/in/foo.c: In function 'Diff':
/in/foo.c:10:13: warning: implicit declaration of function 'isupper' [-Wimplicit-function-declaration]
         if (isupper(*str))
             ^~~~~~~
/in/foo.c:13:20: warning: implicit declaration of function 'islower' [-Wimplicit-function-declaration]
         } else if (islower(*str))
                    ^~~~~~~
/in/foo.c: In function 'main':
/in/foo.c:30:8: warning: implicit declaration of function 'strcspn' [-Wimplicit-function-declaration]
     s1[strcspn(s1, "\n")] = '\0';
        ^~~~~~~
/in/foo.c:30:8: warning: incompatible implicit declaration of built-in function 'strcspn'
/in/foo.c:30:8: note: include '<string.h>' or provide a declaration of 'strcspn'