/in/foo.c: In function 'calculateFactorialDigits':
/in/foo.c:17:18: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
     int digits = strlen(str);
                  ^~~~~~
/in/foo.c:17:18: warning: incompatible implicit declaration of built-in function 'strlen'
/in/foo.c:17:18: note: include '<string.h>' or provide a declaration of 'strlen'