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.\" $FreeBSD: release/9.1.0/share/man/man9/selrecord.9 225177 2011-08-25 15:51:54Z attilio $
.\"
.Dd August 25, 2011
.Dt SELRECORD 9
.Os
.Sh NAME
.Nm seldrain ,
.Nm selrecord ,
.Nm selwakeup
.Nd "record and wakeup select requests"
.Sh SYNOPSIS
.In sys/param.h
.In sys/selinfo.h
.Ft void
.Fn seldrain "struct selinfo *sip"
.Ft void
.Fn selrecord "struct thread *td" "struct selinfo *sip"
.Ft void
.Fn selwakeup "struct selinfo *sip"
.Sh DESCRIPTION
.Fn seldrain ,
.Fn selrecord
and
.Fn selwakeup
are the three central functions used by
.Xr select 2 ,
.Xr poll 2
and the objects that are being selected on.
They handle the task of recording which threads are waiting on which objects
and the waking of the proper threads when an event of interest occurs on an
object.
.Pp
.Fn selrecord
records that the calling thread is interested in events related to a given
object.
If another thread is already waiting on the object a collision will be flagged
in
.Fa sip
which will be later dealt with by
.Fn selwakeup .
.Pp
.Fn selrecord
acquires and releases
.Va sellock .
.Pp
.Fn selwakeup
is called by the underlying object handling code in order to notify any waiting
threads that an event of interest has occurred.
If a collision has occurred,
.Fn selwakeup
will increment
.Va nselcoll ,
and broadcast on the global cv in order to wake all waiting threads so that
they can handle it.
If the thread waiting on the object is not currently sleeping or the wait
channel is not
.Va selwait ,
.Fn selwakeup
will clear the
.Dv TDF_SELECT
flag which should be noted by
.Xr select 2
and
.Xr poll 2
when they wake up.
.Pp
.Fn seldrain
will flush the waiters queue on a specified object before its
destruction.
The object handling code must ensure that
.Fa *sip
cannot be used once
.Fn seldrain
has been called.
.Pp
The contents of
.Fa *sip
must be zeroed, such as by softc initialization, before any call to
.Fn selrecord
or
.Fn selwakeup ,
otherwise a panic may occur.
.Fn selwakeup
acquires and releases
.Va sellock
and may acquire and release
.Va sched_lock .
.Fn seldrain
could usually be just a wrapper for
.Fn selwakeup ,
but consumers should not generally rely on this feature.
.Sh SEE ALSO
.Xr poll 2 ,
.Xr select 2
.Sh AUTHORS
.An -nosplit
This manual page was written by
.An Chad David Aq davidc@FreeBSD.org
and
.An Alfred Perlstein Aq alfred@FreeBSD.org .