/in/foo.c: In function 'main':
/in/foo.c:4:2: warning: implicit declaration of function 'gets' [-Wimplicit-function-declaration]
  gets(a); int i;
  ^~~~
/in/foo.c:14:4: warning: array subscript has type 'char' [-Wchar-subscripts]
   b[a[i]]++;
    ^
/in/foo.c:22:4: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
    printf("%c ",i);
    ^~~~~~
/in/foo.c:22:4: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:22:4: note: include '<stdio.h>' or provide a declaration of 'printf'
/tmp/ccg5rpFN.o: In function `main':
foo.c:(.text.startup+0x1e): warning: the `gets' function is dangerous and should not be used.