7.8.3 Ungetc
The standard library provides a rather restricted version of the function ungetch that we wrote in Chapter 4; it is called ungetc.
int ungetc(int c, FILE *fp)
pushes the character c back onto file fp, and returns either c, or EOF for an error. Only one character of pushback is guaranteed per file. ungetc may be used with any of the input functions like scanf, getc, or getchar.