NAME

     seldrain, selrecord, selwakeup — record and wakeup select requests

SYNOPSIS

     #include <sys/param.h>
     #include <sys/selinfo.h>

     void
     seldrain(struct selinfo *sip);

     void
     selrecord(struct thread *td, struct selinfo *sip);

     void
     selwakeup(struct selinfo *sip);

DESCRIPTION

     seldrain(), selrecord() and selwakeup() are the three central functions used by select(2), 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.

     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 sip which will be later dealt with
     by selwakeup().

     selrecord() acquires and releases sellock.

     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, selwakeup() will increment 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 selwait, selwakeup() will clear
     the TDF_SELECT flag which should be noted by select(2) and poll(2) when they wake up.

     seldrain() will flush the waiters queue on a specified object before its destruction.  The object handling
     code must ensure that *sip cannot be used once seldrain() has been called.

     The contents of *sip must be zeroed, such as by softc initialization, before any call to selrecord() or
     selwakeup(), otherwise a panic may occur.  selwakeup() acquires and releases sellock and may acquire and
     release sched_lock.  seldrain() could usually be just a wrapper for selwakeup(), but consumers should not
     generally rely on this feature.

SEE ALSO

     poll(2), select(2)

AUTHORS

     This manual page was written by Chad David <davidc@FreeBSD.org> and Alfred Perlstein <alfred@FreeBSD.org>.