As discussed in bug 5994 (plus duplicates), POSIX requires fflush
after ungetc to discard pushed-back characters but preserve the file
position indicator. For this purpose, each ungetc decrements the file
position indicator by 1; it is unspecified after ungetc at the start
of the file, and after ungetwc, so no special handling is needed for
either of those cases.
This is fixed with appropriate logic in _IO_new_file_sync. I haven't
made any attempt to test or change things in this area for the "old"
functions; the case of files using mmap is addressed in a subsequent
patch (and there seem to be no problems in this area with files opened
with fmemopen).
Tested for x86_64.
Joseph Myers