c-preprocessor

Can the C preprocessor perform integer arithmetic?

Is the C preprocessor able to perform integer arithmetic?

E.g.:

#define PI 3.1416
#define OP PI/100
#define OP2 PI%100

Is there a way OP and/or OP2 get calculated in the preprocessing phase?

Solution

  • Integer arithmetic? Run the following program to find out:

    #include "stdio.h"
int main() {
    #if 1 + 1 == 2
        printf("1+1==2\n");
    #endif
    #if 1 + 1 == 3
        printf("1+1==3\n");
    #endif
}

    The answer is "yes". There is a way to make the preprocessor perform integer arithmetic, which is to use it in a preprocessor condition.

    Note however that your examples are not integer arithmetic. I just checked, and GCC's preprocessor fails if you try to make it do float comparisons. I haven't checked whether the standard ever allows floating point arithmetic in the preprocessor.

    Regular macro expansion does not evaluate integer expressions. It leaves it to the compiler, as can be seen by preprocessing (-E in GCC) the following:

    #define ONEPLUSONE (1 + 1)
#if ONEPLUSONE == 2
   int i = ONEPLUSONE;
#endif

    The result is int i = (1 + 1); (plus probably some stuff to indicate source file names and line numbers and such).