/in/foo.c: In function 'sushu':
/in/foo.c:19:22: warning: implicit declaration of function 's' [-Wimplicit-function-declaration]
     for (i = 5; i <= s(num); i = i + 6)
                      ^
/tmp/ccdAGBZ5.o: In function `sushu':
foo.c:(.text+0x5d): undefined reference to `s'
collect2: error: ld returned 1 exit status