Description

   When using the ungetc function, the character is pushed back onto
   the file indicated by file_ptr.

   One push-back is guaranteed, even if there has been no previous
   activity on the file. The fseek function erases all memory of
   pushed-back characters. The pushed-back character is not written
   to the underlying file. If the character to be pushed back is
   EOF, the operation fails, the input stream is left unchanged, and
   EOF is returned.

   See also fseek and getc.