Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!lll-winken!taurus!cs!spl
From: spl@cs.nps.navy.mil (Steve Lamont)
Newsgroups: comp.sys.sgi
Subject: Possible C compiler bug?
Message-ID: <1379@cs.nps.navy.mil>
Date: 10 Sep 90 05:14:33 GMT
Reply-To: spl@cs.nps.navy.mil (Steve Lamont)
Organization: Foo Bar Brewers Cooperative
Lines: 91

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. 

Some investigation reveals that floats and doubles are constructed
differently, with 8 bits for exponent and (I think) sign for floats and 12
bits for doubles.  Clearly, referencing a double aliased as a float is a bad
thing.

The following test code shows the problem:

---------------------------------8<-----------------------------------------
#include <stdio.h>
 
main()
 
  {
 
    float x = 123.345;
 
    foo( &x, x );
 
    exit( 0 );
 
  }
 
foo( a, b )
 
  float *a;
  float b;
 
  {
 
    /*
     *	*a and b should be the same.
     */

    fprintf( stderr, "foo: %f %f\n", *a, b );

    /*
     *	Now call with b as a pointer and as a value.
     */

    bar( &b, b );
 
    return;
 
  }
 
bar( a, b )
 
  float *a;
  float b;
 
  {
 
    /*
     *	*a and b should be the same.  On our 4D they ain't.
     */

    fprintf( stderr, "bar: %f %f\n", *a, b );
    return;
 
  }
------------------------------------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

I've looked through K&R (2nd Edition) and can't find anything that
specifically bears upon this problem in the sections on type coersion.  Am I
doing something wrong?

							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"