[1/6] Fix fflush after ungetc on input file (bug 5994)
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Commit Message
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.
---
libio/fileops.c | 5 +++
stdio-common/Makefile | 1 +
stdio-common/tst-ungetc-fflush.c | 64 ++++++++++++++++++++++++++++++++
3 files changed, 70 insertions(+)
create mode 100644 stdio-common/tst-ungetc-fflush.c
Comments
LGTM
Reviewed-by: DJ Delorie <dj@redhat.com>
Joseph Myers <josmyers@redhat.com> writes:
> diff --git a/libio/fileops.c b/libio/fileops.c
> index 775999deb3..27e1977eb6 100644
> --- a/libio/fileops.c
> +++ b/libio/fileops.c
> @@ -799,6 +799,11 @@ _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;
> + }
The normal buffer fills on read, and the file pointer reflects the end
of the already-read data. If we've ungetc'd, we've switched to the
backup buffer and *start* at the end of the buffer, which corresponds to
wherever we were when we started ungetc'ing, and "fills" towards the
beginning.
The new code says "if we had been ungetc'ing, we need to seek backwards
by as much as we'd been ungetc'ing, then switch to the main buffer and
seek backwards for that too" unless we hadn't been ungetc'ing, in which
case it just does the main buffer seek.
Ok.
> if (delta != 0)
> {
> off64_t new_pos = _IO_SYSSEEK (fp, delta, 1);
Then we seek by pre-read data + any ungetc'd data.
> diff --git a/stdio-common/Makefile b/stdio-common/Makefile
> index bbdc1ff709..589b786f45 100644
> --- a/stdio-common/Makefile
> +++ b/stdio-common/Makefile
> @@ -302,6 +302,7 @@ tests := \
> tst-swscanf \
> tst-tmpnam \
> tst-ungetc \
> + tst-ungetc-fflush \
> tst-ungetc-leak \
> tst-ungetc-nomem \
> tst-unlockedio \
Ok.
> diff --git a/stdio-common/tst-ungetc-fflush.c b/stdio-common/tst-ungetc-fflush.c
> +/* Test flushing input file after ungetc (bug 5994).
> + Copyright (C) 2025 Free Software Foundation, Inc.
> + This file is part of the GNU C Library.
> +
> + The GNU C Library is free software; you can redistribute it and/or
> + modify it under the terms of the GNU Lesser General Public
> + License as published by the Free Software Foundation; either
> + version 2.1 of the License, or (at your option) any later version.
> +
> + The GNU C Library is distributed in the hope that it will be useful,
> + but WITHOUT ANY WARRANTY; without even the implied warranty of
> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + Lesser General Public License for more details.
> +
> + You should have received a copy of the GNU Lesser General Public
> + License along with the GNU C Library; if not, see
> + <https://www.gnu.org/licenses/>. */
> +
> +#include <stdio.h>
> +
> +#include <support/check.h>
> +#include <support/temp_file.h>
> +#include <support/xstdio.h>
> +#include <support/xunistd.h>
Ok.
> +int
> +do_test (void)
> +{
> + char *filename = NULL;
> + int fd = create_temp_file ("tst-ungetc-fflush", &filename);
> + TEST_VERIFY_EXIT (fd != -1);
> + xclose (fd);
New file, ok.
> + /* Test as in bug 5994. */
> + FILE *fp = xfopen (filename, "w");
> + TEST_VERIFY_EXIT (fputs ("#include", fp) >= 0);
> + xfclose (fp);
File has [#include]. ok.
> + fp = xfopen (filename, "r");
> + TEST_COMPARE (fgetc (fp), '#');
> + TEST_COMPARE (fgetc (fp), 'i');
> + TEST_COMPARE (ungetc ('@', fp), '@');
back to pos 1, ok
> + TEST_COMPARE (fflush (fp), 0);
> + TEST_COMPARE (lseek (fileno (fp), 0, SEEK_CUR), 1);
Ok.
> + TEST_COMPARE (fgetc (fp), 'i');
This is the char in the file, not the char we ungetc'd, ok.
> + TEST_COMPARE (fgetc (fp), 'n');
> + xfclose (fp);
Ok.
> + /* Test as in bug 12799 (duplicate of 5994). */
> + fp = xfopen (filename, "w+");
> + TEST_VERIFY_EXIT (fputs ("hello world", fp) >= 0);
> + rewind (fp);
> + TEST_VERIFY (fileno (fp) >= 0);
Ok.
> + char buffer[10];
> + TEST_COMPARE (fread (buffer, 1, 5, fp), 5);
[hello] ok
> + TEST_COMPARE (fgetc (fp), ' ');
[ ] ok.
> + TEST_COMPARE (ungetc ('@', fp), '@');
ok.
> + TEST_COMPARE (fflush (fp), 0);
> + TEST_COMPARE (fgetc (fp), ' ');
Re-reads the ' ' from file, ok.
> + xfclose (fp);
> +
> + return 0;
> +}
> +
> +#include <support/test-driver.c>
Ok.
@@ -799,6 +799,11 @@ _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);
@@ -302,6 +302,7 @@ tests := \
tst-swscanf \
tst-tmpnam \
tst-ungetc \
+ tst-ungetc-fflush \
tst-ungetc-leak \
tst-ungetc-nomem \
tst-unlockedio \
new file mode 100644
@@ -0,0 +1,64 @@
+/* Test flushing input file after ungetc (bug 5994).
+ Copyright (C) 2025 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <https://www.gnu.org/licenses/>. */
+
+#include <stdio.h>
+
+#include <support/check.h>
+#include <support/temp_file.h>
+#include <support/xstdio.h>
+#include <support/xunistd.h>
+
+int
+do_test (void)
+{
+ char *filename = NULL;
+ int fd = create_temp_file ("tst-ungetc-fflush", &filename);
+ TEST_VERIFY_EXIT (fd != -1);
+ xclose (fd);
+
+ /* Test as in bug 5994. */
+ FILE *fp = xfopen (filename, "w");
+ TEST_VERIFY_EXIT (fputs ("#include", fp) >= 0);
+ xfclose (fp);
+ fp = xfopen (filename, "r");
+ TEST_COMPARE (fgetc (fp), '#');
+ TEST_COMPARE (fgetc (fp), 'i');
+ TEST_COMPARE (ungetc ('@', fp), '@');
+ TEST_COMPARE (fflush (fp), 0);
+ TEST_COMPARE (lseek (fileno (fp), 0, SEEK_CUR), 1);
+ TEST_COMPARE (fgetc (fp), 'i');
+ TEST_COMPARE (fgetc (fp), 'n');
+ xfclose (fp);
+
+ /* Test as in bug 12799 (duplicate of 5994). */
+ fp = xfopen (filename, "w+");
+ TEST_VERIFY_EXIT (fputs ("hello world", fp) >= 0);
+ rewind (fp);
+ TEST_VERIFY (fileno (fp) >= 0);
+ char buffer[10];
+ TEST_COMPARE (fread (buffer, 1, 5, fp), 5);
+ TEST_COMPARE (fgetc (fp), ' ');
+ TEST_COMPARE (ungetc ('@', fp), '@');
+ TEST_COMPARE (fflush (fp), 0);
+ TEST_COMPARE (fgetc (fp), ' ');
+ xfclose (fp);
+
+ return 0;
+}
+
+#include <support/test-driver.c>