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From: robert@texas.asd.sgi.com (Robert Skinner)
Newsgroups: comp.sys.sgi
Subject: Re: Possible C compiler bug?
Message-ID: <1990Sep11.220607.19831@odin.corp.sgi.com>
Date: 11 Sep 90 22:06:07 GMT
References: <1379@cs.nps.navy.mil>
Sender: news@odin.corp.sgi.com (Net News)
Reply-To: robert@sgi.com
Organization: Silicon Graphics Inc., Entry Systems Division
Lines: 58


In article <1379@cs.nps.navy.mil>, spl@cs.nps.navy.mil (Steve Lamont) writes:
|> I'm having a problem with a piece of C code on a 4D/70GT under 3.2.  I have
|> declared a variable as float (actually Coord but they're effectively the
|> same).  I pass it as a parameter to a function foo() and foo() passes
it as a
|> pointer to function bar().  When bar gets the float and dereferences the
|> pointer, it finds a bogus value.  It seems what C is doing is promoting the
|> variable to a double even though it is declared as a float. 
|> 
|>
------------------------------------8<---------------------------------------
|> 
|> Here's what the SillyG returns:
|> 
|> foo: 123.345001 123.345001
|> bar: 3.481816 123.345001
|> 
|> and here's what a BSD VAX 11/785 sez:
|> 
|> foo: 123.345001 123.345001
|> bar: 123.345001 123.345001
|> 
|> 
|> 							spl (the p stands for
|> 							perplexed over
|> 							precision)
|> -- 
|> Steve Lamont, SciViGuy -- (408) 646-2752 (subject to change at random)
|> NPS Confuser Center / Code 51 / Naval Postgraduate School / Monterey,
CA 93940
|> "You're okay," said Honeysuckle.  "The dogs like you."
|> 			- Charles Bukowski, "How to Get Published"

also, if I remember correctly, the first 4 bytes of a VAX double look 
are exactly the same as the VAX float of the same number.
So I think you could very well say this:

print( float *v );

{
...
	double	pi = 3.1415912937462386;

	print( &pi );
...
}

and 'print' will print the correct single precision value for pi.
So the VAX may be hiding the error from you.

Robert Skinner
robert@sgi.com

	Watch out where the Huskies go,
	and don't you eat that yellow snow.

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