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Revision: 1.2
Committed: Fri Jul 11 10:54:50 2008 UTC (16 years, 4 months ago) by root
Branch: MAIN
CVS Tags: rel-3_16, rel-3_1, rel-3_15, rel-3_07, rel-3_06
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File Contents

# Content
1 =head1 NAME
2
3 libeio - truly asynchronous POSIX I/O
4
5 =head1 SYNOPSIS
6
7 #include <eio.h>
8
9 =head1 DESCRIPTION
10
11 The newest version of this document is also available as an html-formatted
12 web page you might find easier to navigate when reading it for the first
13 time: L<http://pod.tst.eu/http://cvs.schmorp.de/libeio/eio.pod>.
14
15 Note that this library is a by-product of the C<IO::AIO> perl
16 module, and many of the subtler points regarding requets lifetime
17 and so on are only documented in its documentation at the
18 moment: L<http://pod.tst.eu/http://cvs.schmorp.de/IO-AIO/AIO.pm>.
19
20 =head2 FEATURES
21
22 This library provides fully asynchronous versions of most POSIX functions
23 dealign with I/O. Unlike most asynchronous libraries, this not only
24 includes C<read> and C<write>, but also C<open>, C<stat>, C<unlink> and
25 similar functions, as well as less rarely ones such as C<mknod>, C<futime>
26 or C<readlink>.
27
28 It also offers wrappers around C<sendfile> (Solaris, Linux, HP-UX and
29 FreeBSD, with emulation on other platforms) and C<readahead> (Linux, with
30 emulation elsewhere>).
31
32 The goal is to enbale you to write fully non-blocking programs. For
33 example, in a game server, you would not want to freeze for a few seconds
34 just because the server is running a backup and you happen to call
35 C<readdir>.
36
37 =head2 TIME REPRESENTATION
38
39 Libeio represents time as a single floating point number, representing the
40 (fractional) number of seconds since the (POSIX) epoch (somewhere near
41 the beginning of 1970, details are complicated, don't ask). This type is
42 called C<eio_tstamp>, but it is guarenteed to be of type C<double> (or
43 better), so you can freely use C<double> yourself.
44
45 Unlike the name component C<stamp> might indicate, it is also used for
46 time differences throughout libeio.
47
48 =head2 FORK SUPPORT
49
50 Calling C<fork ()> is fully supported by this module. It is implemented in these steps:
51
52 1. wait till all requests in "execute" state have been handled
53 (basically requests that are already handed over to the kernel).
54 2. fork
55 3. in the parent, continue business as usual, done
56 4. in the child, destroy all ready and pending requests and free the
57 memory used by the worker threads. This gives you a fully empty
58 libeio queue.
59
60 =head1 INITIALISATION/INTEGRATION
61
62 Before you can call any eio functions you first have to initialise the
63 library. The library integrates into any event loop, but can also be used
64 without one, including in polling mode.
65
66 You have to provide the necessary glue yourself, however.
67
68 =over 4
69
70 =item int eio_init (void (*want_poll)(void), void (*done_poll)(void))
71
72 This function initialises the library. On success it returns C<0>, on
73 failure it returns C<-1> and sets C<errno> appropriately.
74
75 It accepts two function pointers specifying callbacks as argument, both of
76 which can be C<0>, in which case the callback isn't called.
77
78 =item want_poll callback
79
80 The C<want_poll> callback is invoked whenever libeio wants attention (i.e.
81 it wants to be polled by calling C<eio_poll>). It is "edge-triggered",
82 that is, it will only be called once when eio wants attention, until all
83 pending requests have been handled.
84
85 This callback is called while locks are being held, so I<you must
86 not call any libeio functions inside this callback>. That includes
87 C<eio_poll>. What you should do is notify some other thread, or wake up
88 your event loop, and then call C<eio_poll>.
89
90 =item done_poll callback
91
92 This callback is invoked when libeio detects that all pending requests
93 have been handled. It is "edge-triggered", that is, it will only be
94 called once after C<want_poll>. To put it differently, C<want_poll> and
95 C<done_poll> are invoked in pairs: after C<want_poll> you have to call
96 C<eio_poll ()> until either C<eio_poll> indicates that everything has been
97 handled or C<done_poll> has been called, which signals the same.
98
99 Note that C<eio_poll> might return after C<done_poll> and C<want_poll>
100 have been called again, so watch out for races in your code.
101
102 As with C<want_poll>, this callback is called while lcoks are being held,
103 so you I<must not call any libeio functions form within this callback>.
104
105 =item int eio_poll ()
106
107 This function has to be called whenever there are pending requests that
108 need finishing. You usually call this after C<want_poll> has indicated
109 that you should do so, but you can also call this function regularly to
110 poll for new results.
111
112 If any request invocation returns a non-zero value, then C<eio_poll ()>
113 immediately returns with that value as return value.
114
115 Otherwise, if all requests could be handled, it returns C<0>. If for some
116 reason not all requests have been handled, i.e. some are still pending, it
117 returns C<-1>.
118
119 =back
120
121 For libev, you would typically use an C<ev_async> watcher: the
122 C<want_poll> callback would invoke C<ev_async_send> to wake up the event
123 loop. Inside the callback set for the watcher, one would call C<eio_poll
124 ()> (followed by C<ev_async_send> again if C<eio_poll> indicates that not
125 all requests have been handled yet). The race is taken care of because
126 libev resets/rearms the async watcher before calling your callback,
127 and therefore, before calling C<eio_poll>. This might result in (some)
128 spurious wake-ups, but is generally harmless.
129
130 For most other event loops, you would typically use a pipe - the event
131 loop should be told to wait for read readyness on the read end. In
132 C<want_poll> you would write a single byte, in C<done_poll> you would try
133 to read that byte, and in the callback for the read end, you would call
134 C<eio_poll>. The race is avoided here because the event loop should invoke
135 your callback again and again until the byte has been read (as the pipe
136 read callback does not read it, only C<done_poll>).
137
138 =head2 CONFIGURATION
139
140 The functions in this section can sometimes be useful, but the default
141 configuration will do in most case, so you should skip this section on
142 first reading.
143
144 =over 4
145
146 =item eio_set_max_poll_time (eio_tstamp nseconds)
147
148 This causes C<eio_poll ()> to return after it has detected that it was
149 running for C<nsecond> seconds or longer (this number can be fractional).
150
151 This can be used to limit the amount of time spent handling eio requests,
152 for example, in interactive programs, you might want to limit this time to
153 C<0.01> seconds or so.
154
155 Note that:
156
157 a) libeio doesn't know how long your request callbacks take, so the time
158 spent in C<eio_poll> is up to one callback invocation longer then this
159 interval.
160
161 b) this is implemented by calling C<gettimeofday> after each request,
162 which can be costly.
163
164 c) at least one request will be handled.
165
166 =item eio_set_max_poll_reqs (unsigned int nreqs)
167
168 When C<nreqs> is non-zero, then C<eio_poll> will not handle more than
169 C<nreqs> requests per invocation. This is a less costly way to limit the
170 amount of work done by C<eio_poll> then setting a time limit.
171
172 If you know your callbacks are generally fast, you could use this to
173 encourage interactiveness in your programs by setting it to C<10>, C<100>
174 or even C<1000>.
175
176 =item eio_set_min_parallel (unsigned int nthreads)
177
178 Make sure libeio can handle at least this many requests in parallel. It
179 might be able handle more.
180
181 =item eio_set_max_parallel (unsigned int nthreads)
182
183 Set the maximum number of threads that libeio will spawn.
184
185 =item eio_set_max_idle (unsigned int nthreads)
186
187 Libeio uses threads internally to handle most requests, and will start and stop threads on demand.
188
189 This call can be used to limit the number of idle threads (threads without
190 work to do): libeio will keep some threads idle in preperation for more
191 requests, but never longer than C<nthreads> threads.
192
193 In addition to this, libeio will also stop threads when they are idle for
194 a few seconds, regardless of this setting.
195
196 =item unsigned int eio_nthreads ()
197
198 Return the number of worker threads currently running.
199
200 =item unsigned int eio_nreqs ()
201
202 Return the number of requests currently handled by libeio. This is the
203 total number of requests that have been submitted to libeio, but not yet
204 destroyed.
205
206 =item unsigned int eio_nready ()
207
208 Returns the number of ready requests, i.e. requests that have been
209 submitted but have not yet entered the execution phase.
210
211 =item unsigned int eio_npending ()
212
213 Returns the number of pending requests, i.e. requests that have been
214 executed and have results, but have not been finished yet by a call to
215 C<eio_poll>).
216
217 =back
218
219
220 =head1 ANATOMY OF AN EIO REQUEST
221
222 #TODO
223
224
225 =head1 HIGH LEVEL REQUEST API
226
227 #TODO
228
229 =back
230
231
232 =head1 LOW LEVEL REQUEST API
233
234 #TODO
235
236 =head1 EMBEDDING
237
238 Libeio can be embedded directly into programs. This functionality is not
239 documented and not (yet) officially supported.
240
241 If you need to know how, check the C<IO::AIO> perl module, which does
242 exactly that.
243
244
245 =head1 PORTABILITY REQUIREMENTS
246
247 In addition to a working ISO-C implementation, libeio relies on a few
248 additional extensions:
249
250 =over 4
251
252 =item POSIX threads
253
254 To be portable, this module uses threads, specifically, the POSIX threads
255 library must be available (and working, which partially excludes many xBSD
256 systems, where C<fork ()> is buggy).
257
258 =item POSIX-compatible filesystem API
259
260 This is actually a harder portability requirement: The libeio API is quite
261 demanding regarding POSIX API calls (symlinks, user/group management
262 etc.).
263
264 =item C<double> must hold a time value in seconds with enough accuracy
265
266 The type C<double> is used to represent timestamps. It is required to
267 have at least 51 bits of mantissa (and 9 bits of exponent), which is good
268 enough for at least into the year 4000. This requirement is fulfilled by
269 implementations implementing IEEE 754 (basically all existing ones).
270
271 =back
272
273 If you know of other additional requirements drop me a note.
274
275
276 =head1 AUTHOR
277
278 Marc Lehmann <libeio@schmorp.de>.
279