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Posted to commits@trafficserver.apache.org by jp...@apache.org on 2009/12/18 18:57:52 UTC
svn commit: r892325 [3/8] - in
/incubator/trafficserver/traffic/branches/dev: ./ install/ iocore/cache/
iocore/cluster/ iocore/dns/ iocore/eventsystem/ iocore/hostdb/ iocore/net/
libev/ libev/CVS/ libinktomi++/ proxy/ proxy/config/ proxy/congest/ proxy...
Added: incubator/trafficserver/traffic/branches/dev/libev/ev.3
URL: http://svn.apache.org/viewvc/incubator/trafficserver/traffic/branches/dev/libev/ev.3?rev=892325&view=auto
==============================================================================
--- incubator/trafficserver/traffic/branches/dev/libev/ev.3 (added)
+++ incubator/trafficserver/traffic/branches/dev/libev/ev.3 Fri Dec 18 17:57:46 2009
@@ -0,0 +1,4619 @@
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+.ds ae a\h'-(\w'a'u*4/10)'e
+.ds Ae A\h'-(\w'A'u*4/10)'E
+. \" corrections for vroff
+.if v .ds ~ \\k:\h'-(\\n(.wu*9/10-\*(#H)'\s-2\u~\d\s+2\h'|\\n:u'
+.if v .ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'\v'-.4m'^\v'.4m'\h'|\\n:u'
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+.if \n(.H>23 .if \n(.V>19 \
+\{\
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+.rm #[ #] #H #V #F C
+.\" ========================================================================
+.\"
+.IX Title "LIBEV 3"
+.TH LIBEV 3 "2009-07-27" "libev-3.8" "libev - high performance full featured event loop"
+.\" For nroff, turn off justification. Always turn off hyphenation; it makes
+.\" way too many mistakes in technical documents.
+.if n .ad l
+.nh
+.SH "NAME"
+libev \- a high performance full\-featured event loop written in C
+.SH "SYNOPSIS"
+.IX Header "SYNOPSIS"
+.Vb 1
+\& #include <ev.h>
+.Ve
+.SS "\s-1EXAMPLE\s0 \s-1PROGRAM\s0"
+.IX Subsection "EXAMPLE PROGRAM"
+.Vb 2
+\& // a single header file is required
+\& #include <ev.h>
+\&
+\& #include <stdio.h> // for puts
+\&
+\& // every watcher type has its own typedef\*(Aqd struct
+\& // with the name ev_TYPE
+\& ev_io stdin_watcher;
+\& ev_timer timeout_watcher;
+\&
+\& // all watcher callbacks have a similar signature
+\& // this callback is called when data is readable on stdin
+\& static void
+\& stdin_cb (EV_P_ ev_io *w, int revents)
+\& {
+\& puts ("stdin ready");
+\& // for one\-shot events, one must manually stop the watcher
+\& // with its corresponding stop function.
+\& ev_io_stop (EV_A_ w);
+\&
+\& // this causes all nested ev_loop\*(Aqs to stop iterating
+\& ev_unloop (EV_A_ EVUNLOOP_ALL);
+\& }
+\&
+\& // another callback, this time for a time\-out
+\& static void
+\& timeout_cb (EV_P_ ev_timer *w, int revents)
+\& {
+\& puts ("timeout");
+\& // this causes the innermost ev_loop to stop iterating
+\& ev_unloop (EV_A_ EVUNLOOP_ONE);
+\& }
+\&
+\& int
+\& main (void)
+\& {
+\& // use the default event loop unless you have special needs
+\& struct ev_loop *loop = ev_default_loop (0);
+\&
+\& // initialise an io watcher, then start it
+\& // this one will watch for stdin to become readable
+\& ev_io_init (&stdin_watcher, stdin_cb, /*STDIN_FILENO*/ 0, EV_READ);
+\& ev_io_start (loop, &stdin_watcher);
+\&
+\& // initialise a timer watcher, then start it
+\& // simple non\-repeating 5.5 second timeout
+\& ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
+\& ev_timer_start (loop, &timeout_watcher);
+\&
+\& // now wait for events to arrive
+\& ev_loop (loop, 0);
+\&
+\& // unloop was called, so exit
+\& return 0;
+\& }
+.Ve
+.SH "ABOUT THIS DOCUMENT"
+.IX Header "ABOUT THIS DOCUMENT"
+This document documents the libev software package.
+.PP
+The newest version of this document is also available as an html-formatted
+web page you might find easier to navigate when reading it for the first
+time: <http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod>.
+.PP
+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
+on event-based programming, nor will it introduce event-based programming
+with libev.
+.PP
+Familarity with event based programming techniques in general is assumed
+throughout this document.
+.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
+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 \f(CW\*(C`epoll\*(C'\fR, 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> 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`ev_loop *\*(C'\fR) will not have
+this argument.
+.SS "\s-1TIME\s0 \s-1REPRESENTATION\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 (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. Unlike the name
+component \f(CW\*(C`stamp\*(C'\fR might indicate, it is also used for time differences
+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.
+.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. Basically
+this is a sub-second-resolution \f(CW\*(C`sleep ()\*(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.
+.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. 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
+is the theoretical, all-platform, value. To find which 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)) [\s-1NOT\s0 \s-1REENTRANT\s0]" 4
+.IX Item "ev_set_allocator (void *(*cb)(void *ptr, long size)) [NOT REENTRANT]"
+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: Replace the libev allocator with one that waits a bit and then
+retries (example requires a standards-compliant \f(CW\*(C`realloc\*(C'\fR).
+.Sp
+.Vb 6
+\& static void *
+\& persistent_realloc (void *ptr, size_t size)
+\& {
+\& for (;;)
+\& {
+\& void *newptr = realloc (ptr, size);
+\&
+\& if (newptr)
+\& return newptr;
+\&
+\& sleep (60);
+\& }
+\& }
+\&
+\& ...
+\& ev_set_allocator (persistent_realloc);
+.Ve
+.IP "ev_set_syserr_cb (void (*cb)(const char *msg)); [\s-1NOT\s0 \s-1REENTRANT\s0]" 4
+.IX Item "ev_set_syserr_cb (void (*cb)(const char *msg)); [NOT REENTRANT]"
+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
+.SH "FUNCTIONS CONTROLLING THE EVENT LOOP"
+.IX Header "FUNCTIONS CONTROLLING THE EVENT LOOP"
+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, as there is also an \f(CW\*(C`ev_loop\*(C'\fR
+\&\fIfunction\fR).
+.PP
+The library knows two types of such loops, the \fIdefault\fR loop, which
+supports signals and child events, and dynamically created 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 will initialise the default event loop if it hasn't been initialised
+yet and return it. If the default loop could not be initialised, returns
+false. If it already was initialised it simply returns it (and ignores the
+flags. If that is troubling you, check \f(CW\*(C`ev_backend ()\*(C'\fR afterwards).
+.Sp
+If you don't know what event loop to use, use the one returned from this
+function.
+.Sp
+Note that this function is \fInot\fR thread-safe, so if you want to use it
+from multiple threads, you have to lock (note also that this is unlikely,
+as loops cannot be shared easily between threads anyway).
+.Sp
+The default loop is the only loop that can handle \f(CW\*(C`ev_signal\*(C'\fR and
+\&\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 that 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
+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, or to work
+around bugs.
+.ie n .IP """EVFLAG_FORKCHECK""" 4
+.el .IP "\f(CWEVFLAG_FORKCHECK\fR" 4
+.IX Item "EVFLAG_FORKCHECK"
+Instead of calling \f(CW\*(C`ev_default_fork\*(C'\fR or \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).
+.Sp
+The big advantage of this flag is that you can forget about fork (and
+forget about forgetting to tell libev about forking) when you use this
+flag.
+.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 it's \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_NOSIGNALFD""" 4
+.el .IP "\f(CWEVFLAG_NOSIGNALFD\fR" 4
+.IX Item "EVFLAG_NOSIGNALFD"
+When this flag is specified, then libev will not attempt to use the
+\&\fIsignalfd\fR \s-1API\s0 for it's \f(CW\*(C`ev_signal\*(C'\fR (and \f(CW\*(C`ev_child\*(C'\fR) watchers. This is
+probably only useful to work around any bugs in libev. Consequently, this
+flag might go away once the signalfd functionality is considered stable,
+so it's useful mostly in environment variables and not in program code.
+.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 \fIselect\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 \fIpoll\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)"
+For few fds, this backend is a bit little slower than poll and select,
+but it scales phenomenally better. While poll and select usually scale
+like O(total_fds) where n is the total number of fds (or the highest fd),
+epoll scales either O(1) or O(active_fds).
+.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 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.
+.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
+all kernel versions tested so far.
+.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 (but
+sane, unlike 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\s0 X)) 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
+Please note that Solaris event ports can deliver a lot of spurious
+notifications, so you need to use non-blocking I/O or other means to avoid
+blocking when no data (or space) is available.
+.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, with the exception of the spurious readiness
+notifications, 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
+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.
+.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: 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
+.Sp
+Example: Use whatever libev has to offer, but make sure that kqueue is
+used if available (warning, breaks stuff, best use only with your own
+private event loop and only if you know the \s-1OS\s0 supports your types of
+fds):
+.Sp
+.Vb 1
+\& ev_default_loop (ev_recommended_backends () | EVBACKEND_KQUEUE);
+.Ve
+.RE
+.IP "struct ev_loop *ev_loop_new (unsigned int flags)" 4
+.IX Item "struct ev_loop *ev_loop_new (unsigned int flags)"
+Similar to \f(CW\*(C`ev_default_loop\*(C'\fR, but always creates a new event loop that is
+always distinct from the default loop. Unlike the default loop, it cannot
+handle signal and child watchers, and attempts to do so will be greeted by
+undefined behaviour (or a failed assertion if assertions are enabled).
+.Sp
+Note that this function \fIis\fR thread-safe, and the recommended way to use
+libev with threads is indeed to create one loop per thread, and using the
+default loop in the \*(L"main\*(R" or \*(L"initial\*(R" thread.
+.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
+.IP "ev_default_destroy ()" 4
+.IX Item "ev_default_destroy ()"
+Destroys the default loop again (frees all memory and kernel state
+etc.). None of the active event watchers will be stopped in the normal
+sense, so e.g. \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
+In general it is not advisable to call this function except in the
+rare occasion where you really need to free e.g. the signal handling
+pipe fds. If you need dynamically allocated loops it is better to use
+\&\f(CW\*(C`ev_loop_new\*(C'\fR and \f(CW\*(C`ev_loop_destroy\*(C'\fR).
+.IP "ev_loop_destroy (loop)" 4
+.IX Item "ev_loop_destroy (loop)"
+Like \f(CW\*(C`ev_default_destroy\*(C'\fR, but destroys an event loop created by an
+earlier call to \f(CW\*(C`ev_loop_new\*(C'\fR.
+.IP "ev_default_fork ()" 4
+.IX Item "ev_default_fork ()"
+This function sets a flag that causes subsequent \f(CW\*(C`ev_loop\*(C'\fR iterations
+to reinitialise the kernel state for backends that have one. Despite the
+name, you can call it anytime, but it makes most sense after forking, in
+the child process (or both child and parent, but that again makes little
+sense). You \fImust\fR call it in the child before using any of the libev
+functions, and it will only take effect at the next \f(CW\*(C`ev_loop\*(C'\fR iteration.
+.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 library in the child. If
+you just fork+exec, you don't have to call it at all.
+.Sp
+The function itself is quite fast and it's usually not a problem to call
+it just in case after a fork. To make this easy, the function will fit in
+quite nicely into a call to \f(CW\*(C`pthread_atfork\*(C'\fR:
+.Sp
+.Vb 1
+\& pthread_atfork (0, 0, ev_default_fork);
+.Ve
+.IP "ev_loop_fork (loop)" 4
+.IX Item "ev_loop_fork (loop)"
+Like \f(CW\*(C`ev_default_fork\*(C'\fR, but acts on an event loop created by
+\&\f(CW\*(C`ev_loop_new\*(C'\fR. Yes, you have to call this on every allocated event loop
+after fork that you want to re-use in the child, and how you do this is
+entirely your own problem.
+.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_loop_count (loop)" 4
+.IX Item "unsigned int ev_loop_count (loop)"
+Returns the count of loop iterations for the loop, which is identical to
+the number of times libev did poll for new events. It starts at \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.
+.IP "unsigned int ev_loop_depth (loop)" 4
+.IX Item "unsigned int ev_loop_depth (loop)"
+Returns the number of times \f(CW\*(C`ev_loop\*(C'\fR was entered minus the number of
+times \f(CW\*(C`ev_loop\*(C'\fR was exited, in other words, the recursion depth.
+.Sp
+Outside \f(CW\*(C`ev_loop\*(C'\fR, this number is zero. In a callback, this number is
+\&\f(CW1\fR, unless \f(CW\*(C`ev_loop\*(C'\fR was invoked recursively (or from another thread),
+in which case it is higher.
+.Sp
+Leaving \f(CW\*(C`ev_loop\*(C'\fR abnormally (setjmp/longjmp, cancelling the thread
+etc.), doesn't count as exit.
+.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_loop ()\*(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 a 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
+occured 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 "ev_loop (loop, int flags)" 4
+.IX Item "ev_loop (loop, int flags)"
+Finally, this is it, the event handler. This function usually is called
+after you initialised all your watchers and you want to start handling
+events.
+.Sp
+If the flags argument is specified as \f(CW0\fR, it will not return until
+either no event watchers are active anymore or \f(CW\*(C`ev_unloop\*(C'\fR was called.
+.Sp
+Please note that an explicit \f(CW\*(C`ev_unloop\*(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
+A flags value of \f(CW\*(C`EVLOOP_NONBLOCK\*(C'\fR will look for new events, will handle
+those events and any already outstanding ones, but will not block your
+process in case there are no events and will return after one iteration of
+the loop.
+.Sp
+A flags value of \f(CW\*(C`EVLOOP_ONESHOT\*(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_loop\*(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_loop\*(C'\fR does:
+.Sp
+.Vb 10
+\& \- Before the first iteration, call any pending watchers.
+\& * 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 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, EVLOOP_NONBLOCK or not having
+\& any active watchers at all will result in not sleeping).
+\& \- Sleep if the I/O and timer collect interval say so.
+\& \- Block the process, waiting for any events.
+\& \- Queue all outstanding I/O (fd) events.
+\& \- Update the "event loop time" (ev_now ()), and do time jump adjustments.
+\& \- Queue all expired timers.
+\& \- Queue all expired periodics.
+\& \- Unless any events are pending now, queue all idle watchers.
+\& \- Queue all check watchers.
+\& \- Call all queued watchers in reverse order (i.e. check watchers first).
+\& Signals and child watchers are implemented as I/O watchers, and will
+\& be handled here by queueing them when their watcher gets executed.
+\& \- If ev_unloop has been called, or EVLOOP_ONESHOT or EVLOOP_NONBLOCK
+\& were used, or there are no active watchers, return, otherwise
+\& continue with step *.
+.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_loop (my_loop, 0);
+\& ... jobs done or somebody called unloop. yeah!
+.Ve
+.IP "ev_unloop (loop, how)" 4
+.IX Item "ev_unloop (loop, how)"
+Can be used to make a call to \f(CW\*(C`ev_loop\*(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`EVUNLOOP_ONE\*(C'\fR, which will make the innermost \f(CW\*(C`ev_loop\*(C'\fR call return, or
+\&\f(CW\*(C`EVUNLOOP_ALL\*(C'\fR, which will make all nested \f(CW\*(C`ev_loop\*(C'\fR calls return.
+.Sp
+This \*(L"unloop state\*(R" will be cleared when entering \f(CW\*(C`ev_loop\*(C'\fR again.
+.Sp
+It is safe to call \f(CW\*(C`ev_unloop\*(C'\fR from otuside any \f(CW\*(C`ev_loop\*(C'\fR calls.
+.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_loop\*(C'\fR will not return on its own.
+.Sp
+If you have a watcher you never unregister that should not keep \f(CW\*(C`ev_loop\*(C'\fR
+from returning, call \fIev_unref()\fR after starting, and \fIev_ref()\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_loop\*(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_loop\*(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);
+\& evf_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 be not 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.
+.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 transations 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_loop\*(C'\fR does this automatically when required,
+but when overriding the invoke callback this call comes handy.
+.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_loop\*(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), void (*acquire)(\s-1EV_P\s0))" 4
+.IX Item "ev_set_loop_release_cb (loop, void (*release)(EV_P), void (*acquire)(EV_P))"
+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_loop\*(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 loop via
+\&\f(CW\*(C`ev_unloop\*(C'\fR and \f(CW\*(C`av_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_loop\*(C'\fR when you want it
+to take note of any changes you made.
+.Sp
+In theory, threads executing \f(CW\*(C`ev_loop\*(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 "ev_userdata (loop)" 4
+.IX Item "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_loop_verify (loop)" 4
+.IX Item "ev_loop_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 a structure that you create and register to record your
+interest in some event. For instance, if 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_unloop (loop, EVUNLOOP_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_loop (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 gets invoked each time the event occurs (or, in the case of I/O
+watchers, each time the event loop detects that the file descriptor given
+is readable and/or writable).
+.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_TIMEOUT""" 4
+.el .IP "\f(CWEV_TIMEOUT\fR" 4
+.IX Item "EV_TIMEOUT"
+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_loop\*(C'\fR starts
+to gather new events, and all \f(CW\*(C`ev_check\*(C'\fR watchers are invoked just after
+\&\f(CW\*(C`ev_loop\*(C'\fR has gathered them, but before it invokes any callbacks for any
+received events. Callbacks of both watcher types can start and stop as
+many watchers as they want, and all of them will be taken into account
+(for example, a \f(CW\*(C`ev_prepare\*(C'\fR watcher might start an idle watcher to keep
+\&\f(CW\*(C`ev_loop\*(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_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 \fIread()\fR or \fIwrite()\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\s0 \s-1WATCHER\s0 \s-1FUNCTIONS\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 *, [args])" 4
+.el .IP "\f(CWev_TYPE_set\fR (ev_TYPE *, [args])" 4
+.IX Item "ev_TYPE_set (ev_TYPE *, [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_cb_set (ev_TYPE *watcher, callback)" 4
+.IX Item "ev_cb_set (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, priority)" 4
+.IX Item "ev_set_priority (ev_TYPE *watcher, 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\s0 \s-1PRIORITY\s0 \s-1MODELS\s0\*(R", 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.
+.SS "\s-1ASSOCIATING\s0 \s-1CUSTOM\s0 \s-1DATA\s0 \s-1WITH\s0 A \s-1WATCHER\s0"
+.IX Subsection "ASSOCIATING CUSTOM DATA WITH A WATCHER"
+Each watcher has, by default, a member \f(CW\*(C`void *data\*(C'\fR that you can change
+and read at any time: libev will completely ignore it. This can be used
+to associate arbitrary data with your watcher. If you need more data and
+don't want to allocate memory and store a pointer to it in that data
+member, you can also \*(L"subclass\*(R" the watcher type and provide your own
+data:
+.PP
+.Vb 7
+\& struct my_io
+\& {
+\& ev_io io;
+\& int otherfd;
+\& void *somedata;
+\& struct whatever *mostinteresting;
+\& };
+\&
+\& ...
+\& struct my_io w;
+\& ev_io_init (&w.io, my_cb, fd, EV_READ);
+.Ve
+.PP
+And since your callback will be called with a pointer to the watcher, you
+can cast it back to your own type:
+.PP
+.Vb 5
+\& static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
+\& {
+\& struct my_io *w = (struct my_io *)w_;
+\& ...
+\& }
+.Ve
+.PP
+More interesting and less C\-conformant ways of casting your callback type
+instead have been omitted.
+.PP
+Another common scenario is to use some data structure with multiple
+embedded watchers:
+.PP
+.Vb 6
+\& struct my_biggy
+\& {
+\& int some_data;
+\& ev_timer t1;
+\& ev_timer t2;
+\& }
+.Ve
+.PP
+In this case getting the pointer to \f(CW\*(C`my_biggy\*(C'\fR is a bit more
+complicated: Either you store the address of your \f(CW\*(C`my_biggy\*(C'\fR struct
+in the \f(CW\*(C`data\*(C'\fR member of the watcher (for woozies), or you need to use
+some pointer arithmetic using \f(CW\*(C`offsetof\*(C'\fR inside your watchers (for real
+programmers):
+.PP
+.Vb 1
+\& #include <stddef.h>
+\&
+\& static void
+\& t1_cb (EV_P_ ev_timer *w, int revents)
+\& {
+\& struct my_biggy big = (struct my_biggy *)
+\& (((char *)w) \- offsetof (struct my_biggy, t1));
+\& }
+\&
+\& static void
+\& t2_cb (EV_P_ ev_timer *w, int revents)
+\& {
+\& struct my_biggy big = (struct my_biggy *)
+\& (((char *)w) \- offsetof (struct my_biggy, t2));
+\& }
+.Ve
+.SS "\s-1WATCHER\s0 \s-1PRIORITY\s0 \s-1MODELS\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
+continously 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 ahndle 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
+If you cannot use non-blocking mode, then force the use of a
+known-to-be-good backend (at the time of this writing, this includes only
+\&\f(CW\*(C`EVBACKEND_SELECT\*(C'\fR and \f(CW\*(C`EVBACKEND_POLL\*(C'\fR). The same applies to file
+descriptors for which non-blocking operation makes no sense (such as
+files) \- libev doesn't guarentee any specific behaviour in that case.
+.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. Not only are some backends known to create a
+lot of those (for example Solaris ports), it is very easy to get into
+this situation even with a relatively standard program structure. Thus
+it is best to always use non-blocking I/O: An extra \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 our Xlib example, Xlib 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) 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 fork\fR
+.IX Subsection "The special problem of fork"
+.PP
+Some backends (epoll, kqueue) 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.
+.PP
+To support fork in your programs, you either have to call
+\&\f(CW\*(C`ev_default_fork ()\*(C'\fR or \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
+\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_loop (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). 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_loop\*(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 1
+\& ev_tstamp last_activity; // time of last activity
+\&
+\& static void
+\& callback (EV_P_ ev_timer *w, int revents)
+\& {
+\& ev_tstamp now = ev_now (EV_A);
+\& ev_tstamp timeout = last_activity + 60.;
+\&
+\& // if last_activity + 60. is older than now, we did time out
+\& if (timeout < now)
+\& {
+\& // timeout occured, take action
+\& }
+\& else
+\& {
+\& // callback was invoked, but there was some activity, re\-arm
+\& // the watcher to fire in last_activity + 60, which is
+\& // guaranteed to be in the future, so "again" is positive:
+\& w\->repeat = timeout \- now;
+\& ev_timer_again (EV_A_ w);
+\& }
+\& }
+.Ve
+.Sp
+To summarise the callback: first calculate the real timeout (defined
+as \*(L"60 seconds after the last activity\*(R"), then check if that time has
+been reached, which means something \fIdid\fR, in fact, time out. Otherwise
+the callback was invoked too early (\f(CW\*(C`timeout\*(C'\fR is in the future), so
+re-schedule the timer to fire at that future time, to see if maybe we have
+a timeout then.
+.Sp
+Note how \f(CW\*(C`ev_timer_again\*(C'\fR is used, taking advantage of the
+\&\f(CW\*(C`ev_timer_again\*(C'\fR optimisation when the timer is already running.
+.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 timer, simply initialise the watcher and set \f(CW\*(C`last_activity\*(C'\fR
+to the current time (meaning we just have some activity :), then call the
+callback, which will \*(L"do the right thing\*(R" and start the timer:
+.Sp
+.Vb 3
+\& ev_init (timer, callback);
+\& last_activity = ev_now (loop);
+\& callback (loop, timer, EV_TIMEOUT);
+.Ve
+.Sp
+And 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 1
+\& last_actiivty = ev_now (loop);
+.Ve
+.Sp
+This technique is slightly more complex, but in most cases where the
+time-out is unlikely to be triggered, much more efficient.
+.Sp
+Changing the timeout is trivial as well (if it isn't hard-coded in the
+callback :) \- just change the timeout and invoke the callback, which will
+fix things for you.
+.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
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