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From: tim@ism780c.UUCP (T.W."Tim" Smith, Knowledgian)
Newsgroups: comp.sys.mac.programmer
Subject: Pointer bugs in LightspeedC
Message-ID: <9827@ism780c.UUCP>
Date: 27 Apr 88 17:22:57 GMT
Reply-To: tim@ism780c.UUCP (T.W."Tim" Smith, Knowledgian)
Organization: Suction and Pressure Lab, California Institute of Lawsonomy
Lines: 30

I found an annoying bug in LightspeedC last week.  I had a function that
needed to know it's own return address.  I tried this:

	func( dummy )
	{
		unsigned long RetAddr;

		RetAddr = *(unsigned long *)((char *)&dummy-4);
		.
		.
		.
	}

I Nosy'ed the resultant code and found that LsC had dropped the subtraction
of 4 from the pointer!  It treated the above expression like this:

		RetAddr = *(unsigned long *)((char *)&dummy);

Changing the expression to this made it work:

		RetAddr = (unsigned long)&dummy;
		RetAddr -= 4;
		RetAddr = *(unsigned long *)RetAddr;

This was LsC on a plus.  It was version 2.(something less than the
version that supports the SE and the II).

-- 
Tim Smith				tim@ism780c.isc.com
"I don't practice what I preach because I'm not the
                       kind of person I'm preaching to" -- J.R. "Bob" Dobbs