implicit return value in C. This simple code should not work, but it does [duplicate]

I'm going straight to the point. This simple code should not return any values:

   #include <stdio.h>   int sum (int num) {        static int s = 0;        s+=num;   }   int main() {       printf("%d ",sum(55));       printf("%d ",sum(45));       printf("%d ",sum(50));       return 0;   }

but it actually returns:55 100 150

I'm sorry if I am being just plain stupid and not seeing something obvious, but my limited knowledge says the function "sum" does not return anything.

image for clarification