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From: carl@CITHEX.CALTECH.EDU (Carl J Lydick)
Newsgroups: mod.computers.vax
Subject: Re: VAX C problem (?) - reading single characters with VAX C
Message-ID: <860721065936.00c@CitHex.Caltech.Edu>
Date: Mon, 21-Jul-86 10:00:15 EDT
Article-I.D.: CitHex.860721065936.00c
Posted: Mon Jul 21 10:00:15 1986
Date-Received: Tue, 22-Jul-86 07:22:51 EDT
References: <8607200641.AA22528@csvax.caltech.edu>
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Approved: info-vax@sri-kl.arpa

/*****************************************************************************\
 *  >   ...a program that does the same thing, though often much more        *
 *  >   efficiently, since it buffers up typed-ahead stuff....               *
 *                                                                           *
 *  Not quite the same thing, Jerry; your "more efficient" implementation    *
 *  has the following rather odd properties:                                 *
 *      1)  An image exit may flush any, all, or none of the type-ahead.     *
 *      2)  The amount of type-ahead flushed when the image exits            *
 *          depends, in principle, on every keystroke typed since the        *
 *          first call to your _kbin().  In fact, the dependence is upon     *
 *          the number of characters typed since the last time the           *
 *          terminal's type-ahead queue was empty.                           *
 *  The reason for this, of course, is that you and the terminal driver are  *
 *  both buffering typeahead, but you neither return whatever you've got     *
 *  left at image exit to the terminal data stream (which is reasonable,     *
 *  since the VMS terminal drivers don't have any provision for this) and    *
 *  you don't do anything to force a flush of the terminal's type-ahead      *
 *  buffer at image exit (which is not reasonable, since you can declare     *
 *  your own exit handler to take care of this problem.  What you want is    *
 *  somthing like:                                                           *
\*****************************************************************************/
/* clean up after single-character I/O software */
void flush_stream(status, chan) 
long *status, chan;
{	SYS$QIOW(0, chan, IO$_READVBLK | IO$M_TIMED | IO$M_NOFILTR | 
		IO$M_NOECHO | IO$M_PURGE, 0, 0, 0,0, 0, 0, 0, 0, 0);
	SYS$DASSGN(chan);
}

/* exit handler descriptor block */
struct EXH$DESBLK { 
	long exh$l_flink;
	void (*exh$a_handler)();
	long exh$b_nargs;
	long *exh$a_status;
	long exh$l_chan;
	long exh$l_stat;
} flush = { 0, flush_stream, 2, &(flush.exh$l_stat), 0, 0 };
/*****************************************************************************\
 *   Then add something like:                                                *
 *                                                                           *
 *  	flush.exh$l_chan = chan;                                             *
 *  	SYS$DCLEXH(&flush);                                                  *
 *                                                                           *
 *   to the code that opens the channel on the first call.  When your image  *
 *   exits, flush will be called, and it will flush all typeahead on the     *
 *   terminal channel and deassign the channel.                              *
\*****************************************************************************/