From 4dc30d27d0b48de023a3e1e7fd67ac52273580d1 Mon Sep 17 00:00:00 2001 From: root Date: Fri, 23 Nov 2007 16:17:12 +0000 Subject: add lots of theoretical examples --- ev.html | 230 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 224 insertions(+), 6 deletions(-) (limited to 'ev.html') diff --git a/ev.html b/ev.html index e6f6b48..f8e5829 100644 --- a/ev.html +++ b/ev.html @@ -6,7 +6,7 @@ - + @@ -98,7 +98,12 @@ will not have this argument.

(fractional) number of seconds since the (POSIX) epoch (somewhere near the beginning of 1970, details are complicated, don't ask). This type is called ev_tstamp, which is what you should use too. It usually aliases -to the double type in C.

+to the double type in C, and when you need to do any calculations on +it, you should treat it as such.

+ + + +

GLOBAL FUNCTIONS

Top

@@ -124,6 +129,13 @@ version of the library your program was compiled against.

as this indicates an incompatible change. Minor versions are usually compatible to older versions, so a larger minor version alone is usually not a problem.

Example: make sure we haven't accidentally been linked against the wrong +version:

  assert (("libev version mismatch",
+           ev_version_major () == EV_VERSION_MAJOR
+           && ev_version_minor () >= EV_VERSION_MINOR));
+
+

unsigned int ev_supported_backends ()

@@ -131,6 +143,12 @@ not a problem.

value) compiled into this binary of libev (independent of their availability on the system you are running on). See ev_default_loop for a description of the set values.

Example: make sure we have the epoll method, because yeah this is cool and +a must have and can we have a torrent of it please!!!11

  assert (("sorry, no epoll, no sex",
+           ev_supported_backends () & EVBACKEND_EPOLL));
+
+

unsigned int ev_recommended_backends ()

@@ -151,6 +169,26 @@ destructive action. The default is your system realloc function.

You could override this function in high-availability programs to, say, free some memory if it cannot allocate memory, to use a special allocator, or even to sleep a while and retry until some memory is available.

Example: replace the libev allocator with one that waits a bit and then +retries: better than mine).

   static void *
+   persistent_realloc (void *ptr, long size)
+   {
+     for (;;)
+       {
+         void *newptr = realloc (ptr, size);
+
+         if (newptr)
+           return newptr;
+
+         sleep (60);
+       }
+   }
+
+   ...
+   ev_set_allocator (persistent_realloc);
+
+

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

@@ -161,6 +199,18 @@ callback is set, then libev will expect it to remedy the sitution, no matter what, when it returns. That is, libev will generally retry the requested operation, or, if the condition doesn't go away, do bad stuff (such as abort).

Example: do the same thing as libev does internally:

   static void
+   fatal_error (const char *msg)
+   {
+     perror (msg);
+     abort ();
+   }
+
+   ...
+   ev_set_syserr_cb (fatal_error);
+
+

@@ -295,6 +345,12 @@ event loop and only if you know the OS supports your types of fds):

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).

Example: try to create a event loop that uses epoll and nothing else.

  struct ev_loop *epoller = ev_loop_new (EVBACKEND_EPOLL | EVFLAG_NOENV);
+  if (!epoller)
+    fatal ("no epoll found here, maybe it hides under your chair");
+
+

ev_default_destroy ()

@@ -340,10 +396,10 @@ use.

ev_tstamp ev_now (loop)

Returns the current "event loop time", which is the time the event loop -got 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 -occuring (or more correctly, the mainloop finding out about it).

+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 occuring (or more correctly, libev finding out about it).

ev_loop (loop, int flags)

@@ -352,6 +408,11 @@ after you initialised all your watchers and you want to start handling events.

If the flags argument is specified as 0, it will not return until either no event watchers are active anymore or ev_unloop was called.

Please note that an explicit ev_unloop is usually better than +relying on all watchers to be stopped when deciding when a program has +finished (especially in interactive programs), but having a program that +automatically loops as long as it has to and no longer by virtue of +relying on its watchers stopping correctly is a thing of beauty.

A flags value of EVLOOP_NONBLOCK will look for new events, will handle those events and any outstanding ones, but will not block your process in case there are no events and will return after one iteration of the loop.

@@ -382,6 +443,14 @@ usually a better approach for this kind of thing.

- If ev_unloop has been called or EVLOOP_ONESHOT or EVLOOP_NONBLOCK were used, return, otherwise continue with step *. + +

Example: queue some jobs and then loop until no events are outsanding +anymore.

   ... queue jobs here, make sure they register event watchers as long
+   ... as they still have work to do (even an idle watcher will do..)
+   ev_loop (my_loop, 0);
+   ... jobs done. yeah!
+

ev_unloop (loop, how)

@@ -404,6 +473,19 @@ visible to the libev user and should not keep ev_loop from exiting no event watchers registered by it are active. It is also an excellent way to do this for generic recurring timers or from within third-party libraries. Just remember to unref after start and ref before stop.

Example: create a signal watcher, but keep it from keeping ev_loop +running when nothing else is active.

  struct dv_signal exitsig;
+  ev_signal_init (&exitsig, sig_cb, SIGINT);
+  ev_signal_start (myloop, &exitsig);
+  evf_unref (myloop);
+
+

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

  ev_ref (myloop);
+  ev_signal_stop (myloop, &exitsig);
+
+

@@ -549,6 +631,10 @@ have been omitted....

This section describes each watcher in detail, but will not repeat information given in the last section.

+ + + +

`ev_io` - is this file descriptor readable or writable

@@ -588,6 +674,27 @@ typical ways of handling events, so its a good idea to use non-blocking I/O unconditionally.

Example: call stdin_readable_cb when STDIN_FILENO has become, well +readable, but only once. Since it is likely line-buffered, you could +attempt to read a whole line in the callback:

  static void
+  stdin_readable_cb (struct ev_loop *loop, struct ev_io *w, int revents)
+  {
+     ev_io_stop (loop, w);
+    .. read from stdin here (or from w->fd) and haqndle any I/O errors
+  }
+
+  ...
+  struct ev_loop *loop = ev_default_init (0);
+  struct 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);
+
+
+
+
+

`ev_timer` - relative and optionally recurring timeouts

@@ -641,6 +748,39 @@ state where you do not expect data to travel on the socket, you can stop the timer, and again will automatically restart it if need be.

Example: create a timer that fires after 60 seconds.

  static void
+  one_minute_cb (struct ev_loop *loop, struct ev_timer *w, int revents)
+  {
+    .. one minute over, w is actually stopped right here
+  }
+
+  struct ev_timer mytimer;
+  ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
+  ev_timer_start (loop, &mytimer);
+
+

Example: create a timeout timer that times out after 10 seconds of +inactivity.

  static void
+  timeout_cb (struct ev_loop *loop, struct ev_timer *w, int revents)
+  {
+    .. ten seconds without any activity
+  }
+
+  struct ev_timer mytimer;
+  ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
+  ev_timer_again (&mytimer); /* start timer */
+  ev_loop (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);
+
+
+
+
+

`ev_periodic` - to cron or not to cron

@@ -734,6 +874,42 @@ a different time than the last time it was called (e.g. in a crond like program when the crontabs have changed).

Example: call a callback every hour, or, more precisely, whenever the +system clock is divisible by 3600. The callback invocation times have +potentially a lot of jittering, but good long-term stability.

  static void
+  clock_cb (struct ev_loop *loop, struct ev_io *w, int revents)
+  {
+    ... its now a full hour (UTC, or TAI or whatever your clock follows)
+  }
+
+  struct ev_periodic hourly_tick;
+  ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
+  ev_periodic_start (loop, &hourly_tick);
+
+

Example: the same as above, but use a reschedule callback to do it:

  #include <math.h>
+
+  static ev_tstamp
+  my_scheduler_cb (struct ev_periodic *w, ev_tstamp now)
+  {
+    return fmod (now, 3600.) + 3600.;
+  }
+
+  ev_periodic_init (&hourly_tick, clock_cb, 0., 0., my_scheduler_cb);
+
+

Example: call a callback every hour, starting now:

  struct ev_periodic hourly_tick;
+  ev_periodic_init (&hourly_tick, clock_cb,
+                    fmod (ev_now (loop), 3600.), 3600., 0);
+  ev_periodic_start (loop, &hourly_tick);
+
+
+
+
+

`ev_signal` - signal me when a signal gets signalled

@@ -774,6 +950,21 @@ the status word (use the macros from sys/wait.h and see your system process causing the status change.

Example: try to exit cleanly on SIGINT and SIGTERM.

  static void
+  sigint_cb (struct ev_loop *loop, struct ev_signal *w, int revents)
+  {
+    ev_unloop (loop, EVUNLOOP_ALL);
+  }
+
+  struct ev_signal signal_watcher;
+  ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
+  ev_signal_start (loop, &sigint_cb);
+
+
+
+
+

`ev_idle` - when you've got nothing better to do

@@ -799,6 +990,24 @@ kind. There is a ev_idle_set macro, but using it is utterly pointle believe me.

Example: dynamically allocate an ev_idle, start it, and in the +callback, free it. Alos, use no error checking, as usual.

  static void
+  idle_cb (struct ev_loop *loop, struct ev_idle *w, int revents)
+  {
+    free (w);
+    // now do something you wanted to do when the program has
+    // no longer asnything immediate to do.
+  }
+
+  struct ev_idle *idle_watcher = malloc (sizeof (struct ev_idle));
+  ev_idle_init (idle_watcher, idle_cb);
+  ev_idle_start (loop, idle_cb);
+
+
+
+
+

`ev_prepare` and `ev_check` - customise your event loop

@@ -834,6 +1043,11 @@ parameters of any kind. There are ev_prepare_set and

ev_check
 macros, but using them is utterly, utterly and completely pointless.
 	
 
+Example: *TODO*.
+
+
+
+
 
 
 OTHER FUNCTIONS
Top
@@ -887,6 +1101,10 @@ the given events it.
 	
 
 
+
+
+
+
 
 LIBEVENT EMULATION
Top
 
-- 
cgit v1.2.3

GLOBAL FUNCTIONS

ev_io - is this file descriptor readable or writable

ev_timer - relative and optionally recurring timeouts

ev_periodic - to cron or not to cron

ev_signal - signal me when a signal gets signalled

ev_idle - when you've got nothing better to do

ev_prepare and ev_check - customise your event loop