Q64565: #if Uses Only Lower 2 Bytes of Expression for Test

Article: Q64565
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: 31-AUG-1990

The #if compiler directive uses only the lower 2 bytes of the
evaluated expression when testing for true or false.

The following code, when compiled using the C 6.00 compiler, will
evaluate the #if expression to false, when it should evaluate to
true since the number is non-zero:

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

void main (void)
{
#if 0xFFFF0000L
  printf("TRUE\n");
#else
  printf("FALSE\n");
#endif
}

According to the ANSI standard (Section 3.8.1), any long expression in
an #if directive must be evaluated as if it were a long value.

The workaround for this problem is to only use 2-byte expressions in
an #if directive.

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

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