Conditional Compilation and compile time evaluation of expressions in ANSI C

Question

I would like to do the following, but the compiler doesn't like it:

unsigned short foo = 1;
// do something with foo
#if sizeof(short) * CHAR_BIT > 16
   foo &= 0xffff;
#endif

I know this expression can always be fully evaluated at compile time, but maybe it's only evaluated after the preprocessor does it's thing? Is this possible in ANSI C or do I just have to do the check at run time?

Answer 1

You can't use sizeof in a preprocessor expression. You might want to do something like this instead:

#include <limits.h>

#if SHRT_MAX > 32767
    /* do soemthing */
#endif