ungetc - push character back into input stream
ungetc() pushes the character c back onto an input stream. That char-
acter will be returned by the next getc() call on that stream.
ungetc() returns c, and leaves the file stream unchanged.
One character of pushback is guaranteed provided something has been
read from the stream and the stream is actually buffered. In the case
that stream is stdin, one character may be pushed back onto the buffer
without a previous read statement.
If c equals EOF, ungetc() does nothing to the buffer and returns EOF.
An fseek(3S) erases all memory of pushed back characters.
fseek(3S), getc(3V), setbuf(3V)
ungetc() returns EOF if it cannot push a character back.
18 November 1987 UNGETC(3S)