ungetc - push character back into input stream
Ungetc pushes the character c back on an input stream. That character
will be returned by the next getc call on that stream. Ungetc returns
One character of pushback is guaranteed provided something has been
read from the stream and the stream is actually buffered. Attempts to
push EOF are rejected.
Fseek(3S) erases all memory of pushed back characters.
getc(3S), setbuf(3S), fseek(3S)
Ungetc returns EOF if it can't push a character back.
19 January 1983 UNGETC(3S)