x                  The push-back character.
    WEOF               Indicates that the function cannot push the
                       character back. errno is set to one of the
                       following:

                       o  EBADF - The file descriptor is not valid.

                       o  EALREADY - Operation is already in progress
                          on the same file.

                       o  EILSEQ - Invalid wide-character code
                          detected.