The ungetc() function pushes the byte specified by c (converted to an unsigned char) back onto the input stream pointed to by stream. The pushed-back bytes will be returned by subsequent reads on that stream in the reverse order of their pushing. A successful intervening call (with the stream pointed to by stream) to a file-positioning function ( fseek(3C), fsetpos(3C) or rewind(3C))
discards any pushed-back bytes for the stream. The external storage corresponding to the stream is unchanged.
Four bytes of push-back are guaranteed. If ungetc() is called too many times on the same stream without an intervening read or file-positioning operation on that stream, the operation
If the value of c equals that of the macro EOF, the operation fails and the input stream is unchanged.
A successful call to ungetc() clears the end-of-file indicator for the stream. The value of the file-position indicator for the stream after reading or discarding all pushed-back
bytes will be the same as it was before the bytes were pushed back. The file-position indicator is decremented by each successful call to ungetc(); if its value was 0 before a call, its
value is indeterminate after the call.