… | |
… | |
174 | =item ev_tstamp ev_time () |
174 | =item ev_tstamp ev_time () |
175 | |
175 | |
176 | Returns the current time as libev would use it. Please note that the |
176 | Returns the current time as libev would use it. Please note that the |
177 | C<ev_now> function is usually faster and also often returns the timestamp |
177 | C<ev_now> function is usually faster and also often returns the timestamp |
178 | you actually want to know. Also interesting is the combination of |
178 | you actually want to know. Also interesting is the combination of |
179 | C<ev_update_now> and C<ev_now>. |
179 | C<ev_now_update> and C<ev_now>. |
180 | |
180 | |
181 | =item ev_sleep (ev_tstamp interval) |
181 | =item ev_sleep (ev_tstamp interval) |
182 | |
182 | |
183 | Sleep for the given interval: The current thread will be blocked |
183 | Sleep for the given interval: The current thread will be blocked |
184 | until either it is interrupted or the given time interval has |
184 | until either it is interrupted or the given time interval has |
… | |
… | |
1020 | can be done relatively simply by putting mutex_lock/unlock calls around |
1020 | can be done relatively simply by putting mutex_lock/unlock calls around |
1021 | each call to a libev function. |
1021 | each call to a libev function. |
1022 | |
1022 | |
1023 | However, C<ev_run> can run an indefinite time, so it is not feasible |
1023 | However, C<ev_run> can run an indefinite time, so it is not feasible |
1024 | to wait for it to return. One way around this is to wake up the event |
1024 | to wait for it to return. One way around this is to wake up the event |
1025 | loop via C<ev_break> and C<av_async_send>, another way is to set these |
1025 | loop via C<ev_break> and C<ev_async_send>, another way is to set these |
1026 | I<release> and I<acquire> callbacks on the loop. |
1026 | I<release> and I<acquire> callbacks on the loop. |
1027 | |
1027 | |
1028 | When set, then C<release> will be called just before the thread is |
1028 | When set, then C<release> will be called just before the thread is |
1029 | suspended waiting for new events, and C<acquire> is called just |
1029 | suspended waiting for new events, and C<acquire> is called just |
1030 | afterwards. |
1030 | afterwards. |
… | |
… | |
1771 | detecting time jumps is hard, and some inaccuracies are unavoidable (the |
1771 | detecting time jumps is hard, and some inaccuracies are unavoidable (the |
1772 | monotonic clock option helps a lot here). |
1772 | monotonic clock option helps a lot here). |
1773 | |
1773 | |
1774 | The callback is guaranteed to be invoked only I<after> its timeout has |
1774 | The callback is guaranteed to be invoked only I<after> its timeout has |
1775 | passed (not I<at>, so on systems with very low-resolution clocks this |
1775 | passed (not I<at>, so on systems with very low-resolution clocks this |
1776 | might introduce a small delay). If multiple timers become ready during the |
1776 | might introduce a small delay, see "the special problem of being too |
|
|
1777 | early", below). If multiple timers become ready during the same loop |
1777 | same loop iteration then the ones with earlier time-out values are invoked |
1778 | iteration then the ones with earlier time-out values are invoked before |
1778 | before ones of the same priority with later time-out values (but this is |
1779 | ones of the same priority with later time-out values (but this is no |
1779 | no longer true when a callback calls C<ev_run> recursively). |
1780 | longer true when a callback calls C<ev_run> recursively). |
1780 | |
1781 | |
1781 | =head3 Be smart about timeouts |
1782 | =head3 Be smart about timeouts |
1782 | |
1783 | |
1783 | Many real-world problems involve some kind of timeout, usually for error |
1784 | Many real-world problems involve some kind of timeout, usually for error |
1784 | recovery. A typical example is an HTTP request - if the other side hangs, |
1785 | recovery. A typical example is an HTTP request - if the other side hangs, |
… | |
… | |
1859 | |
1860 | |
1860 | In this case, it would be more efficient to leave the C<ev_timer> alone, |
1861 | In this case, it would be more efficient to leave the C<ev_timer> alone, |
1861 | but remember the time of last activity, and check for a real timeout only |
1862 | but remember the time of last activity, and check for a real timeout only |
1862 | within the callback: |
1863 | within the callback: |
1863 | |
1864 | |
|
|
1865 | ev_tstamp timeout = 60.; |
1864 | ev_tstamp last_activity; // time of last activity |
1866 | ev_tstamp last_activity; // time of last activity |
|
|
1867 | ev_timer timer; |
1865 | |
1868 | |
1866 | static void |
1869 | static void |
1867 | callback (EV_P_ ev_timer *w, int revents) |
1870 | callback (EV_P_ ev_timer *w, int revents) |
1868 | { |
1871 | { |
1869 | ev_tstamp now = ev_now (EV_A); |
1872 | // calculate when the timeout would happen |
1870 | ev_tstamp timeout = last_activity + 60.; |
1873 | ev_tstamp after = last_activity - ev_now (EV_A) + timeout; |
1871 | |
1874 | |
1872 | // if last_activity + 60. is older than now, we did time out |
1875 | // if negative, it means we the timeout already occured |
1873 | if (timeout < now) |
1876 | if (after < 0.) |
1874 | { |
1877 | { |
1875 | // timeout occurred, take action |
1878 | // timeout occurred, take action |
1876 | } |
1879 | } |
1877 | else |
1880 | else |
1878 | { |
1881 | { |
1879 | // callback was invoked, but there was some activity, re-arm |
1882 | // callback was invoked, but there was some recent |
1880 | // the watcher to fire in last_activity + 60, which is |
1883 | // activity. simply restart the timer to time out |
1881 | // guaranteed to be in the future, so "again" is positive: |
1884 | // after "after" seconds, which is the earliest time |
1882 | w->repeat = timeout - now; |
1885 | // the timeout can occur. |
|
|
1886 | ev_timer_set (w, after, 0.); |
1883 | ev_timer_again (EV_A_ w); |
1887 | ev_timer_start (EV_A_ w); |
1884 | } |
1888 | } |
1885 | } |
1889 | } |
1886 | |
1890 | |
1887 | To summarise the callback: first calculate the real timeout (defined |
1891 | To summarise the callback: first calculate in how many seconds the |
1888 | as "60 seconds after the last activity"), then check if that time has |
1892 | timeout will occur (by calculating the absolute time when it would occur, |
1889 | been reached, which means something I<did>, in fact, time out. Otherwise |
1893 | C<last_activity + timeout>, and subtracting the current time, C<ev_now |
1890 | the callback was invoked too early (C<timeout> is in the future), so |
1894 | (EV_A)> from that). |
1891 | re-schedule the timer to fire at that future time, to see if maybe we have |
|
|
1892 | a timeout then. |
|
|
1893 | |
1895 | |
1894 | Note how C<ev_timer_again> is used, taking advantage of the |
1896 | If this value is negative, then we are already past the timeout, i.e. we |
1895 | C<ev_timer_again> optimisation when the timer is already running. |
1897 | timed out, and need to do whatever is needed in this case. |
|
|
1898 | |
|
|
1899 | Otherwise, we now the earliest time at which the timeout would trigger, |
|
|
1900 | and simply start the timer with this timeout value. |
|
|
1901 | |
|
|
1902 | In other words, each time the callback is invoked it will check whether |
|
|
1903 | the timeout cocured. If not, it will simply reschedule itself to check |
|
|
1904 | again at the earliest time it could time out. Rinse. Repeat. |
1896 | |
1905 | |
1897 | This scheme causes more callback invocations (about one every 60 seconds |
1906 | This scheme causes more callback invocations (about one every 60 seconds |
1898 | minus half the average time between activity), but virtually no calls to |
1907 | minus half the average time between activity), but virtually no calls to |
1899 | libev to change the timeout. |
1908 | libev to change the timeout. |
1900 | |
1909 | |
1901 | To start the timer, simply initialise the watcher and set C<last_activity> |
1910 | To start the machinery, simply initialise the watcher and set |
1902 | to the current time (meaning we just have some activity :), then call the |
1911 | C<last_activity> to the current time (meaning there was some activity just |
1903 | callback, which will "do the right thing" and start the timer: |
1912 | now), then call the callback, which will "do the right thing" and start |
|
|
1913 | the timer: |
1904 | |
1914 | |
|
|
1915 | last_activity = ev_now (EV_A); |
1905 | ev_init (timer, callback); |
1916 | ev_init (&timer, callback); |
1906 | last_activity = ev_now (loop); |
1917 | callback (EV_A_ &timer, 0); |
1907 | callback (loop, timer, EV_TIMER); |
|
|
1908 | |
1918 | |
1909 | And when there is some activity, simply store the current time in |
1919 | When there is some activity, simply store the current time in |
1910 | C<last_activity>, no libev calls at all: |
1920 | C<last_activity>, no libev calls at all: |
1911 | |
1921 | |
|
|
1922 | if (activity detected) |
1912 | last_activity = ev_now (loop); |
1923 | last_activity = ev_now (EV_A); |
|
|
1924 | |
|
|
1925 | When your timeout value changes, then the timeout can be changed by simply |
|
|
1926 | providing a new value, stopping the timer and calling the callback, which |
|
|
1927 | will agaion do the right thing (for example, time out immediately :). |
|
|
1928 | |
|
|
1929 | timeout = new_value; |
|
|
1930 | ev_timer_stop (EV_A_ &timer); |
|
|
1931 | callback (EV_A_ &timer, 0); |
1913 | |
1932 | |
1914 | This technique is slightly more complex, but in most cases where the |
1933 | This technique is slightly more complex, but in most cases where the |
1915 | time-out is unlikely to be triggered, much more efficient. |
1934 | time-out is unlikely to be triggered, much more efficient. |
1916 | |
|
|
1917 | Changing the timeout is trivial as well (if it isn't hard-coded in the |
|
|
1918 | callback :) - just change the timeout and invoke the callback, which will |
|
|
1919 | fix things for you. |
|
|
1920 | |
1935 | |
1921 | =item 4. Wee, just use a double-linked list for your timeouts. |
1936 | =item 4. Wee, just use a double-linked list for your timeouts. |
1922 | |
1937 | |
1923 | If there is not one request, but many thousands (millions...), all |
1938 | If there is not one request, but many thousands (millions...), all |
1924 | employing some kind of timeout with the same timeout value, then one can |
1939 | employing some kind of timeout with the same timeout value, then one can |
… | |
… | |
1951 | Method #1 is almost always a bad idea, and buys you nothing. Method #4 is |
1966 | Method #1 is almost always a bad idea, and buys you nothing. Method #4 is |
1952 | rather complicated, but extremely efficient, something that really pays |
1967 | rather complicated, but extremely efficient, something that really pays |
1953 | off after the first million or so of active timers, i.e. it's usually |
1968 | off after the first million or so of active timers, i.e. it's usually |
1954 | overkill :) |
1969 | overkill :) |
1955 | |
1970 | |
|
|
1971 | =head3 The special problem of being too early |
|
|
1972 | |
|
|
1973 | If you ask a timer to call your callback after three seconds, then |
|
|
1974 | you expect it to be invoked after three seconds - but of course, this |
|
|
1975 | cannot be guaranteed to infinite precision. Less obviously, it cannot be |
|
|
1976 | guaranteed to any precision by libev - imagine somebody suspending the |
|
|
1977 | process with a STOP signal for a few hours for example. |
|
|
1978 | |
|
|
1979 | So, libev tries to invoke your callback as soon as possible I<after> the |
|
|
1980 | delay has occurred, but cannot guarantee this. |
|
|
1981 | |
|
|
1982 | A less obvious failure mode is calling your callback too early: many event |
|
|
1983 | loops compare timestamps with a "elapsed delay >= requested delay", but |
|
|
1984 | this can cause your callback to be invoked much earlier than you would |
|
|
1985 | expect. |
|
|
1986 | |
|
|
1987 | To see why, imagine a system with a clock that only offers full second |
|
|
1988 | resolution (think windows if you can't come up with a broken enough OS |
|
|
1989 | yourself). If you schedule a one-second timer at the time 500.9, then the |
|
|
1990 | event loop will schedule your timeout to elapse at a system time of 500 |
|
|
1991 | (500.9 truncated to the resolution) + 1, or 501. |
|
|
1992 | |
|
|
1993 | If an event library looks at the timeout 0.1s later, it will see "501 >= |
|
|
1994 | 501" and invoke the callback 0.1s after it was started, even though a |
|
|
1995 | one-second delay was requested - this is being "too early", despite best |
|
|
1996 | intentions. |
|
|
1997 | |
|
|
1998 | This is the reason why libev will never invoke the callback if the elapsed |
|
|
1999 | delay equals the requested delay, but only when the elapsed delay is |
|
|
2000 | larger than the requested delay. In the example above, libev would only invoke |
|
|
2001 | the callback at system time 502, or 1.1s after the timer was started. |
|
|
2002 | |
|
|
2003 | So, while libev cannot guarantee that your callback will be invoked |
|
|
2004 | exactly when requested, it I<can> and I<does> guarantee that the requested |
|
|
2005 | delay has actually elapsed, or in other words, it always errs on the "too |
|
|
2006 | late" side of things. |
|
|
2007 | |
1956 | =head3 The special problem of time updates |
2008 | =head3 The special problem of time updates |
1957 | |
2009 | |
1958 | Establishing the current time is a costly operation (it usually takes at |
2010 | Establishing the current time is a costly operation (it usually takes |
1959 | least two system calls): EV therefore updates its idea of the current |
2011 | at least one system call): EV therefore updates its idea of the current |
1960 | time only before and after C<ev_run> collects new events, which causes a |
2012 | time only before and after C<ev_run> collects new events, which causes a |
1961 | growing difference between C<ev_now ()> and C<ev_time ()> when handling |
2013 | growing difference between C<ev_now ()> and C<ev_time ()> when handling |
1962 | lots of events in one iteration. |
2014 | lots of events in one iteration. |
1963 | |
2015 | |
1964 | The relative timeouts are calculated relative to the C<ev_now ()> |
2016 | The relative timeouts are calculated relative to the C<ev_now ()> |
… | |
… | |
1970 | ev_timer_set (&timer, after + ev_now () - ev_time (), 0.); |
2022 | ev_timer_set (&timer, after + ev_now () - ev_time (), 0.); |
1971 | |
2023 | |
1972 | If the event loop is suspended for a long time, you can also force an |
2024 | If the event loop is suspended for a long time, you can also force an |
1973 | update of the time returned by C<ev_now ()> by calling C<ev_now_update |
2025 | update of the time returned by C<ev_now ()> by calling C<ev_now_update |
1974 | ()>. |
2026 | ()>. |
|
|
2027 | |
|
|
2028 | =head3 The special problem of unsynchronised clocks |
|
|
2029 | |
|
|
2030 | Modern systems have a variety of clocks - libev itself uses the normal |
|
|
2031 | "wall clock" clock and, if available, the monotonic clock (to avoid time |
|
|
2032 | jumps). |
|
|
2033 | |
|
|
2034 | Neither of these clocks is synchronised with each other or any other clock |
|
|
2035 | on the system, so C<ev_time ()> might return a considerably different time |
|
|
2036 | than C<gettimeofday ()> or C<time ()>. On a GNU/Linux system, for example, |
|
|
2037 | a call to C<gettimeofday> might return a second count that is one higher |
|
|
2038 | than a directly following call to C<time>. |
|
|
2039 | |
|
|
2040 | The moral of this is to only compare libev-related timestamps with |
|
|
2041 | C<ev_time ()> and C<ev_now ()>, at least if you want better precision than |
|
|
2042 | a second or so. |
|
|
2043 | |
|
|
2044 | One more problem arises due to this lack of synchronisation: if libev uses |
|
|
2045 | the system monotonic clock and you compare timestamps from C<ev_time> |
|
|
2046 | or C<ev_now> from when you started your timer and when your callback is |
|
|
2047 | invoked, you will find that sometimes the callback is a bit "early". |
|
|
2048 | |
|
|
2049 | This is because C<ev_timer>s work in real time, not wall clock time, so |
|
|
2050 | libev makes sure your callback is not invoked before the delay happened, |
|
|
2051 | I<measured according to the real time>, not the system clock. |
|
|
2052 | |
|
|
2053 | If your timeouts are based on a physical timescale (e.g. "time out this |
|
|
2054 | connection after 100 seconds") then this shouldn't bother you as it is |
|
|
2055 | exactly the right behaviour. |
|
|
2056 | |
|
|
2057 | If you want to compare wall clock/system timestamps to your timers, then |
|
|
2058 | you need to use C<ev_periodic>s, as these are based on the wall clock |
|
|
2059 | time, where your comparisons will always generate correct results. |
1975 | |
2060 | |
1976 | =head3 The special problems of suspended animation |
2061 | =head3 The special problems of suspended animation |
1977 | |
2062 | |
1978 | When you leave the server world it is quite customary to hit machines that |
2063 | When you leave the server world it is quite customary to hit machines that |
1979 | can suspend/hibernate - what happens to the clocks during such a suspend? |
2064 | can suspend/hibernate - what happens to the clocks during such a suspend? |
… | |
… | |
3402 | ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0); |
3487 | ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0); |
3403 | |
3488 | |
3404 | =item ev_feed_fd_event (loop, int fd, int revents) |
3489 | =item ev_feed_fd_event (loop, int fd, int revents) |
3405 | |
3490 | |
3406 | Feed an event on the given fd, as if a file descriptor backend detected |
3491 | Feed an event on the given fd, as if a file descriptor backend detected |
3407 | the given events it. |
3492 | the given events. |
3408 | |
3493 | |
3409 | =item ev_feed_signal_event (loop, int signum) |
3494 | =item ev_feed_signal_event (loop, int signum) |
3410 | |
3495 | |
3411 | Feed an event as if the given signal occurred. See also C<ev_feed_signal>, |
3496 | Feed an event as if the given signal occurred. See also C<ev_feed_signal>, |
3412 | which is async-safe. |
3497 | which is async-safe. |
… | |
… | |
3485 | t2_cb (EV_P_ ev_timer *w, int revents) |
3570 | t2_cb (EV_P_ ev_timer *w, int revents) |
3486 | { |
3571 | { |
3487 | struct my_biggy big = (struct my_biggy *) |
3572 | struct my_biggy big = (struct my_biggy *) |
3488 | (((char *)w) - offsetof (struct my_biggy, t2)); |
3573 | (((char *)w) - offsetof (struct my_biggy, t2)); |
3489 | } |
3574 | } |
|
|
3575 | |
|
|
3576 | =head2 AVOIDING FINISHING BEFORE RETURNING |
|
|
3577 | |
|
|
3578 | Often you have structures like this in event-based programs: |
|
|
3579 | |
|
|
3580 | callback () |
|
|
3581 | { |
|
|
3582 | free (request); |
|
|
3583 | } |
|
|
3584 | |
|
|
3585 | request = start_new_request (..., callback); |
|
|
3586 | |
|
|
3587 | The intent is to start some "lengthy" operation. The C<request> could be |
|
|
3588 | used to cancel the operation, or do other things with it. |
|
|
3589 | |
|
|
3590 | It's not uncommon to have code paths in C<start_new_request> that |
|
|
3591 | immediately invoke the callback, for example, to report errors. Or you add |
|
|
3592 | some caching layer that finds that it can skip the lengthy aspects of the |
|
|
3593 | operation and simply invoke the callback with the result. |
|
|
3594 | |
|
|
3595 | The problem here is that this will happen I<before> C<start_new_request> |
|
|
3596 | has returned, so C<request> is not set. |
|
|
3597 | |
|
|
3598 | Even if you pass the request by some safer means to the callback, you |
|
|
3599 | might want to do something to the request after starting it, such as |
|
|
3600 | canceling it, which probably isn't working so well when the callback has |
|
|
3601 | already been invoked. |
|
|
3602 | |
|
|
3603 | A common way around all these issues is to make sure that |
|
|
3604 | C<start_new_request> I<always> returns before the callback is invoked. If |
|
|
3605 | C<start_new_request> immediately knows the result, it can artificially |
|
|
3606 | delay invoking the callback by e.g. using a C<prepare> or C<idle> watcher |
|
|
3607 | for example, or more sneakily, by reusing an existing (stopped) watcher |
|
|
3608 | and pushing it into the pending queue: |
|
|
3609 | |
|
|
3610 | ev_set_cb (watcher, callback); |
|
|
3611 | ev_feed_event (EV_A_ watcher, 0); |
|
|
3612 | |
|
|
3613 | This way, C<start_new_request> can safely return before the callback is |
|
|
3614 | invoked, while not delaying callback invocation too much. |
3490 | |
3615 | |
3491 | =head2 MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS |
3616 | =head2 MODEL/NESTED EVENT LOOP INVOCATIONS AND EXIT CONDITIONS |
3492 | |
3617 | |
3493 | Often (especially in GUI toolkits) there are places where you have |
3618 | Often (especially in GUI toolkits) there are places where you have |
3494 | I<modal> interaction, which is most easily implemented by recursively |
3619 | I<modal> interaction, which is most easily implemented by recursively |
… | |
… | |
4527 | With an intelligent-enough linker (gcc+binutils are intelligent enough |
4652 | With an intelligent-enough linker (gcc+binutils are intelligent enough |
4528 | when you use C<-Wl,--gc-sections -ffunction-sections>) functions unused by |
4653 | when you use C<-Wl,--gc-sections -ffunction-sections>) functions unused by |
4529 | your program might be left out as well - a binary starting a timer and an |
4654 | your program might be left out as well - a binary starting a timer and an |
4530 | I/O watcher then might come out at only 5Kb. |
4655 | I/O watcher then might come out at only 5Kb. |
4531 | |
4656 | |
|
|
4657 | =item EV_API_STATIC |
|
|
4658 | |
|
|
4659 | If this symbol is defined (by default it is not), then all identifiers |
|
|
4660 | will have static linkage. This means that libev will not export any |
|
|
4661 | identifiers, and you cannot link against libev anymore. This can be useful |
|
|
4662 | when you embed libev, only want to use libev functions in a single file, |
|
|
4663 | and do not want its identifiers to be visible. |
|
|
4664 | |
|
|
4665 | To use this, define C<EV_API_STATIC> and include F<ev.c> in the file that |
|
|
4666 | wants to use libev. |
|
|
4667 | |
4532 | =item EV_AVOID_STDIO |
4668 | =item EV_AVOID_STDIO |
4533 | |
4669 | |
4534 | If this is set to C<1> at compiletime, then libev will avoid using stdio |
4670 | If this is set to C<1> at compiletime, then libev will avoid using stdio |
4535 | functions (printf, scanf, perror etc.). This will increase the code size |
4671 | functions (printf, scanf, perror etc.). This will increase the code size |
4536 | somewhat, but if your program doesn't otherwise depend on stdio and your |
4672 | somewhat, but if your program doesn't otherwise depend on stdio and your |
… | |
… | |
5061 | good enough for at least into the year 4000 with millisecond accuracy |
5197 | good enough for at least into the year 4000 with millisecond accuracy |
5062 | (the design goal for libev). This requirement is overfulfilled by |
5198 | (the design goal for libev). This requirement is overfulfilled by |
5063 | implementations using IEEE 754, which is basically all existing ones. |
5199 | implementations using IEEE 754, which is basically all existing ones. |
5064 | |
5200 | |
5065 | With IEEE 754 doubles, you get microsecond accuracy until at least the |
5201 | With IEEE 754 doubles, you get microsecond accuracy until at least the |
5066 | year 2255 (and millisecond accuray till the year 287396 - by then, libev |
5202 | year 2255 (and millisecond accuracy till the year 287396 - by then, libev |
5067 | is either obsolete or somebody patched it to use C<long double> or |
5203 | is either obsolete or somebody patched it to use C<long double> or |
5068 | something like that, just kidding). |
5204 | something like that, just kidding). |
5069 | |
5205 | |
5070 | =back |
5206 | =back |
5071 | |
5207 | |