ungetc() pushes characters back onto the input stream for memory files. ungetc() handles only single-byte characters. You can use it to push back as many as four characters onto the ungetc() buffer. For every character pushed back with ungetc(), fflush() backs up the file position by one character and clears all the pushed-back characters from the stream. Backing up the file position may end up going across a record boundary.
If you want fflush() to ignore ungetc() characters, you can set the _EDC_COMPAT environment variable. See Using environment variables for more information.