Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!decwrl!adobe!hawley
From: hawley@adobe.COM (Steve Hawley)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Is there a defacto standard for 'drop ins'?
Message-ID: <13094@adobe.UUCP>
Date: 22 Mar 91 18:14:17 GMT
References: <1CE00001.aolmdnz@tbomb.ice.com>
Reply-To: hawley@adobe.UUCP (Steve Hawley)
Organization: Adobe Systems Incorporated, Mountain View
Lines: 60

In article <1CE00001.aolmdnz@tbomb.ice.com> time@ice.com writes:
>
>In article <1991Mar21.223638.4346@ssd.kodak.com>, ph@ssd.kodak.com (Pete Hoch) writes:
>> So I was wondering, do they all do it the same way?  Should
>> Apple step in and provide a filter manager?  Is this hard to
>> do today without Apples help?  Also, is this a good idea?
>
>Most of these mechanisms use a very similar technique.
>
>However, all differ in content and calling conventions, and it is
>very doubtful that a universal solution would be easily decided upon.
>However, I am certain some could be standardized. For instance, there
>could be a common interface for operations that work on text selections.
>Of course, even then, some cases will still not fit...

Since Pete was interested in a general way of doing filters, why not have
the following (off the top of my head) for byte oriented streams:

typedef struct {
/* stream operations.  These would probably follow Pascal calling conventions */
	boolean (*EndOfStreamFunc)();
	char (*GetCharFunc)();
	OSErr (*PutCharFunc)();
	OSErr (*UngetCharFunc)();
	/* maybe Seek, and Length too */
} StreamOps, *StreamOpsPtr;

/* prototype function to do filtering */
pascal void main(SOps)
StreamOpsPtr SOps;
{
	/* your code to do filtering */
}

The idea is that the parent program passes in pointer to functions to do all
operations one would need for a stream.  The main could then use the ops on
the stream to transform the data as needed:

pascal void main(SOps)
register StreamOpsPtr SOps;
{
	/* convert an uppercase bytes stream to lowercase */
	register char *c;
	while (! (SOps->EndOfStreamFunc)()) {
		c = (SOps->GetCharFunc)();
		if (isupper(c)) c = tolower(c);
		/* whatever the syntax is for CallPascal -- I forget */
		CallPascal1(SOps->PutCharFunc, c);
	}
}

Of course, this will not be particularly speedy because of the overhead added
by the function calling, but you wanted generality, right?

Steve Hawley
hawley@adobe.com
-- 
"Did you know that a cow was *MURDERED* to make that jacket?"
"Yes.    I didn't think there were any witnesses, so I guess I'll have to kill
 you too." -Jake Johansen