Q57780: Casting Pointer Subtraction May Yield Incorrect Code

Article: Q57780
Product(s): See article
Version(s): 2.00 2.01
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 13-FEB-1990

Microsoft QuickC Versions 2.00 and 2.01 will produce incorrect code if
pointer subtraction is immediately cast to a long integer. The
resulting value is not the number of elements between the two
pointers, as it should be.

The following code fragment demonstrates this problem:

#include <stdio.h>
#include <stddef.h>
char
   chArray [20],
   *p1,
   *p2;
long
   pdif;

void main(void)
{
   p2 = &chArray[0];
   p1 = &chArray[19];

   printf("dif:  %lx\n", (long)(p1 - p2)); /* Invalid count */

   pdif = (p1 - p2);
   printf("dif:  %lx\n", pdif);  /* Correct count */
}

In the first printf(), the compiler does not clear out the high-order
word when the cast is performed, so the resulting long value is
incorrect. The workaround is to use the second form and calculate the
count in a temporary variable first.

According to the ANSI standard, pointer subtraction is defined between
two pointers when they both are of the same type and they both point
to the same array. The return value is the number of elements
separating the two pointers. The resulting type is defined inside
"stddef.h" with "ptrdiff_t". Microsoft C and QuickC define "ptrdiff_t"
to be a two-byte signed integer.

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