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     The ungetc function pushes the character <c> back onto <stream>
     and clears the end-of-file indicator. The stream must be open for
     reading. A subsequent read operation on the stream starts with
     <c>. An attempt to push EOF onto the stream using ungetc is
     ignored. The ungetc function returns an error value if nothing has
     yet been read from <stream>, or if <c> cannot be pushed back.
 
     Characters placed on the stream by ungetc may be erased if fflush,
     fseek, fsetpos, or rewind is called before the character is read
     from the stream. The file-position indicator will have the same
     value it had before the characters were pushed back.
 
     On a successful ungetc call against a text stream, the file-
     position indicator is unspecified until all the pushed-back
     characters are read or discarded. On each successful ungetc call
     against a binary stream, the file-position indicator is stepped
     down. If its value was 0 before a call, the value is undefined
     after the call.
 
     Results are unpredictable if the ungetc function is called twice
     without a read operation between the two calls. After a call to
     the fscanf function, a call to ungetc may fail unless another
     read operation (such as the getc function) has been performed.
     This is because the fscanf function itself calls the ungetc
     function.
 
     Return Value
 
     The ungetc function returns the character argument <c>. The return
     value EOF indicates a failure to push back the specified character.
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