C Program Returns NonZero if its Arg is either a Letter or a Digit returns zero

#include <ctype.h> 
 #include <stdio.h> 
 int main(void) 
 { 
     char ch; 
     clrscr(); 
     printf(". to exit:"); 
     for(;;) 
         { 
            ch = getc(stdin); 
            if(ch == '.') 
              break; 
            if(isalnum(ch)) 
              printf("%c is alphanumeric\n", ch); 
         } 
              getch(); 
              return 0; 
   }