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From: wayne@ames.arpa
Newsgroups: comp.lang.c
Subject: Re: Re: Types (about linkers optimizing near/far branches)
Message-ID: <9183@brl-adm.ARPA>
Date: Sat, 5-Sep-87 00:34:28 EDT
Article-I.D.: brl-adm.9183
Posted: Sat Sep  5 00:34:28 1987
Date-Received: Wed, 9-Sep-87 02:22:12 EDT
Sender: news@brl-adm.ARPA
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Carol:

Thanx much for the select poop; VERY thorough for something you "just
slapped together"!  However (isn't there always one?), I've still got
one question (and a comment or two).

First, the question: in your write-up, exactly what do you do to
"terminate select" in the selwakeup routine?  If I understand things,
what you want to do is wake up the one sleeping process given by
db->db_sel (now p); how do you wakeup a single process without waking
everybody else that's sleeping on selwait?  (By the way, my
understanding is that if only one process is sleeping, then you wake
only him; otherwise, you wakeup everybody in the whole system who is
waiting for selects on any device, most of whom will go right back to
sleep.  Trade-off of infrequent inefficiency for simplicity.
Correct?)

Now a couple of comments on the write-up, mostly for feedback and to
see if I really do understand.  First, xxselect is called with "ds" as
param but actually wants "db"; the call should be something like
xxselqueue(ds.db), right?  (Assuming the device buffer is an element
of the device structure.)  Also, at the top of - 3 -, the sentence
"The key is that more than one process can try to select for the same
event on the same process" should end with "device" rather than
"process," no?  Finally, in the code for selwakeup, why bother testing
for SSEL before resetting it?

And also a comment on the stuff being added to the driver-writing
manual.  It contains the sentence "The process number of the current
process is in u.u_procp" which really isn't true (at least if you
think of "process number" as synonymous with "process id"); shouldn't
it be "A pointer to the current process's proc table entry ..."?

Finally, a philosophical item.  How does the xxselect routine know
whether the user is in fact going to sleep?  I mean, to quote your
note, "On a poll, the driver xxselect routine is called just once from
the kernel select routine."  It seems to me the procedures you
outlined will set db->db_sel in the poll case as well, although
admittedly the test in selwakeup for (caddr_t)&selwait will prevent
anything weird happening.  Of course, if there's no way xxselect knows
whether the user is polling or not, then ...

Anyway, thanx again.  Help with "terminate select" please?

wayne