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     seldrain, selrecord, selwakeup — record and wakeup select requests


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

     seldrain(struct selinfo *sip);

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

     selwakeup(struct selinfo *sip);


     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.


     poll(2), select(2)


     This manual page was written by Chad David <> and Alfred Perlstein