.\" -*-nroff-*-
-.TH sel 3 "22 May 1999" mLib
+.TH sel 3 "22 May 1999" "Straylight/Edgeware" "mLib utilities library"
.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
.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 *" 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"
and
.BR conn (3).
.SH "PROGRAMMING INTERFACE"
+.SS "Multiplexors"
A multiplexor is represented using the type
.B sel_state
defined in the
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.
-.PP
+.SS "File selectors"
A file selector is initialized by the
.B sel_initfile
function. This requires a large number of arguments:
.TP
-.I s
+.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
-.I f
+.BI "sel_file *" f
Pointer to the file selector object to be initialized.
.TP
-.I fd
+.BI "int " fd
The file descriptor which the selector is meant to watch.
.TP
-.I mode
+.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
-.I func
+.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
-.I p
+.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
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
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.
-.PP
+.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.
.B sel_addtimer
requires lots of arguments:
.TP
-.I s
+.BI "sel_state *" s
Pointer to the multiplexor to which the selector is to be added.
.TP
-.I t
+.BI "sel_timer *" t
Pointer to the timer selector object being initialized and added.
.TP
-.I tv
-A
-.B "struct timeval"
-object describing when the selector should raise its event. This is an
+.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)
.BR poll (2)
system calls.
.TP
-.I func
+.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
.I p
argument.
.TP
-.I p
+.BI "void *" p
A pointer passed to
.I func
when the timer event occurs. Beyond this, the value of the pointer is
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.
-.PP
+.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
+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
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 fairly trivial to make, and would look just the same
-from the outside.
+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>
+Mark Wooding, <mdw@distorted.org.uk>
~mdw / mLib / blobdiff