\fR watchers is
|
|
usually a better approach for this kind of thing.
|
|
.Sp
|
|
Here are the gory details of what \f(CW\*(C`ev_run\*(C'\fR does (this is for your
|
|
understanding, not a guarantee that things will work exactly like this in
|
|
future versions):
|
|
.Sp
|
|
.Vb 10
|
|
\& \- Increment loop depth.
|
|
\& \- Reset the ev_break status.
|
|
\& \- Before the first iteration, call any pending watchers.
|
|
\& LOOP:
|
|
\& \- If EVFLAG_FORKCHECK was used, check for a fork.
|
|
\& \- If a fork was detected (by any means), queue and call all fork watchers.
|
|
\& \- Queue and call all prepare watchers.
|
|
\& \- If ev_break was called, goto FINISH.
|
|
\& \- If we have been forked, detach and recreate the kernel state
|
|
\& as to not disturb the other process.
|
|
\& \- Update the kernel state with all outstanding changes.
|
|
\& \- Update the "event loop time" (ev_now ()).
|
|
\& \- Calculate for how long to sleep or block, if at all
|
|
\& (active idle watchers, EVRUN_NOWAIT or not having
|
|
\& any active watchers at all will result in not sleeping).
|
|
\& \- Sleep if the I/O and timer collect interval say so.
|
|
\& \- Increment loop iteration counter.
|
|
\& \- Block the process, waiting for any events.
|
|
\& \- Queue all outstanding I/O (fd) events.
|
|
\& \- Update the "event loop time" (ev_now ()), and do time jump adjustments.
|
|
\& \- Queue all expired timers.
|
|
\& \- Queue all expired periodics.
|
|
\& \- Queue all idle watchers with priority higher than that of pending events.
|
|
\& \- Queue all check watchers.
|
|
\& \- Call all queued watchers in reverse order (i.e. check watchers first).
|
|
\& Signals and child watchers are implemented as I/O watchers, and will
|
|
\& be handled here by queueing them when their watcher gets executed.
|
|
\& \- If ev_break has been called, or EVRUN_ONCE or EVRUN_NOWAIT
|
|
\& were used, or there are no active watchers, goto FINISH, otherwise
|
|
\& continue with step LOOP.
|
|
\& FINISH:
|
|
\& \- Reset the ev_break status iff it was EVBREAK_ONE.
|
|
\& \- Decrement the loop depth.
|
|
\& \- Return.
|
|
.Ve
|
|
.Sp
|
|
Example: Queue some jobs and then loop until no events are outstanding
|
|
anymore.
|
|
.Sp
|
|
.Vb 4
|
|
\& ... queue jobs here, make sure they register event watchers as long
|
|
\& ... as they still have work to do (even an idle watcher will do..)
|
|
\& ev_run (my_loop, 0);
|
|
\& ... jobs done or somebody called break. yeah!
|
|
.Ve
|
|
.IP "ev_break (loop, how)" 4
|
|
.IX Item "ev_break (loop, how)"
|
|
Can be used to make a call to \f(CW\*(C`ev_run\*(C'\fR return early (but only after it
|
|
has processed all outstanding events). The \f(CW\*(C`how\*(C'\fR argument must be either
|
|
\&\f(CW\*(C`EVBREAK_ONE\*(C'\fR, which will make the innermost \f(CW\*(C`ev_run\*(C'\fR call return, or
|
|
\&\f(CW\*(C`EVBREAK_ALL\*(C'\fR, which will make all nested \f(CW\*(C`ev_run\*(C'\fR calls return.
|
|
.Sp
|
|
This \*(L"break state\*(R" will be cleared on the next call to \f(CW\*(C`ev_run\*(C'\fR.
|
|
.Sp
|
|
It is safe to call \f(CW\*(C`ev_break\*(C'\fR from outside any \f(CW\*(C`ev_run\*(C'\fR calls, too, in
|
|
which case it will have no effect.
|
|
.IP "ev_ref (loop)" 4
|
|
.IX Item "ev_ref (loop)"
|
|
.PD 0
|
|
.IP "ev_unref (loop)" 4
|
|
.IX Item "ev_unref (loop)"
|
|
.PD
|
|
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, \f(CW\*(C`ev_run\*(C'\fR will not return on its own.
|
|
.Sp
|
|
This is useful when you have a watcher that you never intend to
|
|
unregister, but that nevertheless should not keep \f(CW\*(C`ev_run\*(C'\fR from
|
|
returning. In such a case, call \f(CW\*(C`ev_unref\*(C'\fR after starting, and \f(CW\*(C`ev_ref\*(C'\fR
|
|
before stopping it.
|
|
.Sp
|
|
As an example, libev itself uses this for its internal signal pipe: It
|
|
is not visible to the libev user and should not keep \f(CW\*(C`ev_run\*(C'\fR 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 \fIunref after start\fR and \fIref
|
|
before stop\fR (but only if the watcher wasn't active before, or was active
|
|
before, respectively. Note also that libev might stop watchers itself
|
|
(e.g. non-repeating timers) in which case you have to \f(CW\*(C`ev_ref\*(C'\fR
|
|
in the callback).
|
|
.Sp
|
|
Example: Create a signal watcher, but keep it from keeping \f(CW\*(C`ev_run\*(C'\fR
|
|
running when nothing else is active.
|
|
.Sp
|
|
.Vb 4
|
|
\& ev_signal exitsig;
|
|
\& ev_signal_init (&exitsig, sig_cb, SIGINT);
|
|
\& ev_signal_start (loop, &exitsig);
|
|
\& ev_unref (loop);
|
|
.Ve
|
|
.Sp
|
|
Example: For some weird reason, unregister the above signal handler again.
|
|
.Sp
|
|
.Vb 2
|
|
\& ev_ref (loop);
|
|
\& ev_signal_stop (loop, &exitsig);
|
|
.Ve
|
|
.IP "ev_set_io_collect_interval (loop, ev_tstamp interval)" 4
|
|
.IX Item "ev_set_io_collect_interval (loop, ev_tstamp interval)"
|
|
.PD 0
|
|
.IP "ev_set_timeout_collect_interval (loop, ev_tstamp interval)" 4
|
|
.IX Item "ev_set_timeout_collect_interval (loop, ev_tstamp interval)"
|
|
.PD
|
|
These advanced functions influence the time that libev will spend waiting
|
|
for events. Both time intervals are by default \f(CW0\fR, meaning that libev
|
|
will try to invoke timer/periodic callbacks and I/O callbacks with minimum
|
|
latency.
|
|
.Sp
|
|
Setting these to a higher value (the \f(CW\*(C`interval\*(C'\fR \fImust\fR be >= \f(CW0\fR)
|
|
allows libev to delay invocation of I/O and timer/periodic callbacks
|
|
to increase efficiency of loop iterations (or to increase power-saving
|
|
opportunities).
|
|
.Sp
|
|
The idea is that sometimes your program runs just fast enough to handle
|
|
one (or very few) event(s) per loop iteration. While this makes the
|
|
program responsive, it also wastes a lot of \s-1CPU\s0 time to poll for new
|
|
events, especially with backends like \f(CW\*(C`select ()\*(C'\fR which have a high
|
|
overhead for the actual polling but can deliver many events at once.
|
|
.Sp
|
|
By setting a higher \fIio collect interval\fR 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 \f(CW\*(C`ev_periodic\*(C'\fR and
|
|
\&\f(CW\*(C`ev_timer\*(C'\fR) will not be affected. Setting this to a non-null value will
|
|
introduce an additional \f(CW\*(C`ev_sleep ()\*(C'\fR call into most loop iterations. The
|
|
sleep time ensures that libev will not poll for I/O events more often then
|
|
once per this interval, on average (as long as the host time resolution is
|
|
good enough).
|
|
.Sp
|
|
Likewise, by setting a higher \fItimeout collect interval\fR you allow libev
|
|
to spend more time collecting timeouts, at the expense of increased
|
|
latency/jitter/inexactness (the watcher callback will be called
|
|
later). \f(CW\*(C`ev_io\*(C'\fR watchers will not be affected. Setting this to a non-null
|
|
value will not introduce any overhead in libev.
|
|
.Sp
|
|
Many (busy) programs can usually benefit by setting the I/O collect
|
|
interval to a value near \f(CW0.1\fR 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 \f(CW0.01\fR,
|
|
as this approaches the timing granularity of most systems. Note that if
|
|
you do transactions with the outside world and you can't increase the
|
|
parallelity, then this setting will limit your transaction rate (if you
|
|
need to poll once per transaction and the I/O collect interval is 0.01,
|
|
then you can't do more than 100 transactions per second).
|
|
.Sp
|
|
Setting the \fItimeout collect interval\fR can improve the opportunity for
|
|
saving power, as the program will \*(L"bundle\*(R" timer callback invocations that
|
|
are \*(L"near\*(R" in time together, by delaying some, thus reducing the number of
|
|
times the process sleeps and wakes up again. Another useful technique to
|
|
reduce iterations/wake\-ups is to use \f(CW\*(C`ev_periodic\*(C'\fR watchers and make sure
|
|
they fire on, say, one-second boundaries only.
|
|
.Sp
|
|
Example: we only need 0.1s timeout granularity, and we wish not to poll
|
|
more often than 100 times per second:
|
|
.Sp
|
|
.Vb 2
|
|
\& ev_set_timeout_collect_interval (EV_DEFAULT_UC_ 0.1);
|
|
\& ev_set_io_collect_interval (EV_DEFAULT_UC_ 0.01);
|
|
.Ve
|
|
.IP "ev_invoke_pending (loop)" 4
|
|
.IX Item "ev_invoke_pending (loop)"
|
|
This call will simply invoke all pending watchers while resetting their
|
|
pending state. Normally, \f(CW\*(C`ev_run\*(C'\fR does this automatically when required,
|
|
but when overriding the invoke callback this call comes handy. This
|
|
function can be invoked from a watcher \- this can be useful for example
|
|
when you want to do some lengthy calculation and want to pass further
|
|
event handling to another thread (you still have to make sure only one
|
|
thread executes within \f(CW\*(C`ev_invoke_pending\*(C'\fR or \f(CW\*(C`ev_run\*(C'\fR of course).
|
|
.IP "int ev_pending_count (loop)" 4
|
|
.IX Item "int ev_pending_count (loop)"
|
|
Returns the number of pending watchers \- zero indicates that no watchers
|
|
are pending.
|
|
.IP "ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(\s-1EV_P\s0))" 4
|
|
.IX Item "ev_set_invoke_pending_cb (loop, void (*invoke_pending_cb)(EV_P))"
|
|
This overrides the invoke pending functionality of the loop: Instead of
|
|
invoking all pending watchers when there are any, \f(CW\*(C`ev_run\*(C'\fR will call
|
|
this callback instead. This is useful, for example, when you want to
|
|
invoke the actual watchers inside another context (another thread etc.).
|
|
.Sp
|
|
If you want to reset the callback, use \f(CW\*(C`ev_invoke_pending\*(C'\fR as new
|
|
callback.
|
|
.IP "ev_set_loop_release_cb (loop, void (*release)(\s-1EV_P\s0) throw (), void (*acquire)(\s-1EV_P\s0) throw ())" 4
|
|
.IX Item "ev_set_loop_release_cb (loop, void (*release)(EV_P) throw (), void (*acquire)(EV_P) throw ())"
|
|
Sometimes you want to share the same loop between multiple threads. This
|
|
can be done relatively simply by putting mutex_lock/unlock calls around
|
|
each call to a libev function.
|
|
.Sp
|
|
However, \f(CW\*(C`ev_run\*(C'\fR can run an indefinite time, so it is not feasible
|
|
to wait for it to return. One way around this is to wake up the event
|
|
loop via \f(CW\*(C`ev_break\*(C'\fR and \f(CW\*(C`ev_async_send\*(C'\fR, another way is to set these
|
|
\&\fIrelease\fR and \fIacquire\fR callbacks on the loop.
|
|
.Sp
|
|
When set, then \f(CW\*(C`release\*(C'\fR will be called just before the thread is
|
|
suspended waiting for new events, and \f(CW\*(C`acquire\*(C'\fR is called just
|
|
afterwards.
|
|
.Sp
|
|
Ideally, \f(CW\*(C`release\*(C'\fR will just call your mutex_unlock function, and
|
|
\&\f(CW\*(C`acquire\*(C'\fR will just call the mutex_lock function again.
|
|
.Sp
|
|
While event loop modifications are allowed between invocations of
|
|
\&\f(CW\*(C`release\*(C'\fR and \f(CW\*(C`acquire\*(C'\fR (that's their only purpose after all), no
|
|
modifications done will affect the event loop, i.e. adding watchers will
|
|
have no effect on the set of file descriptors being watched, or the time
|
|
waited. Use an \f(CW\*(C`ev_async\*(C'\fR watcher to wake up \f(CW\*(C`ev_run\*(C'\fR when you want it
|
|
to take note of any changes you made.
|
|
.Sp
|
|
In theory, threads executing \f(CW\*(C`ev_run\*(C'\fR will be async-cancel safe between
|
|
invocations of \f(CW\*(C`release\*(C'\fR and \f(CW\*(C`acquire\*(C'\fR.
|
|
.Sp
|
|
See also the locking example in the \f(CW\*(C`THREADS\*(C'\fR section later in this
|
|
document.
|
|
.IP "ev_set_userdata (loop, void *data)" 4
|
|
.IX Item "ev_set_userdata (loop, void *data)"
|
|
.PD 0
|
|
.IP "void *ev_userdata (loop)" 4
|
|
.IX Item "void *ev_userdata (loop)"
|
|
.PD
|
|
Set and retrieve a single \f(CW\*(C`void *\*(C'\fR associated with a loop. When
|
|
\&\f(CW\*(C`ev_set_userdata\*(C'\fR has never been called, then \f(CW\*(C`ev_userdata\*(C'\fR returns
|
|
\&\f(CW0\fR.
|
|
.Sp
|
|
These two functions can be used to associate arbitrary data with a loop,
|
|
and are intended solely for the \f(CW\*(C`invoke_pending_cb\*(C'\fR, \f(CW\*(C`release\*(C'\fR and
|
|
\&\f(CW\*(C`acquire\*(C'\fR callbacks described above, but of course can be (ab\-)used for
|
|
any other purpose as well.
|
|
.IP "ev_verify (loop)" 4
|
|
.IX Item "ev_verify (loop)"
|
|
This function only does something when \f(CW\*(C`EV_VERIFY\*(C'\fR support has been
|
|
compiled in, which is the default for non-minimal builds. It tries to go
|
|
through all internal structures and checks them for validity. If anything
|
|
is found to be inconsistent, it will print an error message to standard
|
|
error and call \f(CW\*(C`abort ()\*(C'\fR.
|
|
.Sp
|
|
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.
|
|
.SH "ANATOMY OF A WATCHER"
|
|
.IX Header "ANATOMY OF A WATCHER"
|
|
In the following description, uppercase \f(CW\*(C`TYPE\*(C'\fR in names stands for the
|
|
watcher type, e.g. \f(CW\*(C`ev_TYPE_start\*(C'\fR can mean \f(CW\*(C`ev_timer_start\*(C'\fR for timer
|
|
watchers and \f(CW\*(C`ev_io_start\*(C'\fR for I/O watchers.
|
|
.PP
|
|
A watcher is an opaque structure that you allocate and register to record
|
|
your interest in some event. To make a concrete example, imagine you want
|
|
to wait for \s-1STDIN\s0 to become readable, you would create an \f(CW\*(C`ev_io\*(C'\fR watcher
|
|
for that:
|
|
.PP
|
|
.Vb 5
|
|
\& static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
|
|
\& {
|
|
\& ev_io_stop (w);
|
|
\& ev_break (loop, EVBREAK_ALL);
|
|
\& }
|
|
\&
|
|
\& struct ev_loop *loop = ev_default_loop (0);
|
|
\&
|
|
\& ev_io stdin_watcher;
|
|
\&
|
|
\& ev_init (&stdin_watcher, my_cb);
|
|
\& ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
|
|
\& ev_io_start (loop, &stdin_watcher);
|
|
\&
|
|
\& ev_run (loop, 0);
|
|
.Ve
|
|
.PP
|
|
As you can see, you are responsible for allocating the memory for your
|
|
watcher structures (and it is \fIusually\fR a bad idea to do this on the
|
|
stack).
|
|
.PP
|
|
Each watcher has an associated watcher structure (called \f(CW\*(C`struct ev_TYPE\*(C'\fR
|
|
or simply \f(CW\*(C`ev_TYPE\*(C'\fR, as typedefs are provided for all watcher structs).
|
|
.PP
|
|
Each watcher structure must be initialised by a call to \f(CW\*(C`ev_init (watcher
|
|
*, callback)\*(C'\fR, which expects a callback to be provided. This callback is
|
|
invoked each time the event occurs (or, in the case of I/O watchers, each
|
|
time the event loop detects that the file descriptor given is readable
|
|
and/or writable).
|
|
.PP
|
|
Each watcher type further has its own \f(CW\*(C`ev_TYPE_set (watcher *, ...)\*(C'\fR
|
|
macro to configure it, with arguments specific to the watcher type. There
|
|
is also a macro to combine initialisation and setting in one call: \f(CW\*(C`ev_TYPE_init (watcher *, callback, ...)\*(C'\fR.
|
|
.PP
|
|
To make the watcher actually watch out for events, you have to start it
|
|
with a watcher-specific start function (\f(CW\*(C`ev_TYPE_start (loop, watcher
|
|
*)\*(C'\fR), and you can stop watching for events at any time by calling the
|
|
corresponding stop function (\f(CW\*(C`ev_TYPE_stop (loop, watcher *)\*(C'\fR.
|
|
.PP
|
|
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 \f(CW\*(C`ev_TYPE_set\*(C'\fR macro.
|
|
.PP
|
|
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.
|
|
.PP
|
|
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:
|
|
.ie n .IP """EV_READ""" 4
|
|
.el .IP "\f(CWEV_READ\fR" 4
|
|
.IX Item "EV_READ"
|
|
.PD 0
|
|
.ie n .IP """EV_WRITE""" 4
|
|
.el .IP "\f(CWEV_WRITE\fR" 4
|
|
.IX Item "EV_WRITE"
|
|
.PD
|
|
The file descriptor in the \f(CW\*(C`ev_io\*(C'\fR watcher has become readable and/or
|
|
writable.
|
|
.ie n .IP """EV_TIMER""" 4
|
|
.el .IP "\f(CWEV_TIMER\fR" 4
|
|
.IX Item "EV_TIMER"
|
|
The \f(CW\*(C`ev_timer\*(C'\fR watcher has timed out.
|
|
.ie n .IP """EV_PERIODIC""" 4
|
|
.el .IP "\f(CWEV_PERIODIC\fR" 4
|
|
.IX Item "EV_PERIODIC"
|
|
The \f(CW\*(C`ev_periodic\*(C'\fR watcher has timed out.
|
|
.ie n .IP """EV_SIGNAL""" 4
|
|
.el .IP "\f(CWEV_SIGNAL\fR" 4
|
|
.IX Item "EV_SIGNAL"
|
|
The signal specified in the \f(CW\*(C`ev_signal\*(C'\fR watcher has been received by a thread.
|
|
.ie n .IP """EV_CHILD""" 4
|
|
.el .IP "\f(CWEV_CHILD\fR" 4
|
|
.IX Item "EV_CHILD"
|
|
The pid specified in the \f(CW\*(C`ev_child\*(C'\fR watcher has received a status change.
|
|
.ie n .IP """EV_STAT""" 4
|
|
.el .IP "\f(CWEV_STAT\fR" 4
|
|
.IX Item "EV_STAT"
|
|
The path specified in the \f(CW\*(C`ev_stat\*(C'\fR watcher changed its attributes somehow.
|
|
.ie n .IP """EV_IDLE""" 4
|
|
.el .IP "\f(CWEV_IDLE\fR" 4
|
|
.IX Item "EV_IDLE"
|
|
The \f(CW\*(C`ev_idle\*(C'\fR watcher has determined that you have nothing better to do.
|
|
.ie n .IP """EV_PREPARE""" 4
|
|
.el .IP "\f(CWEV_PREPARE\fR" 4
|
|
.IX Item "EV_PREPARE"
|
|
.PD 0
|
|
.ie n .IP """EV_CHECK""" 4
|
|
.el .IP "\f(CWEV_CHECK\fR" 4
|
|
.IX Item "EV_CHECK"
|
|
.PD
|
|
All \f(CW\*(C`ev_prepare\*(C'\fR watchers are invoked just \fIbefore\fR \f(CW\*(C`ev_run\*(C'\fR starts to
|
|
gather new events, and all \f(CW\*(C`ev_check\*(C'\fR watchers are queued (not invoked)
|
|
just after \f(CW\*(C`ev_run\*(C'\fR has gathered them, but before it queues any callbacks
|
|
for any received events. That means \f(CW\*(C`ev_prepare\*(C'\fR watchers are the last
|
|
watchers invoked before the event loop sleeps or polls for new events, and
|
|
\&\f(CW\*(C`ev_check\*(C'\fR watchers will be invoked before any other watchers of the same
|
|
or lower priority within an event loop iteration.
|
|
.Sp
|
|
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
|
|
\&\f(CW\*(C`ev_prepare\*(C'\fR watcher might start an idle watcher to keep \f(CW\*(C`ev_run\*(C'\fR from
|
|
blocking).
|
|
.ie n .IP """EV_EMBED""" 4
|
|
.el .IP "\f(CWEV_EMBED\fR" 4
|
|
.IX Item "EV_EMBED"
|
|
The embedded event loop specified in the \f(CW\*(C`ev_embed\*(C'\fR watcher needs attention.
|
|
.ie n .IP """EV_FORK""" 4
|
|
.el .IP "\f(CWEV_FORK\fR" 4
|
|
.IX Item "EV_FORK"
|
|
The event loop has been resumed in the child process after fork (see
|
|
\&\f(CW\*(C`ev_fork\*(C'\fR).
|
|
.ie n .IP """EV_CLEANUP""" 4
|
|
.el .IP "\f(CWEV_CLEANUP\fR" 4
|
|
.IX Item "EV_CLEANUP"
|
|
The event loop is about to be destroyed (see \f(CW\*(C`ev_cleanup\*(C'\fR).
|
|
.ie n .IP """EV_ASYNC""" 4
|
|
.el .IP "\f(CWEV_ASYNC\fR" 4
|
|
.IX Item "EV_ASYNC"
|
|
The given async watcher has been asynchronously notified (see \f(CW\*(C`ev_async\*(C'\fR).
|
|
.ie n .IP """EV_CUSTOM""" 4
|
|
.el .IP "\f(CWEV_CUSTOM\fR" 4
|
|
.IX Item "EV_CUSTOM"
|
|
Not ever sent (or otherwise used) by libev itself, but can be freely used
|
|
by libev users to signal watchers (e.g. via \f(CW\*(C`ev_feed_event\*(C'\fR).
|
|
.ie n .IP """EV_ERROR""" 4
|
|
.el .IP "\f(CWEV_ERROR\fR" 4
|
|
.IX Item "EV_ERROR"
|
|
An unspecified error has occurred, the watcher has been stopped. This might
|
|
happen because the watcher could not be properly started because libev
|
|
ran out of memory, a file descriptor was found to be closed or any other
|
|
problem. Libev considers these application bugs.
|
|
.Sp
|
|
You best act on it by reporting the problem and somehow coping with the
|
|
watcher being stopped. Note that well-written programs should not receive
|
|
an error ever, so when your watcher receives it, this usually indicates a
|
|
bug in your program.
|
|
.Sp
|
|
Libev will usually signal a few \*(L"dummy\*(R" 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 \fBread()\fR or \fBwrite()\fR. This will not work in multi-threaded
|
|
programs, though, as the fd could already be closed and reused for another
|
|
thing, so beware.
|
|
.SS "\s-1GENERIC WATCHER FUNCTIONS\s0"
|
|
.IX Subsection "GENERIC WATCHER FUNCTIONS"
|
|
.ie n .IP """ev_init"" (ev_TYPE *watcher, callback)" 4
|
|
.el .IP "\f(CWev_init\fR (ev_TYPE *watcher, callback)" 4
|
|
.IX Item "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 \f(CW\*(C`malloc\*(C'\fR will do). Only
|
|
the generic parts of the watcher are initialised, you \fIneed\fR to call
|
|
the type-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR macro afterwards to initialise the
|
|
type-specific parts. For each type there is also a \f(CW\*(C`ev_TYPE_init\*(C'\fR macro
|
|
which rolls both calls into one.
|
|
.Sp
|
|
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.
|
|
.Sp
|
|
The callback is always of type \f(CW\*(C`void (*)(struct ev_loop *loop, ev_TYPE *watcher,
|
|
int revents)\*(C'\fR.
|
|
.Sp
|
|
Example: Initialise an \f(CW\*(C`ev_io\*(C'\fR watcher in two steps.
|
|
.Sp
|
|
.Vb 3
|
|
\& ev_io w;
|
|
\& ev_init (&w, my_cb);
|
|
\& ev_io_set (&w, STDIN_FILENO, EV_READ);
|
|
.Ve
|
|
.ie n .IP """ev_TYPE_set"" (ev_TYPE *watcher, [args])" 4
|
|
.el .IP "\f(CWev_TYPE_set\fR (ev_TYPE *watcher, [args])" 4
|
|
.IX Item "ev_TYPE_set (ev_TYPE *watcher, [args])"
|
|
This macro initialises the type-specific parts of a watcher. You need to
|
|
call \f(CW\*(C`ev_init\*(C'\fR at least once before you call this macro, but you can
|
|
call \f(CW\*(C`ev_TYPE_set\*(C'\fR 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 \f(CW\*(C`ev_init\*(C'\fR macro).
|
|
.Sp
|
|
Although some watcher types do not have type-specific arguments
|
|
(e.g. \f(CW\*(C`ev_prepare\*(C'\fR) you still need to call its \f(CW\*(C`set\*(C'\fR macro.
|
|
.Sp
|
|
See \f(CW\*(C`ev_init\*(C'\fR, above, for an example.
|
|
.ie n .IP """ev_TYPE_init"" (ev_TYPE *watcher, callback, [args])" 4
|
|
.el .IP "\f(CWev_TYPE_init\fR (ev_TYPE *watcher, callback, [args])" 4
|
|
.IX Item "ev_TYPE_init (ev_TYPE *watcher, callback, [args])"
|
|
This convenience macro rolls both \f(CW\*(C`ev_init\*(C'\fR and \f(CW\*(C`ev_TYPE_set\*(C'\fR macro
|
|
calls into a single call. This is the most convenient method to initialise
|
|
a watcher. The same limitations apply, of course.
|
|
.Sp
|
|
Example: Initialise and set an \f(CW\*(C`ev_io\*(C'\fR watcher in one step.
|
|
.Sp
|
|
.Vb 1
|
|
\& ev_io_init (&w, my_cb, STDIN_FILENO, EV_READ);
|
|
.Ve
|
|
.ie n .IP """ev_TYPE_start"" (loop, ev_TYPE *watcher)" 4
|
|
.el .IP "\f(CWev_TYPE_start\fR (loop, ev_TYPE *watcher)" 4
|
|
.IX Item "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.
|
|
.Sp
|
|
Example: Start the \f(CW\*(C`ev_io\*(C'\fR watcher that is being abused as example in this
|
|
whole section.
|
|
.Sp
|
|
.Vb 1
|
|
\& ev_io_start (EV_DEFAULT_UC, &w);
|
|
.Ve
|
|
.ie n .IP """ev_TYPE_stop"" (loop, ev_TYPE *watcher)" 4
|
|
.el .IP "\f(CWev_TYPE_stop\fR (loop, ev_TYPE *watcher)" 4
|
|
.IX Item "ev_TYPE_stop (loop, ev_TYPE *watcher)"
|
|
Stops the given watcher if active, and clears the pending status (whether
|
|
the watcher was active or not).
|
|
.Sp
|
|
It is possible that stopped watchers are pending \- for example,
|
|
non-repeating timers are being stopped when they become pending \- but
|
|
calling \f(CW\*(C`ev_TYPE_stop\*(C'\fR 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 \f(CW\*(C`ev_TYPE_stop\*(C'\fR function.
|
|
.IP "bool ev_is_active (ev_TYPE *watcher)" 4
|
|
.IX Item "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.
|
|
.IP "bool ev_is_pending (ev_TYPE *watcher)" 4
|
|
.IX Item "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
|
|
\&\f(CW\*(C`ev_TYPE_set\*(C'\fR is safe), you must not change its priority, and you must
|
|
make sure the watcher is available to libev (e.g. you cannot \f(CW\*(C`free ()\*(C'\fR
|
|
it).
|
|
.IP "callback ev_cb (ev_TYPE *watcher)" 4
|
|
.IX Item "callback ev_cb (ev_TYPE *watcher)"
|
|
Returns the callback currently set on the watcher.
|
|
.IP "ev_set_cb (ev_TYPE *watcher, callback)" 4
|
|
.IX Item "ev_set_cb (ev_TYPE *watcher, callback)"
|
|
Change the callback. You can change the callback at virtually any time
|
|
(modulo threads).
|
|
.IP "ev_set_priority (ev_TYPE *watcher, int priority)" 4
|
|
.IX Item "ev_set_priority (ev_TYPE *watcher, int priority)"
|
|
.PD 0
|
|
.IP "int ev_priority (ev_TYPE *watcher)" 4
|
|
.IX Item "int ev_priority (ev_TYPE *watcher)"
|
|
.PD
|
|
Set and query the priority of the watcher. The priority is a small
|
|
integer between \f(CW\*(C`EV_MAXPRI\*(C'\fR (default: \f(CW2\fR) and \f(CW\*(C`EV_MINPRI\*(C'\fR
|
|
(default: \f(CW\*(C`\-2\*(C'\fR). Pending watchers with higher priority will be invoked
|
|
before watchers with lower priority, but priority will not keep watchers
|
|
from being executed (except for \f(CW\*(C`ev_idle\*(C'\fR watchers).
|
|
.Sp
|
|
If you need to suppress invocation when higher priority events are pending
|
|
you need to look at \f(CW\*(C`ev_idle\*(C'\fR watchers, which provide this functionality.
|
|
.Sp
|
|
You \fImust not\fR change the priority of a watcher as long as it is active or
|
|
pending.
|
|
.Sp
|
|
Setting a priority outside the range of \f(CW\*(C`EV_MINPRI\*(C'\fR to \f(CW\*(C`EV_MAXPRI\*(C'\fR is
|
|
fine, as long as you do not mind that the priority value you query might
|
|
or might not have been clamped to the valid range.
|
|
.Sp
|
|
The default priority used by watchers when no priority has been set is
|
|
always \f(CW0\fR, which is supposed to not be too high and not be too low :).
|
|
.Sp
|
|
See \*(L"\s-1WATCHER PRIORITY MODELS\*(R"\s0, below, for a more thorough treatment of
|
|
priorities.
|
|
.IP "ev_invoke (loop, ev_TYPE *watcher, int revents)" 4
|
|
.IX Item "ev_invoke (loop, ev_TYPE *watcher, int revents)"
|
|
Invoke the \f(CW\*(C`watcher\*(C'\fR with the given \f(CW\*(C`loop\*(C'\fR and \f(CW\*(C`revents\*(C'\fR. Neither
|
|
\&\f(CW\*(C`loop\*(C'\fR nor \f(CW\*(C`revents\*(C'\fR need to be valid as long as the watcher callback
|
|
can deal with that fact, as both are simply passed through to the
|
|
callback.
|
|
.IP "int ev_clear_pending (loop, ev_TYPE *watcher)" 4
|
|
.IX Item "int ev_clear_pending (loop, ev_TYPE *watcher)"
|
|
If the watcher is pending, this function clears its pending status and
|
|
returns its \f(CW\*(C`revents\*(C'\fR bitset (as if its callback was invoked). If the
|
|
watcher isn't pending it does nothing and returns \f(CW0\fR.
|
|
.Sp
|
|
Sometimes it can be useful to \*(L"poll\*(R" a watcher instead of waiting for its
|
|
callback to be invoked, which can be accomplished with this function.
|
|
.IP "ev_feed_event (loop, ev_TYPE *watcher, int revents)" 4
|
|
.IX Item "ev_feed_event (loop, ev_TYPE *watcher, int revents)"
|
|
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). Obviously you must
|
|
not free the watcher as long as it has pending events.
|
|
.Sp
|
|
Stopping the watcher, letting libev invoke it, or calling
|
|
\&\f(CW\*(C`ev_clear_pending\*(C'\fR will clear the pending event, even if the watcher was
|
|
not started in the first place.
|
|
.Sp
|
|
See also \f(CW\*(C`ev_feed_fd_event\*(C'\fR and \f(CW\*(C`ev_feed_signal_event\*(C'\fR for related
|
|
functions that do not need a watcher.
|
|
.PP
|
|
See also the \*(L"\s-1ASSOCIATING CUSTOM DATA WITH A WATCHER\*(R"\s0 and \*(L"\s-1BUILDING YOUR
|
|
OWN COMPOSITE WATCHERS\*(R"\s0 idioms.
|
|
.SS "\s-1WATCHER STATES\s0"
|
|
.IX Subsection "WATCHER STATES"
|
|
There are various watcher states mentioned throughout this manual \-
|
|
active, pending and so on. In this section these states and the rules to
|
|
transition between them will be described in more detail \- and while these
|
|
rules might look complicated, they usually do \*(L"the right thing\*(R".
|
|
.IP "initialised" 4
|
|
.IX Item "initialised"
|
|
Before a watcher can be registered with the event loop it has to be
|
|
initialised. This can be done with a call to \f(CW\*(C`ev_TYPE_init\*(C'\fR, or calls to
|
|
\&\f(CW\*(C`ev_init\*(C'\fR followed by the watcher-specific \f(CW\*(C`ev_TYPE_set\*(C'\fR function.
|
|
.Sp
|
|
In this state it is simply some block of memory that is suitable for
|
|
use in an event loop. It can be moved around, freed, reused etc. at
|
|
will \- as long as you either keep the memory contents intact, or call
|
|
\&\f(CW\*(C`ev_TYPE_init\*(C'\fR again.
|
|
.IP "started/running/active" 4
|
|
.IX Item "started/running/active"
|
|
Once a watcher has been started with a call to \f(CW\*(C`ev_TYPE_start\*(C'\fR it becomes
|
|
property of the event loop, and is actively waiting for events. While in
|
|
this state it cannot be accessed (except in a few documented ways), moved,
|
|
freed or anything else \- the only legal thing is to keep a pointer to it,
|
|
and call libev functions on it that are documented to work on active watchers.
|
|
.IP "pending" 4
|
|
.IX Item "pending"
|
|
If a watcher is active and libev determines that an event it is interested
|
|
in has occurred (such as a timer expiring), it will become pending. It will
|
|
stay in this pending state until either it is stopped or its callback is
|
|
about to be invoked, so it is not normally pending inside the watcher
|
|
callback.
|
|
.Sp
|
|
The watcher might or might not be active while it is pending (for example,
|
|
an expired non-repeating timer can be pending but no longer active). If it
|
|
is stopped, it can be freely accessed (e.g. by calling \f(CW\*(C`ev_TYPE_set\*(C'\fR),
|
|
but it is still property of the event loop at this time, so cannot be
|
|
moved, freed or reused. And if it is active the rules described in the
|
|
previous item still apply.
|
|
.Sp
|
|
It is also possible to feed an event on a watcher that is not active (e.g.
|
|
via \f(CW\*(C`ev_feed_event\*(C'\fR), in which case it becomes pending without being
|
|
active.
|
|
.IP "stopped" 4
|
|
.IX Item "stopped"
|
|
A watcher can be stopped implicitly by libev (in which case it might still
|
|
be pending), or explicitly by calling its \f(CW\*(C`ev_TYPE_stop\*(C'\fR function. The
|
|
latter will clear any pending state the watcher might be in, regardless
|
|
of whether it was active or not, so stopping a watcher explicitly before
|
|
freeing it is often a good idea.
|
|
.Sp
|
|
While stopped (and not pending) the watcher is essentially in the
|
|
initialised state, that is, it can be reused, moved, modified in any way
|
|
you wish (but when you trash the memory block, you need to \f(CW\*(C`ev_TYPE_init\*(C'\fR
|
|
it again).
|
|
.SS "\s-1WATCHER PRIORITY MODELS\s0"
|
|
.IX Subsection "WATCHER PRIORITY MODELS"
|
|
Many event loops support \fIwatcher priorities\fR, which are usually small
|
|
integers that influence the ordering of event callback invocation
|
|
between watchers in some way, all else being equal.
|
|
.PP
|
|
In libev, Watcher priorities can be set using \f(CW\*(C`ev_set_priority\*(C'\fR. See its
|
|
description for the more technical details such as the actual priority
|
|
range.
|
|
.PP
|
|
There are two common ways how these these priorities are being interpreted
|
|
by event loops:
|
|
.PP
|
|
In the more common lock-out model, higher priorities \*(L"lock out\*(R" invocation
|
|
of lower priority watchers, which means as long as higher priority
|
|
watchers receive events, lower priority watchers are not being invoked.
|
|
.PP
|
|
The less common only-for-ordering model uses priorities solely to order
|
|
callback invocation within a single event loop iteration: Higher priority
|
|
watchers are invoked before lower priority ones, but they all get invoked
|
|
before polling for new events.
|
|
.PP
|
|
Libev uses the second (only-for-ordering) model for all its watchers
|
|
except for idle watchers (which use the lock-out model).
|
|
.PP
|
|
The rationale behind this is that implementing the lock-out model for
|
|
watchers is not well supported by most kernel interfaces, and most event
|
|
libraries will just poll for the same events again and again as long as
|
|
their callbacks have not been executed, which is very inefficient in the
|
|
common case of one high-priority watcher locking out a mass of lower
|
|
priority ones.
|
|
.PP
|
|
Static (ordering) priorities are most useful when you have two or more
|
|
watchers handling the same resource: a typical usage example is having an
|
|
\&\f(CW\*(C`ev_io\*(C'\fR watcher to receive data, and an associated \f(CW\*(C`ev_timer\*(C'\fR to handle
|
|
timeouts. Under load, data might be received while the program handles
|
|
other jobs, but since timers normally get invoked first, the timeout
|
|
handler will be executed before checking for data. In that case, giving
|
|
the timer a lower priority than the I/O watcher ensures that I/O will be
|
|
handled first even under adverse conditions (which is usually, but not
|
|
always, what you want).
|
|
.PP
|
|
Since idle watchers use the \*(L"lock-out\*(R" model, meaning that idle watchers
|
|
will only be executed when no same or higher priority watchers have
|
|
received events, they can be used to implement the \*(L"lock-out\*(R" model when
|
|
required.
|
|
.PP
|
|
For example, to emulate how many other event libraries handle priorities,
|
|
you can associate an \f(CW\*(C`ev_idle\*(C'\fR watcher to each such watcher, and in
|
|
the normal watcher callback, you just start the idle watcher. The real
|
|
processing is done in the idle watcher callback. This causes libev to
|
|
continuously poll and process kernel event data for the watcher, but when
|
|
the lock-out case is known to be rare (which in turn is rare :), this is
|
|
workable.
|
|
.PP
|
|
Usually, however, the lock-out model implemented that way will perform
|
|
miserably under the type of load it was designed to handle. In that case,
|
|
it might be preferable to stop the real watcher before starting the
|
|
idle watcher, so the kernel will not have to process the event in case
|
|
the actual processing will be delayed for considerable time.
|
|
.PP
|
|
Here is an example of an I/O watcher that should run at a strictly lower
|
|
priority than the default, and which should only process data when no
|
|
other events are pending:
|
|
.PP
|
|
.Vb 2
|
|
\& ev_idle idle; // actual processing watcher
|
|
\& ev_io io; // actual event watcher
|
|
\&
|
|
\& static void
|
|
\& io_cb (EV_P_ ev_io *w, int revents)
|
|
\& {
|
|
\& // stop the I/O watcher, we received the event, but
|
|
\& // are not yet ready to handle it.
|
|
\& ev_io_stop (EV_A_ w);
|
|
\&
|
|
\& // start the idle watcher to handle the actual event.
|
|
\& // it will not be executed as long as other watchers
|
|
\& // with the default priority are receiving events.
|
|
\& ev_idle_start (EV_A_ &idle);
|
|
\& }
|
|
\&
|
|
\& static void
|
|
\& idle_cb (EV_P_ ev_idle *w, int revents)
|
|
\& {
|
|
\& // actual processing
|
|
\& read (STDIN_FILENO, ...);
|
|
\&
|
|
\& // have to start the I/O watcher again, as
|
|
\& // we have handled the event
|
|
\& ev_io_start (EV_P_ &io);
|
|
\& }
|
|
\&
|
|
\& // initialisation
|
|
\& ev_idle_init (&idle, idle_cb);
|
|
\& ev_io_init (&io, io_cb, STDIN_FILENO, EV_READ);
|
|
\& ev_io_start (EV_DEFAULT_ &io);
|
|
.Ve
|
|
.PP
|
|
In the \*(L"real\*(R" world, it might also be beneficial to start a timer, so that
|
|
low-priority connections can not be locked out forever under load. This
|
|
enables your program to keep a lower latency for important connections
|
|
during short periods of high load, while not completely locking out less
|
|
important ones.
|
|
.SH "WATCHER TYPES"
|
|
.IX Header "WATCHER TYPES"
|
|
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.
|
|
.PP
|
|
Members are additionally marked with either \fI[read\-only]\fR, 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 \fI[read\-write]\fR, 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.
|
|
.ie n .SS """ev_io"" \- is this file descriptor readable or writable?"
|
|
.el .SS "\f(CWev_io\fP \- is this file descriptor readable or writable?"
|
|
.IX Subsection "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.
|
|
.PP
|
|
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).
|
|
.PP
|
|
Another thing you have to watch out for is that it is quite easy to
|
|
receive \*(L"spurious\*(R" readiness notifications, that is, your callback might
|
|
be called with \f(CW\*(C`EV_READ\*(C'\fR but a subsequent \f(CW\*(C`read\*(C'\fR(2) will actually block
|
|
because there is no data. It is very easy to get into this situation even
|
|
with a relatively standard program structure. Thus it is best to always
|
|
use non-blocking I/O: An extra \f(CW\*(C`read\*(C'\fR(2) returning \f(CW\*(C`EAGAIN\*(C'\fR is far
|
|
preferable to a program hanging until some data arrives.
|
|
.PP
|
|
If you cannot run the fd in non-blocking mode (for example you should
|
|
not play around with an Xlib connection), then you have to separately
|
|
re-test whether a file descriptor is really ready with a known-to-be good
|
|
interface such as poll (fortunately in the case of Xlib, it already does
|
|
this on its own, so its quite safe to use). Some people additionally
|
|
use \f(CW\*(C`SIGALRM\*(C'\fR and an interval timer, just to be sure you won't block
|
|
indefinitely.
|
|
.PP
|
|
But really, best use non-blocking mode.
|
|
.PP
|
|
\fIThe special problem of disappearing file descriptors\fR
|
|
.IX Subsection "The special problem of disappearing file descriptors"
|
|
.PP
|
|
Some backends (e.g. kqueue, epoll, linuxaio) need to be told about closing
|
|
a file descriptor (either due to calling \f(CW\*(C`close\*(C'\fR explicitly or any other
|
|
means, such as \f(CW\*(C`dup2\*(C'\fR). 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.
|
|
.PP
|
|
To avoid having to explicitly tell libev about such cases, libev follows
|
|
the following policy: Each time \f(CW\*(C`ev_io_set\*(C'\fR 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 \fIhave\fR to call \f(CW\*(C`ev_io_set\*(C'\fR (or \f(CW\*(C`ev_io_init\*(C'\fR) when you change the
|
|
descriptor even if the file descriptor number itself did not change.
|
|
.PP
|
|
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.
|
|
.PP
|
|
\fIThe special problem of dup'ed file descriptors\fR
|
|
.IX Subsection "The special problem of dup'ed file descriptors"
|
|
.PP
|
|
Some backends (e.g. epoll), cannot register events for file descriptors,
|
|
but only events for the underlying file descriptions. That means when you
|
|
have \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors or weirder constellations, and register
|
|
events for them, only one file descriptor might actually receive events.
|
|
.PP
|
|
There is no workaround possible except not registering events
|
|
for potentially \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors, or to resort to
|
|
\&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
|
|
.PP
|
|
\fIThe special problem of files\fR
|
|
.IX Subsection "The special problem of files"
|
|
.PP
|
|
Many people try to use \f(CW\*(C`select\*(C'\fR (or libev) on file descriptors
|
|
representing files, and expect it to become ready when their program
|
|
doesn't block on disk accesses (which can take a long time on their own).
|
|
.PP
|
|
However, this cannot ever work in the \*(L"expected\*(R" way \- you get a readiness
|
|
notification as soon as the kernel knows whether and how much data is
|
|
there, and in the case of open files, that's always the case, so you
|
|
always get a readiness notification instantly, and your read (or possibly
|
|
write) will still block on the disk I/O.
|
|
.PP
|
|
Another way to view it is that in the case of sockets, pipes, character
|
|
devices and so on, there is another party (the sender) that delivers data
|
|
on its own, but in the case of files, there is no such thing: the disk
|
|
will not send data on its own, simply because it doesn't know what you
|
|
wish to read \- you would first have to request some data.
|
|
.PP
|
|
Since files are typically not-so-well supported by advanced notification
|
|
mechanism, libev tries hard to emulate \s-1POSIX\s0 behaviour with respect
|
|
to files, even though you should not use it. The reason for this is
|
|
convenience: sometimes you want to watch \s-1STDIN\s0 or \s-1STDOUT,\s0 which is
|
|
usually a tty, often a pipe, but also sometimes files or special devices
|
|
(for example, \f(CW\*(C`epoll\*(C'\fR on Linux works with \fI/dev/random\fR but not with
|
|
\&\fI/dev/urandom\fR), and even though the file might better be served with
|
|
asynchronous I/O instead of with non-blocking I/O, it is still useful when
|
|
it \*(L"just works\*(R" instead of freezing.
|
|
.PP
|
|
So avoid file descriptors pointing to files when you know it (e.g. use
|
|
libeio), but use them when it is convenient, e.g. for \s-1STDIN/STDOUT,\s0 or
|
|
when you rarely read from a file instead of from a socket, and want to
|
|
reuse the same code path.
|
|
.PP
|
|
\fIThe special problem of fork\fR
|
|
.IX Subsection "The special problem of fork"
|
|
.PP
|
|
Some backends (epoll, kqueue, probably linuxaio) do not support \f(CW\*(C`fork ()\*(C'\fR
|
|
at all or exhibit useless behaviour. Libev fully supports fork, but needs
|
|
to be told about it in the child if you want to continue to use it in the
|
|
child.
|
|
.PP
|
|
To support fork in your child processes, you have to call \f(CW\*(C`ev_loop_fork
|
|
()\*(C'\fR after a fork in the child, enable \f(CW\*(C`EVFLAG_FORKCHECK\*(C'\fR, or resort to
|
|
\&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
|
|
.PP
|
|
\fIThe special problem of \s-1SIGPIPE\s0\fR
|
|
.IX Subsection "The special problem of SIGPIPE"
|
|
.PP
|
|
While not really specific to libev, it is easy to forget about \f(CW\*(C`SIGPIPE\*(C'\fR:
|
|
when writing to a pipe whose other end has been closed, your program gets
|
|
sent a \s-1SIGPIPE,\s0 which, by default, aborts your program. For most programs
|
|
this is sensible behaviour, for daemons, this is usually undesirable.
|
|
.PP
|
|
So when you encounter spurious, unexplained daemon exits, make sure you
|
|
ignore \s-1SIGPIPE\s0 (and maybe make sure you log the exit status of your daemon
|
|
somewhere, as that would have given you a big clue).
|
|
.PP
|
|
\fIThe special problem of \f(BIaccept()\fIing when you can't\fR
|
|
.IX Subsection "The special problem of accept()ing when you can't"
|
|
.PP
|
|
Many implementations of the \s-1POSIX\s0 \f(CW\*(C`accept\*(C'\fR function (for example,
|
|
found in post\-2004 Linux) have the peculiar behaviour of not removing a
|
|
connection from the pending queue in all error cases.
|
|
.PP
|
|
For example, larger servers often run out of file descriptors (because
|
|
of resource limits), causing \f(CW\*(C`accept\*(C'\fR to fail with \f(CW\*(C`ENFILE\*(C'\fR but not
|
|
rejecting the connection, leading to libev signalling readiness on
|
|
the next iteration again (the connection still exists after all), and
|
|
typically causing the program to loop at 100% \s-1CPU\s0 usage.
|
|
.PP
|
|
Unfortunately, the set of errors that cause this issue differs between
|
|
operating systems, there is usually little the app can do to remedy the
|
|
situation, and no known thread-safe method of removing the connection to
|
|
cope with overload is known (to me).
|
|
.PP
|
|
One of the easiest ways to handle this situation is to just ignore it
|
|
\&\- when the program encounters an overload, it will just loop until the
|
|
situation is over. While this is a form of busy waiting, no \s-1OS\s0 offers an
|
|
event-based way to handle this situation, so it's the best one can do.
|
|
.PP
|
|
A better way to handle the situation is to log any errors other than
|
|
\&\f(CW\*(C`EAGAIN\*(C'\fR and \f(CW\*(C`EWOULDBLOCK\*(C'\fR, making sure not to flood the log with such
|
|
messages, and continue as usual, which at least gives the user an idea of
|
|
what could be wrong (\*(L"raise the ulimit!\*(R"). For extra points one could stop
|
|
the \f(CW\*(C`ev_io\*(C'\fR watcher on the listening fd \*(L"for a while\*(R", which reduces \s-1CPU\s0
|
|
usage.
|
|
.PP
|
|
If your program is single-threaded, then you could also keep a dummy file
|
|
descriptor for overload situations (e.g. by opening \fI/dev/null\fR), and
|
|
when you run into \f(CW\*(C`ENFILE\*(C'\fR or \f(CW\*(C`EMFILE\*(C'\fR, close it, run \f(CW\*(C`accept\*(C'\fR,
|
|
close that fd, and create a new dummy fd. This will gracefully refuse
|
|
clients under typical overload conditions.
|
|
.PP
|
|
The last way to handle it is to simply log the error and \f(CW\*(C`exit\*(C'\fR, as
|
|
is often done with \f(CW\*(C`malloc\*(C'\fR failures, but this results in an easy
|
|
opportunity for a DoS attack.
|
|
.PP
|
|
\fIWatcher-Specific Functions\fR
|
|
.IX Subsection "Watcher-Specific Functions"
|
|
.IP "ev_io_init (ev_io *, callback, int fd, int events)" 4
|
|
.IX Item "ev_io_init (ev_io *, callback, int fd, int events)"
|
|
.PD 0
|
|
.IP "ev_io_set (ev_io *, int fd, int events)" 4
|
|
.IX Item "ev_io_set (ev_io *, int fd, int events)"
|
|
.PD
|
|
Configures an \f(CW\*(C`ev_io\*(C'\fR watcher. The \f(CW\*(C`fd\*(C'\fR is the file descriptor to
|
|
receive events for and \f(CW\*(C`events\*(C'\fR is either \f(CW\*(C`EV_READ\*(C'\fR, \f(CW\*(C`EV_WRITE\*(C'\fR or
|
|
\&\f(CW\*(C`EV_READ | EV_WRITE\*(C'\fR, to express the desire to receive the given events.
|
|
.IP "int fd [read\-only]" 4
|
|
.IX Item "int fd [read-only]"
|
|
The file descriptor being watched.
|
|
.IP "int events [read\-only]" 4
|
|
.IX Item "int events [read-only]"
|
|
The events being watched.
|
|
.PP
|
|
\fIExamples\fR
|
|
.IX Subsection "Examples"
|
|
.PP
|
|
Example: Call \f(CW\*(C`stdin_readable_cb\*(C'\fR when \s-1STDIN_FILENO\s0 has become, well
|
|
readable, but only once. Since it is likely line-buffered, you could
|
|
attempt to read a whole line in the callback.
|
|
.PP
|
|
.Vb 6
|
|
\& static void
|
|
\& stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
|
|
\& {
|
|
\& ev_io_stop (loop, w);
|
|
\& .. read from stdin here (or from w\->fd) and handle any I/O errors
|
|
\& }
|
|
\&
|
|
\& ...
|
|
\& struct ev_loop *loop = ev_default_init (0);
|
|
\& ev_io stdin_readable;
|
|
\& ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
|
|
\& ev_io_start (loop, &stdin_readable);
|
|
\& ev_run (loop, 0);
|
|
.Ve
|
|
.ie n .SS """ev_timer"" \- relative and optionally repeating timeouts"
|
|
.el .SS "\f(CWev_timer\fP \- relative and optionally repeating timeouts"
|
|
.IX Subsection "ev_timer - relative and optionally repeating timeouts"
|
|
Timer watchers are simple relative timers that generate an event after a
|
|
given time, and optionally repeating in regular intervals after that.
|
|
.PP
|
|
The timers are based on real time, that is, if you register an event that
|
|
times out after an hour and you reset your system clock to January last
|
|
year, it will still time out after (roughly) one hour. \*(L"Roughly\*(R" because
|
|
detecting time jumps is hard, and some inaccuracies are unavoidable (the
|
|
monotonic clock option helps a lot here).
|
|
.PP
|
|
The callback is guaranteed to be invoked only \fIafter\fR its timeout has
|
|
passed (not \fIat\fR, so on systems with very low-resolution clocks this
|
|
might introduce a small delay, see \*(L"the special problem of being too
|
|
early\*(R", below). If multiple timers become ready during the same loop
|
|
iteration then the ones with earlier time-out values are invoked before
|
|
ones of the same priority with later time-out values (but this is no
|
|
longer true when a callback calls \f(CW\*(C`ev_run\*(C'\fR recursively).
|
|
.PP
|
|
\fIBe smart about timeouts\fR
|
|
.IX Subsection "Be smart about timeouts"
|
|
.PP
|
|
Many real-world problems involve some kind of timeout, usually for error
|
|
recovery. A typical example is an \s-1HTTP\s0 request \- if the other side hangs,
|
|
you want to raise some error after a while.
|
|
.PP
|
|
What follows are some ways to handle this problem, from obvious and
|
|
inefficient to smart and efficient.
|
|
.PP
|
|
In the following, a 60 second activity timeout is assumed \- a timeout that
|
|
gets reset to 60 seconds each time there is activity (e.g. each time some
|
|
data or other life sign was received).
|
|
.IP "1. Use a timer and stop, reinitialise and start it on activity." 4
|
|
.IX Item "1. Use a timer and stop, reinitialise and start it on activity."
|
|
This is the most obvious, but not the most simple way: In the beginning,
|
|
start the watcher:
|
|
.Sp
|
|
.Vb 2
|
|
\& ev_timer_init (timer, callback, 60., 0.);
|
|
\& ev_timer_start (loop, timer);
|
|
.Ve
|
|
.Sp
|
|
Then, each time there is some activity, \f(CW\*(C`ev_timer_stop\*(C'\fR it, initialise it
|
|
and start it again:
|
|
.Sp
|
|
.Vb 3
|
|
\& ev_timer_stop (loop, timer);
|
|
\& ev_timer_set (timer, 60., 0.);
|
|
\& ev_timer_start (loop, timer);
|
|
.Ve
|
|
.Sp
|
|
This is relatively simple to implement, but means that each time there is
|
|
some activity, libev will first have to remove the timer from its internal
|
|
data structure and then add it again. Libev tries to be fast, but it's
|
|
still not a constant-time operation.
|
|
.ie n .IP "2. Use a timer and re-start it with ""ev_timer_again"" inactivity." 4
|
|
.el .IP "2. Use a timer and re-start it with \f(CWev_timer_again\fR inactivity." 4
|
|
.IX Item "2. Use a timer and re-start it with ev_timer_again inactivity."
|
|
This is the easiest way, and involves using \f(CW\*(C`ev_timer_again\*(C'\fR instead of
|
|
\&\f(CW\*(C`ev_timer_start\*(C'\fR.
|
|
.Sp
|
|
To implement this, configure an \f(CW\*(C`ev_timer\*(C'\fR with a \f(CW\*(C`repeat\*(C'\fR value
|
|
of \f(CW60\fR and then call \f(CW\*(C`ev_timer_again\*(C'\fR at start and each time you
|
|
successfully read or write some data. If you go into an idle state where
|
|
you do not expect data to travel on the socket, you can \f(CW\*(C`ev_timer_stop\*(C'\fR
|
|
the timer, and \f(CW\*(C`ev_timer_again\*(C'\fR will automatically restart it if need be.
|
|
.Sp
|
|
That means you can ignore both the \f(CW\*(C`ev_timer_start\*(C'\fR function and the
|
|
\&\f(CW\*(C`after\*(C'\fR argument to \f(CW\*(C`ev_timer_set\*(C'\fR, and only ever use the \f(CW\*(C`repeat\*(C'\fR
|
|
member and \f(CW\*(C`ev_timer_again\*(C'\fR.
|
|
.Sp
|
|
At start:
|
|
.Sp
|
|
.Vb 3
|
|
\& ev_init (timer, callback);
|
|
\& timer\->repeat = 60.;
|
|
\& ev_timer_again (loop, timer);
|
|
.Ve
|
|
.Sp
|
|
Each time there is some activity:
|
|
.Sp
|
|
.Vb 1
|
|
\& ev_timer_again (loop, timer);
|
|
.Ve
|
|
.Sp
|
|
It is even possible to change the time-out on the fly, regardless of
|
|
whether the watcher is active or not:
|
|
.Sp
|
|
.Vb 2
|
|
\& timer\->repeat = 30.;
|
|
\& ev_timer_again (loop, timer);
|
|
.Ve
|
|
.Sp
|
|
This is slightly more efficient then stopping/starting the timer each time
|
|
you want to modify its timeout value, as libev does not have to completely
|
|
remove and re-insert the timer from/into its internal data structure.
|
|
.Sp
|
|
It is, however, even simpler than the \*(L"obvious\*(R" way to do it.
|
|
.IP "3. Let the timer time out, but then re-arm it as required." 4
|
|
.IX Item "3. Let the timer time out, but then re-arm it as required."
|
|
This method is more tricky, but usually most efficient: Most timeouts are
|
|
relatively long compared to the intervals between other activity \- in
|
|
our example, within 60 seconds, there are usually many I/O events with
|
|
associated activity resets.
|
|
.Sp
|
|
In this case, it would be more efficient to leave the \f(CW\*(C`ev_timer\*(C'\fR alone,
|
|
but remember the time of last activity, and check for a real timeout only
|
|
within the callback:
|
|
.Sp
|
|
.Vb 3
|
|
\& ev_tstamp timeout = 60.;
|
|
\& ev_tstamp last_activity; // time of last activity
|
|
\& ev_timer timer;
|
|
\&
|
|
\& static void
|
|
\& callback (EV_P_ ev_timer *w, int revents)
|
|
\& {
|
|
\& // calculate when the timeout would happen
|
|
\& ev_tstamp after = last_activity \- ev_now (EV_A) + timeout;
|
|
\&
|
|
\& // if negative, it means we the timeout already occurred
|
|
\& if (after < 0.)
|
|
\& {
|
|
\& // timeout occurred, take action
|
|
\& }
|
|
\& else
|
|
\& {
|
|
\& // callback was invoked, but there was some recent
|
|
\& // activity. simply restart the timer to time out
|
|
\& // after "after" seconds, which is the earliest time
|
|
\& // the timeout can occur.
|
|
\& ev_timer_set (w, after, 0.);
|
|
\& ev_timer_start (EV_A_ w);
|
|
\& }
|
|
\& }
|
|
.Ve
|
|
.Sp
|
|
To summarise the callback: first calculate in how many seconds the
|
|
timeout will occur (by calculating the absolute time when it would occur,
|
|
\&\f(CW\*(C`last_activity + timeout\*(C'\fR, and subtracting the current time, \f(CW\*(C`ev_now
|
|
(EV_A)\*(C'\fR from that).
|
|
.Sp
|
|
If this value is negative, then we are already past the timeout, i.e. we
|
|
timed out, and need to do whatever is needed in this case.
|
|
.Sp
|
|
Otherwise, we now the earliest time at which the timeout would trigger,
|
|
and simply start the timer with this timeout value.
|
|
.Sp
|
|
In other words, each time the callback is invoked it will check whether
|
|
the timeout occurred. If not, it will simply reschedule itself to check
|
|
again at the earliest time it could time out. Rinse. Repeat.
|
|
.Sp
|
|
This scheme causes more callback invocations (about one every 60 seconds
|
|
minus half the average time between activity), but virtually no calls to
|
|
libev to change the timeout.
|
|
.Sp
|
|
To start the machinery, simply initialise the watcher and set
|
|
\&\f(CW\*(C`last_activity\*(C'\fR to the current time (meaning there was some activity just
|
|
now), then call the callback, which will \*(L"do the right thing\*(R" and start
|
|
the timer:
|
|
.Sp
|
|
.Vb 3
|
|
\& last_activity = ev_now (EV_A);
|
|
\& ev_init (&timer, callback);
|
|
\& callback (EV_A_ &timer, 0);
|
|
.Ve
|
|
.Sp
|
|
When there is some activity, simply store the current time in
|
|
\&\f(CW\*(C`last_activity\*(C'\fR, no libev calls at all:
|
|
.Sp
|
|
.Vb 2
|
|
\& if (activity detected)
|
|
\& last_activity = ev_now (EV_A);
|
|
.Ve
|
|
.Sp
|
|
When your timeout value changes, then the timeout can be changed by simply
|
|
providing a new value, stopping the timer and calling the callback, which
|
|
will again do the right thing (for example, time out immediately :).
|
|
.Sp
|
|
.Vb 3
|
|
\& timeout = new_value;
|
|
\& ev_timer_stop (EV_A_ &timer);
|
|
\& callback (EV_A_ &timer, 0);
|
|
.Ve
|
|
.Sp
|
|
This technique is slightly more complex, but in most cases where the
|
|
time-out is unlikely to be triggered, much more efficient.
|
|
.IP "4. Wee, just use a double-linked list for your timeouts." 4
|
|
.IX Item "4. Wee, just use a double-linked list for your timeouts."
|
|
If there is not one request, but many thousands (millions...), all
|
|
employing some kind of timeout with the same timeout value, then one can
|
|
do even better:
|
|
.Sp
|
|
When starting the timeout, calculate the timeout value and put the timeout
|
|
at the \fIend\fR of the list.
|
|
.Sp
|
|
Then use an \f(CW\*(C`ev_timer\*(C'\fR to fire when the timeout at the \fIbeginning\fR of
|
|
the list is expected to fire (for example, using the technique #3).
|
|
.Sp
|
|
When there is some activity, remove the timer from the list, recalculate
|
|
the timeout, append it to the end of the list again, and make sure to
|
|
update the \f(CW\*(C`ev_timer\*(C'\fR if it was taken from the beginning of the list.
|
|
.Sp
|
|
This way, one can manage an unlimited number of timeouts in O(1) time for
|
|
starting, stopping and updating the timers, at the expense of a major
|
|
complication, and having to use a constant timeout. The constant timeout
|
|
ensures that the list stays sorted.
|
|
.PP
|
|
So which method the best?
|
|
.PP
|
|
Method #2 is a simple no-brain-required solution that is adequate in most
|
|
situations. Method #3 requires a bit more thinking, but handles many cases
|
|
better, and isn't very complicated either. In most case, choosing either
|
|
one is fine, with #3 being better in typical situations.
|
|
.PP
|
|
Method #1 is almost always a bad idea, and buys you nothing. Method #4 is
|
|
rather complicated, but extremely efficient, something that really pays
|
|
off after the first million or so of active timers, i.e. it's usually
|
|
overkill :)
|
|
.PP
|
|
\fIThe special problem of being too early\fR
|
|
.IX Subsection "The special problem of being too early"
|
|
.PP
|
|
If you ask a timer to call your callback after three seconds, then
|
|
you expect it to be invoked after three seconds \- but of course, this
|
|
cannot be guaranteed to infinite precision. Less obviously, it cannot be
|
|
guaranteed to any precision by libev \- imagine somebody suspending the
|
|
process with a \s-1STOP\s0 signal for a few hours for example.
|
|
.PP
|
|
So, libev tries to invoke your callback as soon as possible \fIafter\fR the
|
|
delay has occurred, but cannot guarantee this.
|
|
.PP
|
|
A less obvious failure mode is calling your callback too early: many event
|
|
loops compare timestamps with a \*(L"elapsed delay >= requested delay\*(R", but
|
|
this can cause your callback to be invoked much earlier than you would
|
|
expect.
|
|
.PP
|
|
To see why, imagine a system with a clock that only offers full second
|
|
resolution (think windows if you can't come up with a broken enough \s-1OS\s0
|
|
yourself). If you schedule a one-second timer at the time 500.9, then the
|
|
event loop will schedule your timeout to elapse at a system time of 500
|
|
(500.9 truncated to the resolution) + 1, or 501.
|
|
.PP
|
|
If an event library looks at the timeout 0.1s later, it will see \*(L"501 >=
|
|
501\*(R" and invoke the callback 0.1s after it was started, even though a
|
|
one-second delay was requested \- this is being \*(L"too early\*(R", despite best
|
|
intentions.
|
|
.PP
|
|
This is the reason why libev will never invoke the callback if the elapsed
|
|
delay equals the requested delay, but only when the elapsed delay is
|
|
larger than the requested delay. In the example above, libev would only invoke
|
|
the callback at system time 502, or 1.1s after the timer was started.
|
|
.PP
|
|
So, while libev cannot guarantee that your callback will be invoked
|
|
exactly when requested, it \fIcan\fR and \fIdoes\fR guarantee that the requested
|
|
delay has actually elapsed, or in other words, it always errs on the \*(L"too
|
|
late\*(R" side of things.
|
|
.PP
|
|
\fIThe special problem of time updates\fR
|
|
.IX Subsection "The special problem of time updates"
|
|
.PP
|
|
Establishing the current time is a costly operation (it usually takes
|
|
at least one system call): \s-1EV\s0 therefore updates its idea of the current
|
|
time only before and after \f(CW\*(C`ev_run\*(C'\fR collects new events, which causes a
|
|
growing difference between \f(CW\*(C`ev_now ()\*(C'\fR and \f(CW\*(C`ev_time ()\*(C'\fR when handling
|
|
lots of events in one iteration.
|
|
.PP
|
|
The relative timeouts are calculated relative to the \f(CW\*(C`ev_now ()\*(C'\fR
|
|
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 \fIneed\fR to base the
|
|
timeout on the current time, use something like the following to adjust
|
|
for it:
|
|
.PP
|
|
.Vb 1
|
|
\& ev_timer_set (&timer, after + (ev_time () \- ev_now ()), 0.);
|
|
.Ve
|
|
.PP
|
|
If the event loop is suspended for a long time, you can also force an
|
|
update of the time returned by \f(CW\*(C`ev_now ()\*(C'\fR by calling \f(CW\*(C`ev_now_update
|
|
()\*(C'\fR, although that will push the event time of all outstanding events
|
|
further into the future.
|
|
.PP
|
|
\fIThe special problem of unsynchronised clocks\fR
|
|
.IX Subsection "The special problem of unsynchronised clocks"
|
|
.PP
|
|
Modern systems have a variety of clocks \- libev itself uses the normal
|
|
\&\*(L"wall clock\*(R" clock and, if available, the monotonic clock (to avoid time
|
|
jumps).
|
|
.PP
|
|
Neither of these clocks is synchronised with each other or any other clock
|
|
on the system, so \f(CW\*(C`ev_time ()\*(C'\fR might return a considerably different time
|
|
than \f(CW\*(C`gettimeofday ()\*(C'\fR or \f(CW\*(C`time ()\*(C'\fR. On a GNU/Linux system, for example,
|
|
a call to \f(CW\*(C`gettimeofday\*(C'\fR might return a second count that is one higher
|
|
than a directly following call to \f(CW\*(C`time\*(C'\fR.
|
|
.PP
|
|
The moral of this is to only compare libev-related timestamps with
|
|
\&\f(CW\*(C`ev_time ()\*(C'\fR and \f(CW\*(C`ev_now ()\*(C'\fR, at least if you want better precision than
|
|
a second or so.
|
|
.PP
|
|
One more problem arises due to this lack of synchronisation: if libev uses
|
|
the system monotonic clock and you compare timestamps from \f(CW\*(C`ev_time\*(C'\fR
|
|
or \f(CW\*(C`ev_now\*(C'\fR from when you started your timer and when your callback is
|
|
invoked, you will find that sometimes the callback is a bit \*(L"early\*(R".
|
|
.PP
|
|
This is because \f(CW\*(C`ev_timer\*(C'\fRs work in real time, not wall clock time, so
|
|
libev makes sure your callback is not invoked before the delay happened,
|
|
\&\fImeasured according to the real time\fR, not the system clock.
|
|
.PP
|
|
If your timeouts are based on a physical timescale (e.g. \*(L"time out this
|
|
connection after 100 seconds\*(R") then this shouldn't bother you as it is
|
|
exactly the right behaviour.
|
|
.PP
|
|
If you want to compare wall clock/system timestamps to your timers, then
|
|
you need to use \f(CW\*(C`ev_periodic\*(C'\fRs, as these are based on the wall clock
|
|
time, where your comparisons will always generate correct results.
|
|
.PP
|
|
\fIThe special problems of suspended animation\fR
|
|
.IX Subsection "The special problems of suspended animation"
|
|
.PP
|
|
When you leave the server world it is quite customary to hit machines that
|
|
can suspend/hibernate \- what happens to the clocks during such a suspend?
|
|
.PP
|
|
Some quick tests made with a Linux 2.6.28 indicate that a suspend freezes
|
|
all processes, while the clocks (\f(CW\*(C`times\*(C'\fR, \f(CW\*(C`CLOCK_MONOTONIC\*(C'\fR) continue
|
|
to run until the system is suspended, but they will not advance while the
|
|
system is suspended. That means, on resume, it will be as if the program
|
|
was frozen for a few seconds, but the suspend time will not be counted
|
|
towards \f(CW\*(C`ev_timer\*(C'\fR when a monotonic clock source is used. The real time
|
|
clock advanced as expected, but if it is used as sole clocksource, then a
|
|
long suspend would be detected as a time jump by libev, and timers would
|
|
be adjusted accordingly.
|
|
.PP
|
|
I would not be surprised to see different behaviour in different between
|
|
operating systems, \s-1OS\s0 versions or even different hardware.
|
|
.PP
|
|
The other form of suspend (job control, or sending a \s-1SIGSTOP\s0) will see a
|
|
time jump in the monotonic clocks and the realtime clock. If the program
|
|
is suspended for a very long time, and monotonic clock sources are in use,
|
|
then you can expect \f(CW\*(C`ev_timer\*(C'\fRs to expire as the full suspension time
|
|
will be counted towards the timers. When no monotonic clock source is in
|
|
use, then libev will again assume a timejump and adjust accordingly.
|
|
.PP
|
|
It might be beneficial for this latter case to call \f(CW\*(C`ev_suspend\*(C'\fR
|
|
and \f(CW\*(C`ev_resume\*(C'\fR in code that handles \f(CW\*(C`SIGTSTP\*(C'\fR, to at least get
|
|
deterministic behaviour in this case (you can do nothing against
|
|
\&\f(CW\*(C`SIGSTOP\*(C'\fR).
|
|
.PP
|
|
\fIWatcher-Specific Functions and Data Members\fR
|
|
.IX Subsection "Watcher-Specific Functions and Data Members"
|
|
.IP "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)" 4
|
|
.IX Item "ev_timer_init (ev_timer *, callback, ev_tstamp after, ev_tstamp repeat)"
|
|
.PD 0
|
|
.IP "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)" 4
|
|
.IX Item "ev_timer_set (ev_timer *, ev_tstamp after, ev_tstamp repeat)"
|
|
.PD
|
|
Configure the timer to trigger after \f(CW\*(C`after\*(C'\fR seconds (fractional and
|
|
negative values are supported). If \f(CW\*(C`repeat\*(C'\fR is \f(CW0.\fR, 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 \f(CW\*(C`repeat\*(C'\fR
|
|
seconds later, again, and again, until stopped manually.
|
|
.Sp
|
|
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.
|
|
.IP "ev_timer_again (loop, ev_timer *)" 4
|
|
.IX Item "ev_timer_again (loop, ev_timer *)"
|
|
This will act as if the timer timed out, and restarts it again if it is
|
|
repeating. It basically works like calling \f(CW\*(C`ev_timer_stop\*(C'\fR, updating the
|
|
timeout to the \f(CW\*(C`repeat\*(C'\fR value and calling \f(CW\*(C`ev_timer_start\*(C'\fR.
|
|
.Sp
|
|
The exact semantics are as in the following rules, all of which will be
|
|
applied to the watcher:
|
|
.RS 4
|
|
.IP "If the timer is pending, the pending status is always cleared." 4
|
|
.IX Item "If the timer is pending, the pending status is always cleared."
|
|
.PD 0
|
|
.IP "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)." 4
|
|
.IX Item "If the timer is started but non-repeating, stop it (as if it timed out, without invoking it)."
|
|
.ie n .IP "If the timer is repeating, make the ""repeat"" value the new timeout and start the timer, if necessary." 4
|
|
.el .IP "If the timer is repeating, make the \f(CWrepeat\fR value the new timeout and start the timer, if necessary." 4
|
|
.IX Item "If the timer is repeating, make the repeat value the new timeout and start the timer, if necessary."
|
|
.RE
|
|
.RS 4
|
|
.PD
|
|
.Sp
|
|
This sounds a bit complicated, see \*(L"Be smart about timeouts\*(R", above, for a
|
|
usage example.
|
|
.RE
|
|
.IP "ev_tstamp ev_timer_remaining (loop, ev_timer *)" 4
|
|
.IX Item "ev_tstamp ev_timer_remaining (loop, ev_timer *)"
|
|
Returns the remaining time until a timer fires. If the timer is active,
|
|
then this time is relative to the current event loop time, otherwise it's
|
|
the timeout value currently configured.
|
|
.Sp
|
|
That is, after an \f(CW\*(C`ev_timer_set (w, 5, 7)\*(C'\fR, \f(CW\*(C`ev_timer_remaining\*(C'\fR returns
|
|
\&\f(CW5\fR. When the timer is started and one second passes, \f(CW\*(C`ev_timer_remaining\*(C'\fR
|
|
will return \f(CW4\fR. When the timer expires and is restarted, it will return
|
|
roughly \f(CW7\fR (likely slightly less as callback invocation takes some time,
|
|
too), and so on.
|
|
.IP "ev_tstamp repeat [read\-write]" 4
|
|
.IX Item "ev_tstamp repeat [read-write]"
|
|
The current \f(CW\*(C`repeat\*(C'\fR value. Will be used each time the watcher times out
|
|
or \f(CW\*(C`ev_timer_again\*(C'\fR is called, and determines the next timeout (if any),
|
|
which is also when any modifications are taken into account.
|
|
.PP
|
|
\fIExamples\fR
|
|
.IX Subsection "Examples"
|
|
.PP
|
|
Example: Create a timer that fires after 60 seconds.
|
|
.PP
|
|
.Vb 5
|
|
\& static void
|
|
\& one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
|
|
\& {
|
|
\& .. one minute over, w is actually stopped right here
|
|
\& }
|
|
\&
|
|
\& ev_timer mytimer;
|
|
\& ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
|
|
\& ev_timer_start (loop, &mytimer);
|
|
.Ve
|
|
.PP
|
|
Example: Create a timeout timer that times out after 10 seconds of
|
|
inactivity.
|
|
.PP
|
|
.Vb 5
|
|
\& static void
|
|
\& timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
|
|
\& {
|
|
\& .. ten seconds without any activity
|
|
\& }
|
|
\&
|
|
\& ev_timer mytimer;
|
|
\& ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
|
|
\& ev_timer_again (&mytimer); /* start timer */
|
|
\& ev_run (loop, 0);
|
|
\&
|
|
\& // and in some piece of code that gets executed on any "activity":
|
|
\& // reset the timeout to start ticking again at 10 seconds
|
|
\& ev_timer_again (&mytimer);
|
|
.Ve
|
|
.ie n .SS """ev_periodic"" \- to cron or not to cron?"
|
|
.el .SS "\f(CWev_periodic\fP \- to cron or not to cron?"
|
|
.IX Subsection "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).
|
|
.PP
|
|
Unlike \f(CW\*(C`ev_timer\*(C'\fR, periodic watchers are not based on real time (or
|
|
relative time, the physical time that passes) but on wall clock time
|
|
(absolute time, the thing you can read on your calendar or clock). The
|
|
difference is that wall clock time can run faster or slower than real
|
|
time, and time jumps are not uncommon (e.g. when you adjust your
|
|
wrist-watch).
|
|
.PP
|
|
You can tell a periodic watcher to trigger after some specific point
|
|
in time: for example, if you tell a periodic watcher to trigger \*(L"in 10
|
|
seconds\*(R" (by specifying e.g. \f(CW\*(C`ev_now () + 10.\*(C'\fR, that is, an absolute time
|
|
not a delay) and then reset your system clock to January of the previous
|
|
year, then it will take a year or more to trigger the event (unlike an
|
|
\&\f(CW\*(C`ev_timer\*(C'\fR, which would still trigger roughly 10 seconds after starting
|
|
it, as it uses a relative timeout).
|
|
.PP
|
|
\&\f(CW\*(C`ev_periodic\*(C'\fR watchers can also be used to implement vastly more complex
|
|
timers, such as triggering an event on each \*(L"midnight, local time\*(R", or
|
|
other complicated rules. This cannot easily be done with \f(CW\*(C`ev_timer\*(C'\fR
|
|
watchers, as those cannot react to time jumps.
|
|
.PP
|
|
As with timers, the callback is guaranteed to be invoked only when the
|
|
point in time where it is supposed to trigger has passed. If multiple
|
|
timers become ready during the same loop iteration then the ones with
|
|
earlier time-out values are invoked before ones with later time-out values
|
|
(but this is no longer true when a callback calls \f(CW\*(C`ev_run\*(C'\fR recursively).
|
|
.PP
|
|
\fIWatcher-Specific Functions and Data Members\fR
|
|
.IX Subsection "Watcher-Specific Functions and Data Members"
|
|
.IP "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
|
|
.IX Item "ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
|
|
.PD 0
|
|
.IP "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)" 4
|
|
.IX Item "ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)"
|
|
.PD
|
|
Lots of arguments, let's sort it out... There are basically three modes of
|
|
operation, and we will explain them from simplest to most complex:
|
|
.RS 4
|
|
.IP "\(bu" 4
|
|
absolute timer (offset = absolute time, interval = 0, reschedule_cb = 0)
|
|
.Sp
|
|
In this configuration the watcher triggers an event after the wall clock
|
|
time \f(CW\*(C`offset\*(C'\fR has passed. It will not repeat and will not adjust when a
|
|
time jump occurs, that is, if it is to be run at January 1st 2011 then it
|
|
will be stopped and invoked when the system clock reaches or surpasses
|
|
this point in time.
|
|
.IP "\(bu" 4
|
|
repeating interval timer (offset = offset within interval, interval > 0, reschedule_cb = 0)
|
|
.Sp
|
|
In this mode the watcher will always be scheduled to time out at the next
|
|
\&\f(CW\*(C`offset + N * interval\*(C'\fR time (for some integer N, which can also be
|
|
negative) and then repeat, regardless of any time jumps. The \f(CW\*(C`offset\*(C'\fR
|
|
argument is merely an offset into the \f(CW\*(C`interval\*(C'\fR periods.
|
|
.Sp
|
|
This can be used to create timers that do not drift with respect to the
|
|
system clock, for example, here is an \f(CW\*(C`ev_periodic\*(C'\fR that triggers each
|
|
hour, on the hour (with respect to \s-1UTC\s0):
|
|
.Sp
|
|
.Vb 1
|
|
\& ev_periodic_set (&periodic, 0., 3600., 0);
|
|
.Ve
|
|
.Sp
|
|
This doesn't mean there will always be 3600 seconds in between triggers,
|
|
but only that the callback will be called when the system time shows a
|
|
full hour (\s-1UTC\s0), or more correctly, when the system time is evenly divisible
|
|
by 3600.
|
|
.Sp
|
|
Another way to think about it (for the mathematically inclined) is that
|
|
\&\f(CW\*(C`ev_periodic\*(C'\fR will try to run the callback in this mode at the next possible
|
|
time where \f(CW\*(C`time = offset (mod interval)\*(C'\fR, regardless of any time jumps.
|
|
.Sp
|
|
The \f(CW\*(C`interval\*(C'\fR \fI\s-1MUST\s0\fR be positive, and for numerical stability, the
|
|
interval value should be higher than \f(CW\*(C`1/8192\*(C'\fR (which is around 100
|
|
microseconds) and \f(CW\*(C`offset\*(C'\fR should be higher than \f(CW0\fR and should have
|
|
at most a similar magnitude as the current time (say, within a factor of
|
|
ten). Typical values for offset are, in fact, \f(CW0\fR or something between
|
|
\&\f(CW0\fR and \f(CW\*(C`interval\*(C'\fR, which is also the recommended range.
|
|
.Sp
|
|
Note also that there is an upper limit to how often a timer can fire (\s-1CPU\s0
|
|
speed for example), so if \f(CW\*(C`interval\*(C'\fR is very small then timing stability
|
|
will of course deteriorate. Libev itself tries to be exact to be about one
|
|
millisecond (if the \s-1OS\s0 supports it and the machine is fast enough).
|
|
.IP "\(bu" 4
|
|
manual reschedule mode (offset ignored, interval ignored, reschedule_cb = callback)
|
|
.Sp
|
|
In this mode the values for \f(CW\*(C`interval\*(C'\fR and \f(CW\*(C`offset\*(C'\fR 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.
|
|
.Sp
|
|
\&\s-1NOTE:\s0 \fIThis callback \s-1MUST NOT\s0 stop or destroy any periodic watcher, ever,
|
|
or make \s-1ANY\s0 other event loop modifications whatsoever, unless explicitly
|
|
allowed by documentation here\fR.
|
|
.Sp
|
|
If you need to stop it, return \f(CW\*(C`now + 1e30\*(C'\fR (or so, fudge fudge) and stop
|
|
it afterwards (e.g. by starting an \f(CW\*(C`ev_prepare\*(C'\fR watcher, which is the
|
|
only event loop modification you are allowed to do).
|
|
.Sp
|
|
The callback prototype is \f(CW\*(C`ev_tstamp (*reschedule_cb)(ev_periodic
|
|
*w, ev_tstamp now)\*(C'\fR, e.g.:
|
|
.Sp
|
|
.Vb 5
|
|
\& static ev_tstamp
|
|
\& my_rescheduler (ev_periodic *w, ev_tstamp now)
|
|
\& {
|
|
\& return now + 60.;
|
|
\& }
|
|
.Ve
|
|
.Sp
|
|
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.
|
|
.Sp
|
|
\&\s-1NOTE:\s0 \fIThis callback must always return a time that is higher than or
|
|
equal to the passed \f(CI\*(C`now\*(C'\fI value\fR.
|
|
.Sp
|
|
This can be used to create very complex timers, such as a timer that
|
|
triggers on \*(L"next midnight, local time\*(R". To do this, you would calculate
|
|
the next midnight after \f(CW\*(C`now\*(C'\fR and return the timestamp value for
|
|
this. Here is a (completely untested, no error checking) example on how to
|
|
do this:
|
|
.Sp
|
|
.Vb 1
|
|
\& #include <time.h>
|
|
\&
|
|
\& static ev_tstamp
|
|
\& my_rescheduler (ev_periodic *w, ev_tstamp now)
|
|
\& {
|
|
\& time_t tnow = (time_t)now;
|
|
\& struct tm tm;
|
|
\& localtime_r (&tnow, &tm);
|
|
\&
|
|
\& tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day
|
|
\& ++tm.tm_mday; // midnight next day
|
|
\&
|
|
\& return mktime (&tm);
|
|
\& }
|
|
.Ve
|
|
.Sp
|
|
Note: this code might run into trouble on days that have more then two
|
|
midnights (beginning and end).
|
|
.RE
|
|
.RS 4
|
|
.RE
|
|
.IP "ev_periodic_again (loop, ev_periodic *)" 4
|
|
.IX Item "ev_periodic_again (loop, ev_periodic *)"
|
|
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).
|
|
.IP "ev_tstamp ev_periodic_at (ev_periodic *)" 4
|
|
.IX Item "ev_tstamp ev_periodic_at (ev_periodic *)"
|
|
When active, returns the absolute time that the watcher is supposed
|
|
to trigger next. This is not the same as the \f(CW\*(C`offset\*(C'\fR argument to
|
|
\&\f(CW\*(C`ev_periodic_set\*(C'\fR, but indeed works even in interval and manual
|
|
rescheduling modes.
|
|
.IP "ev_tstamp offset [read\-write]" 4
|
|
.IX Item "ev_tstamp offset [read-write]"
|
|
When repeating, this contains the offset value, otherwise this is the
|
|
absolute point in time (the \f(CW\*(C`offset\*(C'\fR value passed to \f(CW\*(C`ev_periodic_set\*(C'\fR,
|
|
although libev might modify this value for better numerical stability).
|
|
.Sp
|
|
Can be modified any time, but changes only take effect when the periodic
|
|
timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
|
|
.IP "ev_tstamp interval [read\-write]" 4
|
|
.IX Item "ev_tstamp interval [read-write]"
|
|
The current interval value. Can be modified any time, but changes only
|
|
take effect when the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being
|
|
called.
|
|
.IP "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read\-write]" 4
|
|
.IX Item "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]"
|
|
The current reschedule callback, or \f(CW0\fR, if this functionality is
|
|
switched off. Can be changed any time, but changes only take effect when
|
|
the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
|
|
.PP
|
|
\fIExamples\fR
|
|
.IX Subsection "Examples"
|
|
.PP
|
|
Example: Call a callback every hour, or, more precisely, whenever the
|
|
system time is divisible by 3600. The callback invocation times have
|
|
potentially a lot of jitter, but good long-term stability.
|
|
.PP
|
|
.Vb 5
|
|
\& static void
|
|
\& clock_cb (struct ev_loop *loop, ev_periodic *w, int revents)
|
|
\& {
|
|
\& ... its now a full hour (UTC, or TAI or whatever your clock follows)
|
|
\& }
|
|
\&
|
|
\& ev_periodic hourly_tick;
|
|
\& ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
|
|
\& ev_periodic_start (loop, &hourly_tick);
|
|
.Ve
|
|
.PP
|
|
Example: The same as above, but use a reschedule callback to do it:
|
|
.PP
|
|
.Vb 1
|
|
\& #include <math.h>
|
|
\&
|
|
\& static ev_tstamp
|
|
\& my_scheduler_cb (ev_periodic *w, ev_tstamp now)
|
|
\& {
|
|
\& return now + (3600. \- fmod (now, 3600.));
|
|
\& }
|
|
\&
|
|
\& ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
|
|
.Ve
|
|
.PP
|
|
Example: Call a callback every hour, starting now:
|
|
.PP
|
|
.Vb 4
|
|
\& ev_periodic hourly_tick;
|
|
\& ev_periodic_init (&hourly_tick, clock_cb,
|
|
\& fmod (ev_now (loop), 3600.), 3600., 0);
|
|
\& ev_periodic_start (loop, &hourly_tick);
|
|
.Ve
|
|
.ie n .SS """ev_signal"" \- signal me when a signal gets signalled!"
|
|
.el .SS "\f(CWev_signal\fP \- signal me when a signal gets signalled!"
|
|
.IX Subsection "ev_signal - signal me when a signal gets signalled!"
|
|
Signal watchers will trigger an event when the process receives a specific
|
|
signal one or more times. Even though signals are very asynchronous, libev
|
|
will try its best to deliver signals synchronously, i.e. as part of the
|
|
normal event processing, like any other event.
|
|
.PP
|
|
If you want signals to be delivered truly asynchronously, just use
|
|
\&\f(CW\*(C`sigaction\*(C'\fR as you would do without libev and forget about sharing
|
|
the signal. You can even use \f(CW\*(C`ev_async\*(C'\fR from a signal handler to
|
|
synchronously wake up an event loop.
|
|
.PP
|
|
You can configure as many watchers as you like for the same signal, but
|
|
only within the same loop, i.e. you can watch for \f(CW\*(C`SIGINT\*(C'\fR in your
|
|
default loop and for \f(CW\*(C`SIGIO\*(C'\fR in another loop, but you cannot watch for
|
|
\&\f(CW\*(C`SIGINT\*(C'\fR in both the default loop and another loop at the same time. At
|
|
the moment, \f(CW\*(C`SIGCHLD\*(C'\fR is permanently tied to the default loop.
|
|
.PP
|
|
Only after the first watcher for a signal is started will libev actually
|
|
register something with the kernel. It thus coexists with your own signal
|
|
handlers as long as you don't register any with libev for the same signal.
|
|
.PP
|
|
If possible and supported, libev will install its handlers with
|
|
\&\f(CW\*(C`SA_RESTART\*(C'\fR (or equivalent) behaviour enabled, so system calls should
|
|
not be unduly interrupted. If you have a problem with system calls getting
|
|
interrupted by signals you can block all signals in an \f(CW\*(C`ev_check\*(C'\fR watcher
|
|
and unblock them in an \f(CW\*(C`ev_prepare\*(C'\fR watcher.
|
|
.PP
|
|
\fIThe special problem of inheritance over fork/execve/pthread_create\fR
|
|
.IX Subsection "The special problem of inheritance over fork/execve/pthread_create"
|
|
.PP
|
|
Both the signal mask (\f(CW\*(C`sigprocmask\*(C'\fR) and the signal disposition
|
|
(\f(CW\*(C`sigaction\*(C'\fR) are unspecified after starting a signal watcher (and after
|
|
stopping it again), that is, libev might or might not block the signal,
|
|
and might or might not set or restore the installed signal handler (but
|
|
see \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR).
|
|
.PP
|
|
While this does not matter for the signal disposition (libev never
|
|
sets signals to \f(CW\*(C`SIG_IGN\*(C'\fR, so handlers will be reset to \f(CW\*(C`SIG_DFL\*(C'\fR on
|
|
\&\f(CW\*(C`execve\*(C'\fR), this matters for the signal mask: many programs do not expect
|
|
certain signals to be blocked.
|
|
.PP
|
|
This means that before calling \f(CW\*(C`exec\*(C'\fR (from the child) you should reset
|
|
the signal mask to whatever \*(L"default\*(R" you expect (all clear is a good
|
|
choice usually).
|
|
.PP
|
|
The simplest way to ensure that the signal mask is reset in the child is
|
|
to install a fork handler with \f(CW\*(C`pthread_atfork\*(C'\fR that resets it. That will
|
|
catch fork calls done by libraries (such as the libc) as well.
|
|
.PP
|
|
In current versions of libev, the signal will not be blocked indefinitely
|
|
unless you use the \f(CW\*(C`signalfd\*(C'\fR \s-1API\s0 (\f(CW\*(C`EV_SIGNALFD\*(C'\fR). While this reduces
|
|
the window of opportunity for problems, it will not go away, as libev
|
|
\&\fIhas\fR to modify the signal mask, at least temporarily.
|
|
.PP
|
|
So I can't stress this enough: \fIIf you do not reset your signal mask when
|
|
you expect it to be empty, you have a race condition in your code\fR. This
|
|
is not a libev-specific thing, this is true for most event libraries.
|
|
.PP
|
|
\fIThe special problem of threads signal handling\fR
|
|
.IX Subsection "The special problem of threads signal handling"
|
|
.PP
|
|
\&\s-1POSIX\s0 threads has problematic signal handling semantics, specifically,
|
|
a lot of functionality (sigfd, sigwait etc.) only really works if all
|
|
threads in a process block signals, which is hard to achieve.
|
|
.PP
|
|
When you want to use sigwait (or mix libev signal handling with your own
|
|
for the same signals), you can tackle this problem by globally blocking
|
|
all signals before creating any threads (or creating them with a fully set
|
|
sigprocmask) and also specifying the \f(CW\*(C`EVFLAG_NOSIGMASK\*(C'\fR when creating
|
|
loops. Then designate one thread as \*(L"signal receiver thread\*(R" which handles
|
|
these signals. You can pass on any signals that libev might be interested
|
|
in by calling \f(CW\*(C`ev_feed_signal\*(C'\fR.
|
|
.PP
|
|
\fIWatcher-Specific Functions and Data Members\fR
|
|
.IX Subsection "Watcher-Specific Functions and Data Members"
|
|
.IP "ev_signal_init (ev_signal *, callback, int signum)" 4
|
|
.IX Item "ev_signal_init (ev_signal *, callback, int signum)"
|
|
.PD 0
|
|
.IP "ev_signal_set (ev_signal *, int signum)" 4
|
|
.IX Item "ev_signal_set (ev_signal *, int signum)"
|
|
.PD
|
|
Configures the watcher to trigger on the given signal number (usually one
|
|
of the \f(CW\*(C`SIGxxx\*(C'\fR constants).
|
|
.IP "int signum [read\-only]" 4
|
|
.IX Item "int signum [read-only]"
|
|
The signal the watcher watches out for.
|
|
.PP
|
|
\fIExamples\fR
|
|
.IX Subsection "Examples"
|
|
.PP
|
|
Example: Try to exit cleanly on \s-1SIGINT.\s0
|
|
.PP
|
|
.Vb 5
|
|
\& static void
|
|
\& sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
|
|
\& {
|
|
\& ev_break (loop, EVBREAK_ALL);
|
|
\& }
|
|
\&
|
|
\& ev_signal signal_watcher;
|
|
\& ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
|
|
\& ev_signal_start (loop, &signal_watcher);
|
|
.Ve
|
|
.ie n .SS """ev_child"" \- watch out for process status changes"
|
|
.el .SS "\f(CWev_child\fP \- watch out for process status changes"
|
|
.IX Subsection "ev_child - watch out for process status changes"
|
|
Child watchers trigger when your process receives a \s-1SIGCHLD\s0 in response to
|
|
some child status changes (most typically when a child of yours dies or
|
|
exits). It is permissible to install a child watcher \fIafter\fR the child
|
|
has been forked (which implies it might have already exited), as long
|
|
as the event loop isn't entered (or is continued from a watcher), i.e.,
|
|
forking and then immediately registering a watcher for the child is fine,
|
|
but forking and registering a watcher a few event loop iterations later or
|
|
in the next callback invocation is not.
|
|
.PP
|
|
Only the default event loop is capable of handling signals, and therefore
|
|
you can only register child watchers in the default event loop.
|
|
.PP
|
|
Due to some design glitches inside libev, child watchers will always be
|
|
handled at maximum priority (their priority is set to \f(CW\*(C`EV_MAXPRI\*(C'\fR by
|
|
libev)
|
|
.PP
|
|
\fIProcess Interaction\fR
|
|
.IX Subsection "Process Interaction"
|
|
.PP
|
|
Libev grabs \f(CW\*(C`SIGCHLD\*(C'\fR as soon as the default event loop is
|
|
initialised. This is necessary to guarantee proper behaviour even if the
|
|
first child watcher is started after the child exits. The occurrence
|
|
of \f(CW\*(C`SIGCHLD\*(C'\fR 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.
|
|
.PP
|
|
\fIOverriding the Built-In Processing\fR
|
|
.IX Subsection "Overriding the Built-In Processing"
|
|
.PP
|
|
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
|
|
\&\f(CW\*(C`SIGCHLD\*(C'\fR 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 \f(CW\*(C`ev_child\*(C'\fR watchers freely.
|
|
.PP
|
|
\fIStopping the Child Watcher\fR
|
|
.IX Subsection "Stopping the Child Watcher"
|
|
.PP
|
|
Currently, the child watcher never gets stopped, even when the
|
|
child terminates, so normally one needs to stop the watcher in the
|
|
callback. Future versions of libev might stop the watcher automatically
|
|
when a child exit is detected (calling \f(CW\*(C`ev_child_stop\*(C'\fR twice is not a
|
|
problem).
|
|
.PP
|
|
\fIWatcher-Specific Functions and Data Members\fR
|
|
.IX Subsection "Watcher-Specific Functions and Data Members"
|
|
.IP "ev_child_init (ev_child *, callback, int pid, int trace)" 4
|
|
.IX Item "ev_child_init (ev_child *, callback, int pid, int trace)"
|
|
.PD 0
|
|
.IP "ev_child_set (ev_child *, int pid, int trace)" 4
|
|
.IX Item "ev_child_set (ev_child *, int pid, int trace)"
|
|
.PD
|
|
Configures the watcher to wait for status changes of process \f(CW\*(C`pid\*(C'\fR (or
|
|
\&\fIany\fR process if \f(CW\*(C`pid\*(C'\fR is specified as \f(CW0\fR). The callback can look
|
|
at the \f(CW\*(C`rstatus\*(C'\fR member of the \f(CW\*(C`ev_child\*(C'\fR watcher structure to see
|
|
the status word (use the macros from \f(CW\*(C`sys/wait.h\*(C'\fR and see your systems
|
|
\&\f(CW\*(C`waitpid\*(C'\fR documentation). The \f(CW\*(C`rpid\*(C'\fR member contains the pid of the
|
|
process causing the status change. \f(CW\*(C`trace\*(C'\fR must be either \f(CW0\fR (only
|
|
activate the watcher when the process terminates) or \f(CW1\fR (additionally
|
|
activate the watcher when the process is stopped or continued).
|
|
.IP "int pid [read\-only]" 4
|
|
.IX Item "int pid [read-only]"
|
|
The process id this watcher watches out for, or \f(CW0\fR, meaning any process id.
|
|
.IP "int rpid [read\-write]" 4
|
|
.IX Item "int rpid [read-write]"
|
|
The process id that detected a status change.
|
|
.IP "int rstatus [read\-write]" 4
|
|
.IX Item "int rstatus [read-write]"
|
|
The process exit/trace status caused by \f(CW\*(C`rpid\*(C'\fR (see your systems
|
|
\&\f(CW\*(C`waitpid\*(C'\fR and \f(CW\*(C`sys/wait.h\*(C'\fR documentation for details).
|
|
.PP
|
|
\fIExamples\fR
|
|
.IX Subsection "Examples"
|
|
.PP
|
|
Example: \f(CW\*(C`fork()\*(C'\fR a new process and install a child handler to wait for
|
|
its completion.
|
|
.PP
|
|
.Vb 1
|
|
\& ev_child cw;
|
|
\&
|
|
\& static void
|
|
\& child_cb (EV_P_ ev_child *w, int revents)
|
|
\& {
|
|
\& ev_child_stop (EV_A_ w);
|
|
\& printf ("process %d exited with status %x\en", 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);
|
|
\& }
|
|
.Ve
|
|
.ie n .SS """ev_stat"" \- did the file attributes just change?"
|
|
.el .SS "\f(CWev_stat\fP \- did the file attributes just change?"
|
|
.IX Subsection "ev_stat - did the file attributes just change?"
|
|
This watches a file system path for attribute changes. That is, it calls
|
|
\&\f(CW\*(C`stat\*(C'\fR on that path in regular intervals (or when the \s-1OS\s0 says it changed)
|
|
and sees if it changed compared to the last time, invoking the callback
|
|
if it did. Starting the watcher \f(CW\*(C`stat\*(C'\fR's the file, so only changes that
|
|
happen after the watcher has been started will be reported.
|
|
.PP
|
|
The path does not need to exist: changing from \*(L"path exists\*(R" to \*(L"path does
|
|
not exist\*(R" is a status change like any other. The condition \*(L"path does not
|
|
exist\*(R" (or more correctly \*(L"path cannot be stat'ed\*(R") is signified by the
|
|
\&\f(CW\*(C`st_nlink\*(C'\fR 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.
|
|
.PP
|
|
The path \fImust not\fR end in a slash or contain special components such as
|
|
\&\f(CW\*(C`.\*(C'\fR or \f(CW\*(C`..\*(C'\fR. The path \fIshould\fR be absolute: If it is relative and
|
|
your working directory changes, then the behaviour is undefined.
|
|
.PP
|
|
Since there is no portable change notification interface available, the
|
|
portable implementation simply calls \f(CWstat(2)\fR 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 \f(CW0\fR (highly
|
|
recommended!) then a \fIsuitable, unspecified default\fR 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 \f(CW0.1\fR, but that's usually overkill.
|
|
.PP
|
|
This watcher type is not meant for massive numbers of stat watchers,
|
|
as even with OS-supported change notifications, this can be
|
|
resource-intensive.
|
|
.PP
|
|
At the time of this writing, the only OS-specific interface implemented
|
|
is the Linux inotify interface (implementing kqueue support is left as an
|
|
exercise for the reader. Note, however, that the author sees no way of
|
|
implementing \f(CW\*(C`ev_stat\*(C'\fR semantics with kqueue, except as a hint).
|
|
.PP
|
|
\fI\s-1ABI\s0 Issues (Largefile Support)\fR
|
|
.IX Subsection "ABI Issues (Largefile Support)"
|
|
.PP
|
|
Libev by default (unless the user overrides this) uses the default
|
|
compilation environment, which means that on systems with large file
|
|
support disabled by default, you get the 32 bit version of the stat
|
|
structure. When using the library from programs that change the \s-1ABI\s0 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 \s-1ABI,\s0 but the problem is
|
|
most noticeably displayed with ev_stat and large file support.
|
|
.PP
|
|
The solution for this is to lobby your distribution maker to make large
|
|
file interfaces available by default (as e.g. FreeBSD does) and not
|
|
optional. Libev cannot simply switch on large file support because it has
|
|
to exchange stat structures with application programs compiled using the
|
|
default compilation environment.
|
|
.PP
|
|
\fIInotify and Kqueue\fR
|
|
.IX Subsection "Inotify and Kqueue"
|
|
.PP
|
|
When \f(CW\*(C`inotify (7)\*(C'\fR support has been compiled into libev and present at
|
|
runtime, it will be used to speed up change detection where possible. The
|
|
inotify descriptor will be created lazily when the first \f(CW\*(C`ev_stat\*(C'\fR
|
|
watcher is being started.
|
|
.PP
|
|
Inotify presence does not change the semantics of \f(CW\*(C`ev_stat\*(C'\fR watchers
|
|
except that changes might be detected earlier, and in some cases, to avoid
|
|
making regular \f(CW\*(C`stat\*(C'\fR calls. Even in the presence of inotify support
|
|
there are many cases where libev has to resort to regular \f(CW\*(C`stat\*(C'\fR polling,
|
|
but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too
|
|
many bugs), the path exists (i.e. stat succeeds), and the path resides on
|
|
a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and
|
|
xfs are fully working) libev usually gets away without polling.
|
|
.PP
|
|
There is no support for kqueue, as apparently it cannot be used to
|
|
implement this functionality, due to the requirement of having a file
|
|
descriptor open on the object at all times, and detecting renames, unlinks
|
|
etc. is difficult.
|
|
.PP
|
|
\fI\f(CI\*(C`stat ()\*(C'\fI is a synchronous operation\fR
|
|
.IX Subsection "stat () is a synchronous operation"
|
|
.PP
|
|
Libev doesn't normally do any kind of I/O itself, and so is not blocking
|
|
the process. The exception are \f(CW\*(C`ev_stat\*(C'\fR watchers \- those call \f(CW\*(C`stat
|
|
()\*(C'\fR, which is a synchronous operation.
|
|
.PP
|
|
For local paths, this usually doesn't matter: unless the system is very
|
|
busy or the intervals between stat's are large, a stat call will be fast,
|
|
as the path data is usually in memory already (except when starting the
|
|
watcher).
|
|
.PP
|
|
For networked file systems, calling \f(CW\*(C`stat ()\*(C'\fR can block an indefinite
|
|
time due to network issues, and even under good conditions, a stat call
|
|
often takes multiple milliseconds.
|
|
.PP
|
|
Therefore, it is best to avoid using \f(CW\*(C`ev_stat\*(C'\fR watchers on networked
|
|
paths, although this is fully supported by libev.
|
|
.PP
|
|
\fIThe special problem of stat time resolution\fR
|
|
.IX Subsection "The special problem of stat time resolution"
|
|
.PP
|
|
The \f(CW\*(C`stat ()\*(C'\fR system call only supports full-second resolution portably,
|
|
and even on systems where the resolution is higher, most file systems
|
|
still only support whole seconds.
|
|
.PP
|
|
That means that, if the time is the only thing that changes, you can
|
|
easily miss updates: on the first update, \f(CW\*(C`ev_stat\*(C'\fR detects a change and
|
|
calls your callback, which does something. When there is another update
|
|
within the same second, \f(CW\*(C`ev_stat\*(C'\fR will be unable to detect unless the
|
|
stat data does change in other ways (e.g. file size).
|
|
.PP
|
|
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 \f(CW\*(C`ev_timer\*(C'\fR (e.g. \f(CW\*(C`ev_timer_set (w, 0., 1.02);
|
|
ev_timer_again (loop, w)\*(C'\fR).
|
|
.PP
|
|
The \f(CW.02\fR 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
|
|
\&\f(CW\*(C`gettimeofday\*(C'\fR might return a timestamp with a full second later than
|
|
a subsequent \f(CW\*(C`time\*(C'\fR call \- if the equivalent of \f(CW\*(C`time ()\*(C'\fR 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).
|
|
.PP
|
|
\fIWatcher-Specific Functions and Data Members\fR
|
|
.IX Subsection "Watcher-Specific Functions and Data Members"
|
|
.IP "ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)" 4
|
|
.IX Item "ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)"
|
|
.PD 0
|
|
.IP "ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)" 4
|
|
.IX Item "ev_stat_set (ev_stat *, const char *path, ev_tstamp interval)"
|
|
.PD
|
|
Configures the watcher to wait for status changes of the given
|
|
\&\f(CW\*(C`path\*(C'\fR. The \f(CW\*(C`interval\*(C'\fR is a hint on how quickly a change is expected to
|
|
be detected and should normally be specified as \f(CW0\fR to let libev choose
|
|
a suitable value. The memory pointed to by \f(CW\*(C`path\*(C'\fR must point to the same
|
|
path for as long as the watcher is active.
|
|
.Sp
|
|
The callback will receive an \f(CW\*(C`EV_STAT\*(C'\fR event when a change was detected,
|
|
relative to the attributes at the time the watcher was started (or the
|
|
last change was detected).
|
|
.IP "ev_stat_stat (loop, ev_stat *)" 4
|
|
.IX Item "ev_stat_stat (loop, ev_stat *)"
|
|
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.
|
|
.IP "ev_statdata attr [read\-only]" 4
|
|
.IX Item "ev_statdata attr [read-only]"
|
|
The most-recently detected attributes of the file. Although the type is
|
|
\&\f(CW\*(C`ev_statdata\*(C'\fR, this is usually the (or one of the) \f(CW\*(C`struct stat\*(C'\fR types
|
|
suitable for your system, but you can only rely on the POSIX-standardised
|
|
members to be present. If the \f(CW\*(C`st_nlink\*(C'\fR member is \f(CW0\fR, then there was
|
|
some error while \f(CW\*(C`stat\*(C'\fRing the file.
|
|
.IP "ev_statdata prev [read\-only]" 4
|
|
.IX Item "ev_statdata prev [read-only]"
|
|
The previous attributes of the file. The callback gets invoked whenever
|
|
\&\f(CW\*(C`prev\*(C'\fR != \f(CW\*(C`attr\*(C'\fR, or, more precisely, one or more of these members
|
|
differ: \f(CW\*(C`st_dev\*(C'\fR, \f(CW\*(C`st_ino\*(C'\fR, \f(CW\*(C`st_mode\*(C'\fR, \f(CW\*(C`st_nlink\*(C'\fR, \f(CW\*(C`st_uid\*(C'\fR,
|
|
\&\f(CW\*(C`st_gid\*(C'\fR, \f(CW\*(C`st_rdev\*(C'\fR, \f(CW\*(C`st_size\*(C'\fR, \f(CW\*(C`st_atime\*(C'\fR, \f(CW\*(C`st_mtime\*(C'\fR, \f(CW\*(C`st_ctime\*(C'\fR.
|
|
.IP "ev_tstamp interval [read\-only]" 4
|
|
.IX Item "ev_tstamp interval [read-only]"
|
|
The specified interval.
|
|
.IP "const char *path [read\-only]" 4
|
|
.IX Item "const char *path [read-only]"
|
|
The file system path that is being watched.
|
|
.PP
|
|
\fIExamples\fR
|
|
.IX Subsection "Examples"
|
|
.PP
|
|
Example: Watch \f(CW\*(C`/etc/passwd\*(C'\fR for attribute changes.
|
|
.PP
|
|
.Vb 10
|
|
\& 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\en", (long)w\->attr.st_size);
|
|
\& printf ("passwd current atime %ld\en", (long)w\->attr.st_mtime);
|
|
\& printf ("passwd current mtime %ld\en", (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\en");
|
|
\& }
|
|
\&
|
|
\& ...
|
|
\& ev_stat passwd;
|
|
\&
|
|
\& ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
|
|
\& ev_stat_start (loop, &passwd);
|
|
.Ve
|
|
.PP
|
|
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 \f(CW\*(C`ev_stat\*(C'\fR callback invocation \fIand\fR on
|
|
\&\f(CW\*(C`ev_timer\*(C'\fR callback invocation).
|
|
.PP
|
|
.Vb 2
|
|
\& 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\*(Aqs 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);
|
|
.Ve
|
|
.ie n .SS """ev_idle"" \- when you've got nothing better to do..."
|
|
.el .SS "\f(CWev_idle\fP \- when you've got nothing better to do..."
|
|
.IX Subsection "ev_idle - when you've got nothing better to do..."
|
|
Idle watchers trigger events when no other events of the same or higher
|
|
priority are pending (prepare, check and other idle watchers do not count
|
|
as receiving \*(L"events\*(R").
|
|
.PP
|
|
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.
|
|
.PP
|
|
The most noteworthy effect is that as long as any idle watchers are
|
|
active, the process will not block when waiting for new events.
|
|
.PP
|
|
Apart from keeping your process non-blocking (which is a useful
|
|
effect on its own sometimes), idle watchers are a good place to do
|
|
\&\*(L"pseudo-background processing\*(R", or delay processing stuff to after the
|
|
event loop has handled all outstanding events.
|
|
.PP
|
|
\fIAbusing an \f(CI\*(C`ev_idle\*(C'\fI watcher for its side-effect\fR
|
|
.IX Subsection "Abusing an ev_idle watcher for its side-effect"
|
|
.PP
|
|
As long as there is at least one active idle watcher, libev will never
|
|
sleep unnecessarily. Or in other words, it will loop as fast as possible.
|
|
For this to work, the idle watcher doesn't need to be invoked at all \- the
|
|
lowest priority will do.
|
|
.PP
|
|
This mode of operation can be useful together with an \f(CW\*(C`ev_check\*(C'\fR watcher,
|
|
to do something on each event loop iteration \- for example to balance load
|
|
between different connections.
|
|
.PP
|
|
See \*(L"Abusing an ev_check watcher for its side-effect\*(R" for a longer
|
|
example.
|
|
.PP
|
|
\fIWatcher-Specific Functions and Data Members\fR
|
|
.IX Subsection "Watcher-Specific Functions and Data Members"
|
|
.IP "ev_idle_init (ev_idle *, callback)" 4
|
|
.IX Item "ev_idle_init (ev_idle *, callback)"
|
|
Initialises and configures the idle watcher \- it has no parameters of any
|
|
kind. There is a \f(CW\*(C`ev_idle_set\*(C'\fR macro, but using it is utterly pointless,
|
|
believe me.
|
|
.PP
|
|
\fIExamples\fR
|
|
.IX Subsection "Examples"
|
|
.PP
|
|
Example: Dynamically allocate an \f(CW\*(C`ev_idle\*(C'\fR watcher, start it, and in the
|
|
callback, free it. Also, use no error checking, as usual.
|
|
.PP
|
|
.Vb 5
|
|
\& static void
|
|
\& idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
|
|
\& {
|
|
\& // stop the watcher
|
|
\& ev_idle_stop (loop, w);
|
|
\&
|
|
\& // now we can free it
|
|
\& free (w);
|
|
\&
|
|
\& // now do something you wanted to do when the program has
|
|
\& // no longer anything immediate to do.
|
|
\& }
|
|
\&
|
|
\& ev_idle *idle_watcher = malloc (sizeof (ev_idle));
|
|
\& ev_idle_init (idle_watcher, idle_cb);
|
|
\& ev_idle_start (loop, idle_watcher);
|
|
.Ve
|
|
.ie n .SS """ev_prepare"" and ""ev_check"" \- customise your event loop!"
|
|
.el .SS "\f(CWev_prepare\fP and \f(CWev_check\fP \- customise your event loop!"
|
|
.IX Subsection "ev_prepare and ev_check - customise your event loop!"
|
|
Prepare and check watchers are often (but not always) used in pairs:
|
|
prepare watchers get invoked before the process blocks and check watchers
|
|
afterwards.
|
|
.PP
|
|
You \fImust not\fR call \f(CW\*(C`ev_run\*(C'\fR (or similar functions that enter the
|
|
current event loop) or \f(CW\*(C`ev_loop_fork\*(C'\fR from either \f(CW\*(C`ev_prepare\*(C'\fR or
|
|
\&\f(CW\*(C`ev_check\*(C'\fR 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
|
|
\&\f(CW\*(C`ev_prepare\*(C'\fR, blocking, \f(CW\*(C`ev_check\*(C'\fR so if you have one watcher of each
|
|
kind they will always be called in pairs bracketing the blocking call.
|
|
.PP
|
|
Their main purpose is to integrate other event mechanisms into libev and
|
|
their use is somewhat advanced. They could be used, for example, to track
|
|
variable changes, implement your own watchers, integrate net-snmp or a
|
|
coroutine library and lots more. They are also occasionally useful if
|
|
you cache some data and want to flush it before blocking (for example,
|
|
in X programs you might want to do an \f(CW\*(C`XFlush ()\*(C'\fR in an \f(CW\*(C`ev_prepare\*(C'\fR
|
|
watcher).
|
|
.PP
|
|
This is done by examining in each prepare call which file descriptors
|
|
need to be watched by the other library, registering \f(CW\*(C`ev_io\*(C'\fR watchers
|
|
for them and starting an \f(CW\*(C`ev_timer\*(C'\fR watcher for any timeouts (many
|
|
libraries provide exactly this functionality). Then, in the check watcher,
|
|
you check for any events that occurred (by checking the pending status
|
|
of all watchers and stopping them) and call back into the library. The
|
|
I/O and timer callbacks will never actually be called (but must be valid
|
|
nevertheless, because you never know, you know?).
|
|
.PP
|
|
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).
|
|
.PP
|
|
When used for this purpose, it is recommended to give \f(CW\*(C`ev_check\*(C'\fR watchers
|
|
highest (\f(CW\*(C`EV_MAXPRI\*(C'\fR) priority, to ensure that they are being run before
|
|
any other watchers after the poll (this doesn't matter for \f(CW\*(C`ev_prepare\*(C'\fR
|
|
watchers).
|
|
.PP
|
|
Also, \f(CW\*(C`ev_check\*(C'\fR watchers (and \f(CW\*(C`ev_prepare\*(C'\fR watchers, too) should not
|
|
activate (\*(L"feed\*(R") events into libev. While libev fully supports this, they
|
|
might get executed before other \f(CW\*(C`ev_check\*(C'\fR watchers did their job. As
|
|
\&\f(CW\*(C`ev_check\*(C'\fR watchers are often used to embed other (non-libev) event
|
|
loops those other event loops might be in an unusable state until their
|
|
\&\f(CW\*(C`ev_check\*(C'\fR |