cc-preprocessorpreprocessor-directive

Is this a C preprocessor bug?

#include <stdio.h>
#define n 16
#define a -2.0
#define b 2.0
#define c (b-a)/n
int main() {
    printf("%lf\n",5.0*c);
}

Outputs 1.250000 But if i change (b-a)/n to b/n-a/n it outputs 0.750000.

#include <stdio.h>
#define n 16
#define a -2.0
#define b 2.0
#define c b/n-a/n
int main() {
    printf("%lf\t%lf\n",c,5.0*c/5.0);
}

Outputs 0.250000 0.650000

So if i print c alone it's correct, but if i actually perform some operation with it, it turns out that c is different.

Update: It seems to require the expression to be in parentheses. (b/n-a/n)

Solution

  • It's not a bug. Macros perform direct token substitution. So this:

    printf("%lf\t%lf\n",c,5.0*c/5.0);

    Expands to this:

    printf("%lf\t%lf\n",2.0/16 - -2.0/16,5.0*2.0/16 - -2.0/16/5.0);

    This demonstrates why identifiers in macros should always be parenthesized:

    #define c (((b)/(n))-((a)/(n)))