Q62912: Bad Code Generated for Difference Between Huge Pointers

Article: Q62912
Product(s): See article
Version(s): 6.00   | 6.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | buglist6.00 | mspl13_c
Last Modified: 13-JUN-1990

The following code demonstrates a case in which the C Compiler version
6.00 generates incorrect results when calculating the difference
between two huge pointers.

Note that the pointer must address an array element inside of a
structure for this problem to occur. Changing the types of the array
elements, etc., does not solve the problem.

Sample Code
-----------

#include <stdio.h>

struct s_type {
            int ary[2]; /* Note: MUST point to array element inside
                                                     struct */
            int l;
       } s, *sptr;

int * lptr1;
long long2;

void main ( void )
{
    sptr = &s;
    printf ( " &(sptr->ary[1]) is at %p\n", &(sptr->ary[1]) ) ;

    lptr1 = & ( sptr -> ary[1] ) ;
    printf ( "lptr1 is at %p\n\n", lptr1 ) ;

    long2 =  lptr1 - &(sptr->ary[1]) ;  /* This should be 0 */
    printf ( "difference is %ld bytes\n", long2 ) ;

}

Compile the above program with the following:

   cl /AH /W4 /Od test.c

When you run the program, the first two addresses should be the same
because they are pointing to the same location. The third printf()
should return 0 (zero) bytes. However, under a huge model, you will
get an incorrect return value.

One workaround is to use the quick compiler (/qc option), as follows:

   cl /qc /AH /W4 /Od test.c

This will generate the correct code.

Microsoft has confirmed this to be a problem in C version 6.00. We are
researching this problem and will post new information here as it
becomes available.

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