New function `sel_force' to force a descriptor to be `selected'.

[mLib] / man / sel.3

.\" -*-nroff-*-
.TH sel 3 "22 May 1999" mLib
.SH NAME
sel \- low level interface for waiting for I/O
.\" @sel_init
.\" @sel_initfile
.\" @sel_addfile
.\" @sel_force
.\" @sel_rmfile
.\" @sel_addtimer
.\" @sel_rmtimer
.\" @sel_addhook
.\" @sel_rmhook
.\" @sel_fdmerge
.\" @sel_select
.SH SYNOPSIS
.nf
.B "#include <mLib/sel.h>"

.BI "void sel_init(sel_state *" s );

.BI "void sel_initfile(sel_state *" s ", sel_file *" f ,
.BI "                  int " fd ", unsigned " mode ,
.BI "                  void (*" func ")(int " fd ", unsigned " mode ", void *" p ),
.BI "                  void *" p );
.BI "void sel_addfile(sel_file *" f );
.BI "void sel_force(sel_file *" f );
.BI "void sel_rmfile(sel_file *" f );

.BI "void sel_addtimer(sel_state *" s ", sel_timer *" t ,
.BI "                  struct timeval *" tv ,
.BI "                  void (*" func ")(struct timeval *" tv ", void *" p ),
.BI "void sel_rmtimer(sel_timer *" t );

.BI "void sel_addhook(sel_state *" s ", sel_hook *" h ,
.BI "                 sel_hookfn " before ", sel_hookfn " after ,
.BI "                 void *" p );
.BI "void sel_rmhook(sel_hook *" h );

.BI "int sel_fdmerge(fd_set *" dest ", fd_set *" fd ", int " maxfd );

.BI "int sel_select(sel_state *" s );
.fi
.SH "OVERVIEW"
The
.B sel
subsystem provides a structured way of handling I/O in a non-blocking
event-driven sort of a way, for single-threaded programs.  (Although
there's no reason at all why multithreaded programs shouldn't use 
.BR sel ,
it's much less useful.)
.PP
The
.B sel
subsystem does no memory allocation, and has no static state.  All
of its data is stored in structures allocated by the caller.  I'll
explain how this fits in nicely with typical calling sequences below.
.PP
Although all the data structures are exposed in the header file, you
should consider
.BR sel 's
data structures to be opaque except where described here, and not fiddle
around inside them.  Some things may become more sophisticated later.
.SH "IMPORTANT CONCEPTS"
The system is based around two concepts:
.I multiplexors
and
.IR selectors .
.PP
A
.I selector
is interested in some sort of I/O event, which might be something like
`my socket has become readable', or `the time is now half past three on
the third of June 2013'.  It has a handler function attached to it,
which is called when the appropriate event occurs.  Some events happen
once only ever; some events happen over and over again.  For example, a
socket might become readable many times, but it's only half-past three
on the third of June 2013 once.
.PP
When a selector is initialized, the caller describes the event the
selector is interested in, and specifies which function should handle
the event.  Also, it must specify an arbitrary pointer which is passed
to the handler function when the event occurs.  This is typically some
sort of pointer to instance data of some kind, providing more
information about the event (`it's
.I this
socket that's become readable'), or what to do about it.
.PP
A multiplexor gathers information about who's interested in what.  It
maintains lists of selectors.  Selectors must be added to a
mulitplexor before the events they're interested in are actually watched
for.  Selectors can be removed again when their events aren't
interesting any more.  Apart from adding and removing selectors, you can
.I select
on a multiplexor.  This waits for something interesting to happen and
then fires off all the selectors which have events waiting for them.
.PP
You can have lots of multiplexors in your program if you like.  You can
only ask for events from one of them at a time, though.
.PP
There are currently two types of selector understood by the low-level
.B sel
system: file selectors and timer selectors.  These two types of
selectors react to corresponding different types of events.  A file
event indicates that a file is now ready for reading or writing.  A
timer event indicates that a particular time has now passed (useful for
implementing timeouts).  More sophisticated selectors can be constructed
using
.BR sel 's
interface.  For examples, see
.BR selbuf (3)
and
.BR conn (3).
.SH "PROGRAMMING INTERFACE"
.SS "Multiplexors"
A multiplexor is represented using the type
.B sel_state
defined in the
.B <mLib/sel.h>
header file.  Before use, a
.B sel_state
must be initialized, by passing it to the
.B sel_init
function.  The header file talks about `state blocks' a lot \- that's
because it was written before I thought the word `multiplexor' was
nicer.
.PP
File selectors are represented by the type
.BR sel_file .
The interface provides three operations on file selectors:
initialization, addition to multiplexor, and removal from a
multiplexor.  It's convenient to separate addition and removal from
initialization because file selectors often get added and removed many
times over during their lifetimes.
.SS "File selectors"
A file selector is initialized by the
.B sel_initfile
function.  This requires a large number of arguments:
.TP
.BI "sel_state *" s
A pointer to the multiplexor with which the file selector will be
associated.  This is stored in the selector so that the multiplexor
argument can be omitted from later calls.
.TP
.BI "sel_file *" f
Pointer to the file selector object to be initialized.
.TP
.BI "int " fd
The file descriptor which the selector is meant to watch.
.TP
.BI "unsigned " mode
A constant describing which condition the selector is interested in.
This must be one of the
.B SEL_
constants described below.
.TP
.BI "void (*" func ")(int " fd ", unsigned " mode ", void *" p );
The handler function which is called when the appropriate condition
occurs on the file.  This function's interface is described in more
detail below.
.TP
.BI "void *" p
An arbitrary pointer argument passed to
.I func
when it's called.  Beyond this, no meaning is attached to the value of
the pointer.  If you don't care about it, just leave it as null.
.PP
The mode argument is one of the following constants:
.TP
.B SEL_READ
Raise an event when the file is ready to be read from.
.TP
.B SEL_WRITE
Raise an event when the file is ready to be written to.
.TP
.B SEL_EXC
Raise an event when the file has an `exceptional condition'.
.PP
The constant
.B SEL_MODES
contains the number of possible file modes.  This is useful internally
for allocating arrays of the right size.
.PP
The functions
.B sel_addfile
and
.B sel_rmfile
perform the addition and removal operations on file selectors.  They are
passed only the actual selector object, since the selector already knows
which multiplexor it's associated with.  A newly initialized file
selector is not added to its multiplexor: this must be done explicitly.
.PP
The handler function for a file multiplexor is passed three arguments:
the file descriptor for the file, a mode argument which describes the
file's new condition, and the pointer argument set up at initialization
time.
.PP
The function
.B sel_force
will sometimes be useful while a
.B sel_select
call (see below) is in progress.  It marks a file selector as being
ready even if it's not really.  This is most useful when dynamically
adding a write selector: it's likely that the write will succeed
immediately, so it's worth trying.  This will only work properly if
the write is non-blocking.
.PP
The member
.B fd
of the
.B sel_file
structure is exported.  It contains the file descriptor in which the
selector is interested.  You may not modify this value, but it's useful
to be able to read it out \- it saves having to keep a copy.
.SS "Timer selectors"
Timer selectors are simpler.  There are only two operations provided on
timer selectors: addition and removal.  Initialization is performed as
part of the addition operation.
.PP
A timer selector is represented by an object of time
.BR sel_timer .
.PP
The function
.B sel_addtimer
requires lots of arguments:
.TP
.BI "sel_state *" s
Pointer to the multiplexor to which the selector is to be added.
.TP
.BI "sel_timer *" t
Pointer to the timer selector object being initialized and added.
.TP
.BI "struct timeval " tv
When the selector should raise its event.  This is an
.I absolute
time, not a relative time as required by the traditional
.BR select (2)
and
.BR poll (2)
system calls.
.TP
.BI "void (*" func ")(struct timeval *" tv ", void *" p )
A handler function to be called when the event occurs.  The function is
passed the
.I current
time, and the arbitrary pointer passed to
.B sel_addtimer
as the
.I p
argument.
.TP
.BI "void *" p
A pointer passed to
.I func
when the timer event occurs.  Beyond this, the value of the pointer is
not inspected.
.PP
The function
.B sel_rmtimer
removes a timer selector.  It is passed only the selector object.
.PP
Note that timer events are a one-shot thing.  Once they've happened, the
timer selector is removed and the event can't happen again.  This is
normally what you want.  Removing a timer is only useful (or safe!)
before the timer event has been sent.
.SS "Performing I/O"
Finally, the function
.B sel_select
is passed a multiplexor object.  It waits for something interesting to
happen, informs the appropriate selector handlers, and returns.  If
everything went according to plan,
.B sel_select
returns zero.  Otherwise it returns -1, and the global variable
.B errno
is set appropriately.
.SS "Hook functions"
In order to interface other I/O multiplexing systems to this one, it's
possible to register
.I hook
functions which are called before and after each
.BR select (2)
system call.
.PP
The function
.B sel_addhook
registers a pair of hook functions.  It is passed the pointer to the
multiplexor which is being hooked, the address of a
.B sel_hook
structure which will be used to record the hook information, the two
hook functions (either of which may be a null pointer, signifying no
action to be taken), and a pointer argument to be passed to the hook
functions.
.PP
The function
.B sel_rmhook
removes a pair of hooks given the address of the
.B sel_hook
structure which recorded their registration.
.PP
A
.I "hook function"
is passed three arguments:
.TP
.BI "sel_state *" s
A pointer to the multiplexor block.  This probably isn't very useful,
actually.
.TP
.BI "sel_args *" a
A pointer to a block containing proposed arguments for, or results from,
.BR select (2).
The format of this block is described below.
.TP
.BI "void *" p
A pointer argument set up in the call to
.B sel_addhook
to provide the hook function with some context.
.PP
The argument block contains the following members:
.TP
.B "int maxfd"
One greater than the highest-numbered file descriptor to be examined.
This may need to be modified if the file descriptor sets are altered.
.TP
.B "fd_set fd[SEL_MODES]"
A file descriptor set for each of
.BR SEL_READ ,
.B SEL_WRITE
and
.BR SEL_EXC .
Before the
.B select
call, these may be modified to register an interest in other file
descriptors.  Afterwards, they may be examined to decide which file
descriptors are active.
.TP
.B "struct timeval tv, *tvp"
Before the
.B select
call, these specify the time after which to return even if no files are
active.  If
.B tvp
is null, there is no timeout, and
.B select
should wait forever if necessary.  Otherwise
.B tvp
should contain the address of
.BR tv ,
and
.B tv
should contain the timeout.  After the
.B select
call, the contents of
.B tv
are undefined.
.TP
.B "struct timeval now"
Before the
.B select
call, contains the current time.  After the call, this will have been
updated to reflect the new current time only if there was a timeout
set before the call.
.PP
Hook functions may find the call
.B sel_fdmerge
useful.  Given two file descriptor sets
.I dest
and
.IR fd ,
and a possibly overestimated highest file descriptor in
.IR fd ,
the function sets in
.I dest
all of the descriptors set in
.I fd
and returns an accurate file descriptor count as its result.
.SH "OTHER NOTES"
Although the naming seems to suggest that this is all
based around the BSD-ish
.BR select (2)
system call (and indeed it is), the interface is actually a good deal
more general than that.  An implementation which worked off System V-ish
.BR poll (2)
instead would be possible to make, and would look just the same from the
outside.  Some work would be needed to make the hook functions work,
though.
.SH "SEE ALSO"
.BR select (2),
.BR poll (2),
.BR conn (3),
.BR selbuf (3),
.BR mLib (3).
.SH AUTHOR
Mark Wooding, <mdw@nsict.org>