Xref: utzoo comp.lang.c:9495 comp.sys.ibm.pc:14679
Path: utzoo!mnetor!uunet!husc6!im4u!demelo
From: demelo@im4u.UUCP (Julio De Melo)
Newsgroups: comp.lang.c,comp.sys.ibm.pc
Subject: Turbo C bug with unsigned long integer.
Message-ID: <2658@im4u.UUCP>
Date: 20 Apr 88 22:42:52 GMT
Organization: U. Texas CS Dept., Austin, Texas
Lines: 58
Keywords: Turbo C, bug.


	I haven't read this news group for a while, and for this reason
I am not sure whether this bug has been reported or not. My apologies if so.

	I wrote a program in Turbo C, which runs on an 8088, that needs an
unsigned long integer (32 bits) that I am using simply as a bit pattern. I
initially load this variable with a single bit one in a specific position
out of the 32 possible. To do this I wrote:

		unsigned long variable;

		...

		variable = 1 << num_shift;

	where :  num_shift < 32;

	The problem is that if num_shift is higher than 14, I get
(I suppose so) unnexpected results. The table below show these
results:

	num_shift	result (variable = ...)

	   14		  16384 (0x4000)	OK!
	   15	       -32768 (0xffff8000)	(???, variable was declared
						 as unsigned)
	   16		    0			(???, variable is long)
	 17-31		    0			(same problem)

	What seems to be happening is that "variable" is treated in memory
as two separate 16-bit words, such that when a shift is performed that
crosses the boundary between them, the one in the MSB of the lower word
is lost.

	To confirm that this is actually happening, I wrote the code below
to test whether "variable" is actually interpreted by the compiler as a
long integer. It works well with the exception of the case for "num_shift"
equal to 31, when the result is  -2^^31, instead of  2^^31 (the hex result
is actually what I intend it to be, 0x80000000, but when printing it I
should get a positive number as output).


      if (num_shift != 15)		/* avoid the -32768 result */
	{
	  variable = 1 << num_shift;	/* shift lower word */
          if (!variable)		/* if zero, shift upper word */
	    variable = 0x10000 << (num_shift - 16);
        }
      else
	variable = 0x8000;


	I have no idea whether this works or not on Quick C.
	Any comments?

		Julio de Melo
		UTexas - Austin
		julio17@emx.utexas.edu