Q67740: Dividing Negative Integers May Give Positive Results

Article: Q67740
Product(s): See article
Version(s): 5.10 6.00 6.00a | 5.10 6.00 6.00a
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | buglist5.10 buglist6.00 buglist6.00a | mspl13_c
Last Modified: 30-JAN-1991

In some situations, dividing a negative integer by a positive integer
may give positive results. For example, when the following code is
compiled with the /Od option, both printf() statements will generate
positive answers.

Code Example
------------

void foo(void)
{
    int num1 = -32768,
        num2 = -32751;

    printf(" %d ",  num1 / 16);
    printf(" %d ", (num2 & 0xfff0) / 16);
}

When these calculations are performed, the sign bit is lost. The
output is incorrect for the first printf() statement when -32768 is
being divided by a power of 2 (for example, 4,8,16,32,64,...16384).

Workarounds for the first printf() statement are:

1. Compile without the /Od option.

2. Compile with the /qc option.

With the second printf(), the value (num2 & 0xfff0) must be typecast
to an integer or separated into another statement. For example,

   printf(" %d ", (int)(num2 & 0xfff0) / 16);

-or-

   num3 = num2 & 0xfff0;
   printf(" %d ", num3 / 16);

Microsoft has confirmed this to be a problem in the C versions 5.10,
6.00, and 6.00a. We are researching this problem and will post new
information here as it becomes available.

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