diff options
Diffstat (limited to 'ev.pod')
| -rw-r--r-- | ev.pod | 119 |
1 files changed, 78 insertions, 41 deletions
@@ -1612,11 +1612,12 @@ resource-intensive. At the time of this writing, only the Linux inotify interface is implemented (implementing kqueue support is left as an exercise for the -reader). Inotify will be used to give hints only and should not change the -semantics of C<ev_stat> watchers, which means that libev sometimes needs -to fall back to regular polling again even with inotify, but changes are -usually detected immediately, and if the file exists there will be no -polling. +reader, note, however, that the author sees no way of implementing ev_stat +semantics with kqueue). Inotify will be used to give hints only and should +not change the semantics of C<ev_stat> watchers, which means that libev +sometimes needs to fall back to regular polling again even with inotify, +but changes are usually detected immediately, and if the file exists there +will be no polling. =head3 ABI Issues (Largefile Support) @@ -1651,16 +1652,25 @@ The C<stat ()> syscall only supports full-second resolution portably, and even on systems where the resolution is higher, many filesystems still only support whole seconds. -That means that, if the time is the only thing that changes, you might -miss updates: on the first update, C<ev_stat> detects a change and calls -your callback, which does something. When there is another update within -the same second, C<ev_stat> will be unable to detect it. - -The solution to this is to delay acting on a change for a second (or till -the next second boundary), using a roughly one-second delay C<ev_timer> -(C<ev_timer_set (w, 0., 1.01); ev_timer_again (loop, w)>). The C<.01> -is added to work around small timing inconsistencies of some operating -systems. +That means that, if the time is the only thing that changes, you can +easily miss updates: on the first update, C<ev_stat> detects a change and +calls your callback, which does something. When there is another update +within the same second, C<ev_stat> will be unable to detect it as the stat +data does not change. + +The solution to this is to delay acting on a change for slightly more +than second (or till slightly after the next full second boundary), using +a roughly one-second-delay C<ev_timer> (e.g. C<ev_timer_set (w, 0., 1.02); +ev_timer_again (loop, w)>). + +The C<.02> offset is added to work around small timing inconsistencies +of some operating systems (where the second counter of the current time +might be be delayed. One such system is the Linux kernel, where a call to +C<gettimeofday> might return a timestamp with a full second later than +a subsequent C<time> call - if the equivalent of C<time ()> is used to +update file times then there will be a small window where the kernel uses +the previous second to update file times but libev might already execute +the timer callback). =head3 Watcher-Specific Functions and Data Members @@ -1676,28 +1686,32 @@ be detected and should normally be specified as C<0> to let libev choose a suitable value. The memory pointed to by C<path> must point to the same path for as long as the watcher is active. -The callback will be receive C<EV_STAT> when a change was detected, -relative to the attributes at the time the watcher was started (or the -last change was detected). +The callback will receive C<EV_STAT> when a change was detected, relative +to the attributes at the time the watcher was started (or the last change +was detected). =item ev_stat_stat (loop, ev_stat *) Updates the stat buffer immediately with new values. If you change the -watched path in your callback, you could call this fucntion to avoid -detecting this change (while introducing a race condition). Can also be -useful simply to find out the new values. +watched path in your callback, you could call this function to avoid +detecting this change (while introducing a race condition if you are not +the only one changing the path). Can also be useful simply to find out the +new values. =item ev_statdata attr [read-only] -The most-recently detected attributes of the file. Although the type is of +The most-recently detected attributes of the file. Although the type is C<ev_statdata>, this is usually the (or one of the) C<struct stat> types -suitable for your system. If the C<st_nlink> member is C<0>, then there -was some error while C<stat>ing the file. +suitable for your system, but you can only rely on the POSIX-standardised +members to be present. If the C<st_nlink> member is C<0>, then there was +some error while C<stat>ing the file. =item ev_statdata prev [read-only] The previous attributes of the file. The callback gets invoked whenever -C<prev> != C<attr>. +C<prev> != C<attr>, or, more precisely, one or more of these members +differ: C<st_dev>, C<st_ino>, C<st_mode>, C<st_nlink>, C<st_uid>, +C<st_gid>, C<st_rdev>, C<st_size>, C<st_atime>, C<st_mtime>, C<st_ctime>. =item ev_tstamp interval [read-only] @@ -1761,7 +1775,7 @@ C<ev_timer> callback invocation). ... ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.); ev_stat_start (loop, &passwd); - ev_timer_init (&timer, timer_cb, 0., 1.01); + ev_timer_init (&timer, timer_cb, 0., 1.02); =head2 C<ev_idle> - when you've got nothing better to do... @@ -1859,7 +1873,7 @@ It is recommended to give C<ev_check> watchers highest (C<EV_MAXPRI>) priority, to ensure that they are being run before any other watchers after the poll. Also, C<ev_check> watchers (and C<ev_prepare> watchers, too) should not activate ("feed") events into libev. While libev fully -supports this, they will be called before other C<ev_check> watchers +supports this, they might get executed before other C<ev_check> watchers did their job. As C<ev_check> watchers are often used to embed other (non-libev) event loops those other event loops might be in an unusable state until their C<ev_check> watcher ran (always remind yourself to @@ -1884,9 +1898,9 @@ macros, but using them is utterly, utterly and completely pointless. There are a number of principal ways to embed other event loops or modules into libev. Here are some ideas on how to include libadns into libev (there is a Perl module named C<EV::ADNS> that does this, which you could -use for an actually working example. Another Perl module named C<EV::Glib> -embeds a Glib main context into libev, and finally, C<Glib::EV> embeds EV -into the Glib event loop). +use as a working example. Another Perl module named C<EV::Glib> embeds a +Glib main context into libev, and finally, C<Glib::EV> embeds EV into the +Glib event loop). Method 1: Add IO watchers and a timeout watcher in a prepare handler, and in a check watcher, destroy them and call into libadns. What follows @@ -3204,15 +3218,21 @@ the form of the C<EVBACKEND_SELECT> backend, and only supports socket descriptors. This only applies when using Win32 natively, not when using e.g. cygwin. +Lifting these limitations would basically require the full +re-implementation of the I/O system. If you are into these kinds of +things, then note that glib does exactly that for you in a very portable +way (note also that glib is the slowest event library known to man). + There is no supported compilation method available on windows except embedding it into other applications. -Due to the many, low, and arbitrary limits on the win32 platform and the -abysmal performance of winsockets, using a large number of sockets is not -recommended (and not reasonable). If your program needs to use more than -a hundred or so sockets, then likely it needs to use a totally different -implementation for windows, as libev offers the POSIX model, which cannot -be implemented efficiently on windows (microsoft monopoly games). +Due to the many, low, and arbitrary limits on the win32 platform and +the abysmal performance of winsockets, using a large number of sockets +is not recommended (and not reasonable). If your program needs to use +more than a hundred or so sockets, then likely it needs to use a totally +different implementation for windows, as libev offers the POSIX readyness +notification model, which cannot be implemented efficiently on windows +(microsoft monopoly games). =over 4 @@ -3236,11 +3256,13 @@ complexity in the O(n²) range when using win32. =item Limited number of file descriptors -Windows has numerous arbitrary (and low) limits on things. Early versions -of winsocket's select only supported waiting for a max. of C<64> handles -(probably owning to the fact that all windows kernels can only wait for -C<64> things at the same time internally; microsoft recommends spawning a -chain of threads and wait for 63 handles and the previous thread in each). +Windows has numerous arbitrary (and low) limits on things. + +Early versions of winsocket's select only supported waiting for a maximum +of C<64> handles (probably owning to the fact that all windows kernels +can only wait for C<64> things at the same time internally; microsoft +recommends spawning a chain of threads and wait for 63 handles and the +previous thread in each. Great). Newer versions support more handles, but you need to define C<FD_SETSIZE> to some high number (e.g. C<2048>) before compiling the winsocket select @@ -3289,6 +3311,21 @@ The most portable way to handle signals is to block signals in all threads except the initial one, and run the default loop in the initial thread as well. +=item C<long> must be large enough for common memory allocation sizes + +To improve portability and simplify using libev, libev uses C<long> +internally instead of C<size_t> when allocating its data structures. On +non-POSIX systems (Microsoft...) this might be unexpectedly low, but +is still at least 31 bits everywhere, which is enough for hundreds of +millions of watchers. + +=item C<double> must hold a time value in seconds with enough accuracy + +The type C<double> is used to represent timestamps. It is required to have +at least 51 bits of mantissa, which is good enough for at least into the +year 4000. This requirement is fulfilled by implementations implementing +IEEE 754 (basically all existing ones). + =back If you know of other additional requirements drop me a note. |