freebsd-skq/contrib/libbegemot/rpoll.man

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'\" $Begemot: libbegemot/rpoll.man,v 1.4 2004/09/21 15:59:00 brandt Exp $
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.TH rpoll 3 "21 Oct 1996" "BEGEMOT" "BEGEMOT Library"
.SH NAME
rpoll - callback functions for file descriptors and timers
.SH SYNOPSIS
.LP
.B "# include <rpoll.h>"
.LP
.BR "typedef void (*poll_f)(int " "fd" ", int " "mask" ", void *" "arg);"
.br
.BR "typedef void (*timer_f)(int " "tid" ", void *" "arg);"
.LP
.BR "int poll_register(int " "fd" ", poll_f "
.RB "func" ", void *" "arg" ", int " "mask" ");"
.LP
.BR "void poll_unregister(int " "handle" ");"
.LP
.BR "int poll_start_timer(u_int " "msecs" ", int " "repeat" ", timer_f " "func,"
.if n .ti +.5i
.BR "void *" "arg);"
.LP
.BR "void poll_stop_timer(int " "handle" ");"
.LP
.BR "void poll_dispatch(int " "wait" ");"
.SH DESCRIPTION
Many programs need to read from several file descriptors at the same time.
Typically in these programs one of
.BR select (3c)
or
.BR poll (2)
is used.
These calls are however clumsy to use and the usage of one of these calls is
probably not portable to other systems - not all systems support both calls.
.LP
The
.BR rpoll (l)
family of functions is designed to overcome these restrictions.
They support the well known and understood technique of event driven
programing and, in addition to
.BR select (3c)
and
.BR poll (2)
also support timers.
.LP
Each event on a file descriptor or each timer event is translated into a call to a user
defined callback function. These functions need to be registered.
A file descriptor is registered with
.BR poll_register .
.I fd
is the file descriptor to watch,
.I mask
is an event mask.
It may be any combination of
.B POLL_IN
to get informed when input on the file descriptor is possible,
.B POLL_OUT
to get informed when output is possible or
.B POLL_EXCEPT
to get informed when an exceptional condition occures.
An example of an exceptional condition is the arrival of urgent data.
(Note, that an end of file condition is signaled via POLL_IN).
.I func
is the user function to be called and
.I arg
is a user supplied argument for this function.
The callback functions is called with the file descriptor, a mask
describing the actual events (from the set supplied in the registration) and
the user argument.
.B poll_register
returns a handle, which may be used later to de-register the file descriptor.
A file descriptor may be registered more than once, if the function, the user arguments
or both differ in the call to
.BR poll_register .
If
.I func
and
.I arg
are the same, then no new registration is done, instead the event mask of the registration
is changed to reflect the new mask.
.LP
A registered file descriptor may be de-registered by calling
.B poll_unregister
with the handle returned by
.BR poll_register .
.LP
A timer is created with
.BR poll_start_timer .
.I msecs
is the number of milliseconds, after which the timer event will be generated.
.I repeat
selects one-short behavior (if 0) or a repeatable timer (if not 0). A one-short timer
will automatically unregistered after expiry.
.I func
is the user function which will be called with a timer id and the user supplied
.IR arg .
.B poll_start_timer
returnes a timer id, which may be used to cancel the timer with
.BR poll_stop_timer .
A one-short timer should be canceled only if it has not yet fired.
.LP
.B poll_dispatch
must be called to actually dispatch events. 
.I wait
is a flag, which should be 0, if only a poll should be done. In this case, the function returns,
after polling the registered file descriptors and timers. If
.I wait
is not 0,
.B poll_dispatch
waits until an event occures. All events are dispatch (i.e. callback functions called) and
.B poll_dispatch returns.
.LP
Typical use is:
.LP
.RS
.nf
.ft 3
while(1)
	poll_dispatch(1);
.ft 1
.fi
.RE
.SH "SEE ALSO"
.BR poll (2), select (3C)
.SH "RETURN VALUES"
.B poll_register
and
.B poll_start_timer
return a handle which may be used to unregister the file descriptor or cancel the timer.
.LP
Both functions and
.B poll_dispatch
call
.BR xrealloc (l)
and can end in
.BR panic (l).
.SH "ERRORS"
System call or memory allocation errors are fatal and are handle by calling
.BR panic (l).
The one exception is a return of EINTR from
.BR select (3c)
or
.BR poll (2)
in
.BR poll_dispatch .
In this case
.B poll_dispatch
simply returns.
.SH "BUGS"
Obscure sequences of
.B poll_start_timer
and
.B poll_stop_timer
in callback functions may probably break the code.
.LP
The semantics of
.B POLL_EXCEPT
are not clear.
.SH AUTHORS
Hartmut Brandt, harti@freebsd.org