BUG: Incorrect Code Generated for Long Constant

The information in this article applies to:

• The Microsoft C/C++ Compiler (CL.EXE), included with:

      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
  

SYMPTOMS

Incorrect code is generated when using a long constant in a conditional expression in a for or a while statement.

If attempting to compile using Microsoft Visual C++ versions 1.0 or 1.5, the following warning may be generated under warning level 2 or higher

   warning C4309: '*' : truncation of constant value

There is no warning or error message with version 7.0 of the C/C++ compiler; however, the problem still occurs.

CAUSE

The compiler generates incorrect code only if the following conditions are met:

• The fast compiler is used. Under debug mode, this is the default.
• The code is compiled as a C++ module.
• A long constant is used in a relational expression in a for or a while statement.
• The long constant has a value greater than the range of integer values (greater than +32767 or less than -32768).

RESOLUTION

To avoid the problem, you can do any of the following:

• Compile without using the fast compiler (/f-).
• Because the error occurs only with a signed long constant, either use a variable or an unsigned long constant.

STATUS

Microsoft has confirmed this to be a problem in C/C++ version 7.0 and Visual C++ for Windows, versions 1.0 and 1.5. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

This is not a problem in the 32-bit compilers.

MORE INFORMATION

The following sample code will produce a warning at the line indicated when compiled with Visual C++ version 1.0 or 1.5 under warning level 2 or above. No warning will be generated with version 7.0. In either case, the body of the for loop will not be executed.

Sample Code

/* Compile options needed: /f and </W3 or /W4>

*/

#include <stdio.h>

void main()

     for (long l = 0L; l < bignum; l++)
     {
          printf("%ld\n",l);     // This line never executes.
     }     // warning C4309: '<' : truncation of constant value
}