2 .TH sel 3 "22 May 1999" "Straylight/Edgeware" "mLib utilities library"
4 sel \- low level interface for waiting for I/O
18 .B "#include <mLib/sel.h>"
20 .BI "void sel_init(sel_state *" s );
22 .BI "void sel_initfile(sel_state *" s ", sel_file *" f ,
23 .BI " int " fd ", unsigned " mode ,
24 .BI " void (*" func ")(int " fd ", unsigned " mode ", void *" p ),
26 .BI "void sel_addfile(sel_file *" f );
27 .BI "void sel_force(sel_file *" f );
28 .BI "void sel_rmfile(sel_file *" f );
30 .BI "void sel_addtimer(sel_state *" s ", sel_timer *" t ,
31 .BI " struct timeval *" tv ,
32 .BI " void (*" func ")(struct timeval *" tv ", void *" p ),
34 .BI "void sel_rmtimer(sel_timer *" t );
36 .BI "void sel_addhook(sel_state *" s ", sel_hook *" h ,
37 .BI " sel_hookfn " before ", sel_hookfn " after ,
39 .BI "void sel_rmhook(sel_hook *" h );
41 .BI "int sel_fdmerge(fd_set *" dest ", fd_set *" fd ", int " maxfd );
43 .BI "int sel_select(sel_state *" s );
48 subsystem provides a structured way of handling I/O in a non-blocking
49 event-driven sort of a way, for single-threaded programs. (Although
50 there's no reason at all why multithreaded programs shouldn't use
52 it's much less useful.)
56 subsystem does no memory allocation, and has no static state. All
57 of its data is stored in structures allocated by the caller. I'll
58 explain how this fits in nicely with typical calling sequences below.
60 Although all the data structures are exposed in the header file, you
63 data structures to be opaque except where described here, and not fiddle
64 around inside them. Some things may become more sophisticated later.
65 .SH "IMPORTANT CONCEPTS"
66 The system is based around two concepts:
73 is interested in some sort of I/O event, which might be something like
74 `my socket has become readable', or `the time is now half past three on
75 the third of June 2013'. It has a handler function attached to it,
76 which is called when the appropriate event occurs. Some events happen
77 once only ever; some events happen over and over again. For example, a
78 socket might become readable many times, but it's only half-past three
79 on the third of June 2013 once.
81 When a selector is initialized, the caller describes the event the
82 selector is interested in, and specifies which function should handle
83 the event. Also, it must specify an arbitrary pointer which is passed
84 to the handler function when the event occurs. This is typically some
85 sort of pointer to instance data of some kind, providing more
86 information about the event (`it's
88 socket that's become readable'), or what to do about it.
90 A multiplexor gathers information about who's interested in what. It
91 maintains lists of selectors. Selectors must be added to a
92 mulitplexor before the events they're interested in are actually watched
93 for. Selectors can be removed again when their events aren't
94 interesting any more. Apart from adding and removing selectors, you can
96 on a multiplexor. This waits for something interesting to happen and
97 then fires off all the selectors which have events waiting for them.
99 You can have lots of multiplexors in your program if you like. You can
100 only ask for events from one of them at a time, though.
102 There are currently two types of selector understood by the low-level
104 system: file selectors and timer selectors. These two types of
105 selectors react to corresponding different types of events. A file
106 event indicates that a file is now ready for reading or writing. A
107 timer event indicates that a particular time has now passed (useful for
108 implementing timeouts). More sophisticated selectors can be constructed
111 interface. For examples, see
115 .SH "PROGRAMMING INTERFACE"
117 A multiplexor is represented using the type
121 header file. Before use, a
123 must be initialized, by passing it to the
125 function. The header file talks about `state blocks' a lot \- that's
126 because it was written before I thought the word `multiplexor' was
129 File selectors are represented by the type
131 The interface provides three operations on file selectors:
132 initialization, addition to multiplexor, and removal from a
133 multiplexor. It's convenient to separate addition and removal from
134 initialization because file selectors often get added and removed many
135 times over during their lifetimes.
137 A file selector is initialized by the
139 function. This requires a large number of arguments:
142 A pointer to the multiplexor with which the file selector will be
143 associated. This is stored in the selector so that the multiplexor
144 argument can be omitted from later calls.
147 Pointer to the file selector object to be initialized.
150 The file descriptor which the selector is meant to watch.
153 A constant describing which condition the selector is interested in.
154 This must be one of the
156 constants described below.
158 .BI "void (*" func ")(int " fd ", unsigned " mode ", void *" p );
159 The handler function which is called when the appropriate condition
160 occurs on the file. This function's interface is described in more
164 An arbitrary pointer argument passed to
166 when it's called. Beyond this, no meaning is attached to the value of
167 the pointer. If you don't care about it, just leave it as null.
169 The mode argument is one of the following constants:
172 Raise an event when the file is ready to be read from.
175 Raise an event when the file is ready to be written to.
178 Raise an event when the file has an `exceptional condition'.
182 contains the number of possible file modes. This is useful internally
183 for allocating arrays of the right size.
189 perform the addition and removal operations on file selectors. They are
190 passed only the actual selector object, since the selector already knows
191 which multiplexor it's associated with. A newly initialized file
192 selector is not added to its multiplexor: this must be done explicitly.
194 The handler function for a file multiplexor is passed three arguments:
195 the file descriptor for the file, a mode argument which describes the
196 file's new condition, and the pointer argument set up at initialization
201 will sometimes be useful while a
203 call (see below) is in progress. It marks a file selector as being
204 ready even if it's not really. This is most useful when dynamically
205 adding a write selector: it's likely that the write will succeed
206 immediately, so it's worth trying. This will only work properly if
207 the write is non-blocking.
213 structure is exported. It contains the file descriptor in which the
214 selector is interested. You may not modify this value, but it's useful
215 to be able to read it out \- it saves having to keep a copy.
216 .SS "Timer selectors"
217 Timer selectors are simpler. There are only two operations provided on
218 timer selectors: addition and removal. Initialization is performed as
219 part of the addition operation.
221 A timer selector is represented by an object of time
226 requires lots of arguments:
229 Pointer to the multiplexor to which the selector is to be added.
232 Pointer to the timer selector object being initialized and added.
234 .BI "struct timeval " tv
235 When the selector should raise its event. This is an
237 time, not a relative time as required by the traditional
243 .BI "void (*" func ")(struct timeval *" tv ", void *" p )
244 A handler function to be called when the event occurs. The function is
247 time, and the arbitrary pointer passed to
256 when the timer event occurs. Beyond this, the value of the pointer is
261 removes a timer selector. It is passed only the selector object.
263 Note that timer events are a one-shot thing. Once they've happened, the
264 timer selector is removed and the event can't happen again. This is
265 normally what you want. Removing a timer is only useful (or safe!)
266 before the timer event has been sent.
268 Finally, the function
270 is passed a multiplexor object. It waits for something interesting to
271 happen, informs the appropriate selector handlers, and returns. If
272 everything went according to plan,
274 returns zero. Otherwise it returns \-1, and the global variable
276 is set appropriately.
278 In order to interface other I/O multiplexing systems to this one, it's
281 functions which are called before and after each
287 registers a pair of hook functions. It is passed the pointer to the
288 multiplexor which is being hooked, the address of a
290 structure which will be used to record the hook information, the two
291 hook functions (either of which may be a null pointer, signifying no
292 action to be taken), and a pointer argument to be passed to the hook
297 removes a pair of hooks given the address of the
299 structure which recorded their registration.
303 is passed three arguments:
306 A pointer to the multiplexor block. This probably isn't very useful,
310 A pointer to a block containing proposed arguments for, or results from,
312 The format of this block is described below.
315 A pointer argument set up in the call to
317 to provide the hook function with some context.
319 The argument block contains the following members:
322 One greater than the highest-numbered file descriptor to be examined.
323 This may need to be modified if the file descriptor sets are altered.
325 .B "fd_set fd[SEL_MODES]"
326 A file descriptor set for each of
333 call, these may be modified to register an interest in other file
334 descriptors. Afterwards, they may be examined to decide which file
335 descriptors are active.
337 .B "struct timeval tv, *tvp"
340 call, these specify the time after which to return even if no files are
343 is null, there is no timeout, and
345 should wait forever if necessary. Otherwise
347 should contain the address of
351 should contain the timeout. After the
353 call, the contents of
357 .B "struct timeval now"
360 call, contains the current time. After the call, this will have been
361 updated to reflect the new current time only if there was a timeout
364 Hook functions may find the call
366 useful. Given two file descriptor sets
370 and a possibly overestimated highest file descriptor in
374 all of the descriptors set in
376 and returns an accurate file descriptor count as its result.
378 Although the naming seems to suggest that this is all
379 based around the BSD-ish
381 system call (and indeed it is), the interface is actually a good deal
382 more general than that. An implementation which worked off System V-ish
384 instead would be possible to make, and would look just the same from the
385 outside. Some work would be needed to make the hook functions work,
394 Mark Wooding, <mdw@distorted.org.uk>
~mdw / mLib / blob