/in/foo.c: In function 'main':
/in/foo.c:8:13: warning: format '%s' expects argument of type 'char *', but argument 2 has type 'char (*)[255]' [-Wformat=]
     scanf("%s",&a);
             ^
/in/foo.c:9:15: error: 's' undeclared (first use in this function)
  int n=strlen(s);
               ^
/in/foo.c:9:15: note: each undeclared identifier is reported only once for each function it appears in