2 .TH sel 3 "22 May 1999" mLib
4 sel \- low level interface for waiting for I/O
14 .B "#include <mLib/sel.h>"
16 .BI "void sel_init(sel_state *" s );
18 .BI "void sel_initfile(sel_state *" s ", sel_file *" f ,
19 .BI " int " fd ", unsigned " mode ,
20 .BI " void (*" func ")(int " fd ", unsigned " mode ", void *" p ),
22 .BI "void sel_addfile(sel_file *" f );
23 .BI "void sel_rmfile(sel_file *" f );
25 .BI "void sel_addtimer(sel_state *" s ", sel_timer *" t ,
26 .BI " struct timeval *" tv ,
27 .BI " void (*" func ")(struct timeval *" tv ", void *" p ),
28 .BI "void sel_rmtimer(sel_timer *" t );
30 .BI "int sel_select(sel_state *" s );
35 subsystem provides a structured way of handling I/O in a non-blocking
36 event-driven sort of a way, for single-threaded programs. (Although
37 there's no reason at all why multithreaded programs shouldn't use
39 it's much less useful.)
43 subsystem does no memory allocation, and has no static state. All
44 of its data is stored in structures allocated by the caller. I'll
45 explain how this fits in nicely with typical calling sequences below.
47 Although all the data structures are exposed in the header file, you
50 data structures to be opaque except where described here, and not fiddle
51 around inside them. Some things may become more sophisticated later.
52 .SH "IMPORTANT CONCEPTS"
53 The system is based around two concepts:
60 is interested in some sort of I/O event, which might be something like
61 `my socket has become readable', or `the time is now half past three on
62 the third of June 2013'. It has a handler function attached to it,
63 which is called when the appropriate event occurs. Some events happen
64 once only ever; some events happen over and over again. For example, a
65 socket might become readable many times, but it's only half-past three
66 on the third of June 2013 once.
68 When a selector is initialized, the caller describes the event the
69 selector is interested in, and specifies which function should handle
70 the event. Also, it must specify an arbitrary pointer which is passed
71 to the handler function when the event occurs. This is typically some
72 sort of pointer to instance data of some kind, providing more
73 information about the event (`it's
75 socket that's become readable'), or what to do about it.
77 A multiplexor gathers information about who's interested in what. It
78 maintains lists of selectors. Selectors must be added to a
79 mulitplexor before the events they're interested in are actually watched
80 for. Selectors can be removed again when their events aren't
81 interesting any more. Apart from adding and removing selectors, you can
83 on a multiplexor. This waits for something interesting to happen and
84 then fires off all the selectors which have events waiting for them.
86 You can have lots of multiplexors in your program if you like. You can
87 only ask for events from one of them at a time, though.
89 There are currently two types of selector understood by the low-level
91 system: file selectors and timer selectors. These two types of
92 selectors react to corresponding different types of events. A file
93 event indicates that a file is now ready for reading or writing. A
94 timer event indicates that a particular time has now passed (useful for
95 implementing timeouts). More sophisticated selectors can be constructed
98 interface. For examples, see
102 .SH "PROGRAMMING INTERFACE"
103 A multiplexor is represented using the type
107 header file. Before use, a
109 must be initialized, by passing it to the
111 function. The header file talks about `state blocks' a lot \- that's
112 because it was written before I thought the word `multiplexor' was
115 File selectors are represented by the type
117 The interface provides three operations on file selectors:
118 initialization, addition to multiplexor, and removal from a
119 multiplexor. It's convenient to separate addition and removal from
120 initialization because file selectors often get added and removed many
121 times over during their lifetimes.
123 A file selector is initialized by the
125 function. This requires a large number of arguments:
128 A pointer to the multiplexor with which the file selector will be
129 associated. This is stored in the selector so that the multiplexor
130 argument can be omitted from later calls.
133 Pointer to the file selector object to be initialized.
136 The file descriptor which the selector is meant to watch.
139 A constant describing which condition the selector is interested in.
140 This must be one of the
142 constants described below.
145 The handler function which is called when the appropriate condition
146 occurs on the file. This function's interface is described in more
150 An arbitrary pointer argument passed to
152 when it's called. Beyond this, no meaning is attached to the value of
153 the pointer. If you don't care about it, just leave it as null.
155 The mode argument is one of the following constants:
158 Raise an event when the file is ready to be read from.
161 Raise an event when the file is ready to be written to.
164 Raise an event when the file has an `exceptional condition'.
168 contains the number of possible file modes. This is useful internally
169 for allocating arrays of the right size.
175 perform the addition and removal operations on file selectors. They are
176 passed only the actual selector object, since the selector already knows
177 which multiplexor it's associated with. A newly initialized file
178 selector is not added to its multiplexor: this must be done explicitly.
180 The handler function for a file multiplexor is passed three arguments:
181 the file descriptor for the file, a mode argument which descibes the
182 file's new condition, and the pointer argument set up at initialization
189 structure is exported. It contains the file descriptor in which the
190 selector is interested. You may not modify this value, but it's useful
191 to be able to read it out \- it saves having to keep a copy.
193 Timer selectors are simpler. There are only two operations provided on
194 timer selectors: addition and removal. Initialization is performed as
195 part of the addition operation.
197 A timer selector is represented by an object of time
202 requires lots of arguments:
205 Pointer to the multiplexor to which the selector is to be added.
208 Pointer to the timer selector object being initialized and added.
213 object describing when the selector should raise its event. This is an
215 time, not a relative time as required by the traditional
222 A handler function to be called when the event occurs. The function is
225 time, and the arbitrary pointer passed to
234 when the timer event occurs. Beyond this, the value of the pointer is
239 removes a timer selector. It is passed only the selector object.
241 Note that timer events are a one-shot thing. Once they've happened, the
242 timer selector is removed and the event can't happen again. This is
243 normally what you want. Removing a timer is only useful (or safe!)
244 before the timer event has been sent.
246 Finally, the function
248 is passed a multiplexor object. It waits for something interesting to
249 happen, informs the appropriate selector handlers, and returns. If
250 everything went according to plan,
252 returns zero. Otherwise it returns -1, and the global variable
254 is set appropriately.
256 Although the naming seems to suggest that this is all
257 based around the BSD-ish
259 system call (and indeed it is), the interface is actually a good deal
260 more general than that. An implementation which worked off System V-ish
262 instead would be fairly trivial to make, and would look just the same
269 Mark Wooding, <mdw@nsict.org>
~mdw / mLib / blob