Fix fflush after ungetc on input file (bug 5994)

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.

1 files changed, 5 insertions, 0 deletions

diff --git a/libio/fileops.c b/libio/fileops.c
index 12b440b09d..42e695265d 100644
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -800,6 +800,11 @@ _IO_new_file_sync (FILE *fp)
   if (fp->_IO_write_ptr > fp->_IO_write_base)
     if (_IO_do_flush(fp)) return EOF;
   delta = fp->_IO_read_ptr - fp->_IO_read_end;
+  if (_IO_in_backup (fp))
+    {
+      _IO_switch_to_main_get_area (fp);
+      delta += fp->_IO_read_ptr - fp->_IO_read_end;
+    }
   if (delta != 0)
     {
       off64_t new_pos = _IO_SYSSEEK (fp, delta, 1);