Lets us consider this snippet:

int s;
scanf("%c",&s);

Here I have used int, and not char, for variable s, now for using s for character conversion safely I have to make it char again because when scanf reads a character it only overwrites one byte of the variable it is assigning it to, and not all four that int has.

For conversion I could use s = (char)s; as the next line, but is it possible to implement the same by subtracting something from s ?