[3/6] Make fflush (NULL) flush input files (bug 32369)
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Commit Message
As discussed in bug 32369 and required by POSIX, the POSIX feature
fflush (NULL) should flush input files, not just output files. The
POSIX requirement is that "fflush() shall perform this flushing action
on all streams for which the behavior is defined above", and the
definition for input files is for "a stream open for reading with an
underlying file description, if the file is not already at EOF, and
the file is one capable of seeking".
Implement this requirement in glibc. (The underlying flushing
implementation is what deals with avoiding errors for seeking on an
unseekable file.)
Tested for x86_64.
---
libio/genops.c | 7 ++++
stdio-common/Makefile | 1 +
stdio-common/tst-fflush-all-input.c | 53 +++++++++++++++++++++++++++++
3 files changed, 61 insertions(+)
create mode 100644 stdio-common/tst-fflush-all-input.c
Comments
I was surprised at the way fwprintf worked, perhaps a comment explaining
that we expect the wide result to be the same as the narrow result
because we expect the wide chars to be converted to multibyte chars?
I'm used to seeing 16-bit unicode in files, and expected to see that
here too...
We don't have a separate test case for when ungetc() triggers this logic
but I don't think that's needed.
LGTM
Reviewed-by: DJ Delorie <dj@redhat.com>
Joseph Myers <josmyers@redhat.com> writes:
> diff --git a/libio/genops.c b/libio/genops.c
> @@ -730,6 +730,13 @@ _IO_flush_all (void)
> )
> && _IO_OVERFLOW (fp, EOF) == EOF)
> result = EOF;
> + if (_IO_fileno (fp) >= 0
> + && ((fp->_mode <= 0 && fp->_IO_read_ptr < fp->_IO_read_end)
> + || (_IO_vtable_offset (fp) == 0
> + && fp->_mode > 0 && (fp->_wide_data->_IO_read_ptr
> + < fp->_wide_data->_IO_read_end)))
> + && _IO_SYNC (fp) != 0)
> + result = EOF;
This checks for read or wide read buffers having data; if we ungetc'd
we'd still evaluate true here. Ok.
> diff --git a/stdio-common/Makefile b/stdio-common/Makefile
> index b5f78c365b..48fbf05a85 100644
> --- a/stdio-common/Makefile
> +++ b/stdio-common/Makefile
> @@ -238,6 +238,7 @@ tests := \
> tst-fdopen \
> tst-fdopen2 \
> tst-ferror \
> + tst-fflush-all-input \
> tst-fgets \
> tst-fgets2 \
> tst-fileno \
Ok.
> diff --git a/stdio-common/tst-fflush-all-input.c b/stdio-common/tst-fflush-all-input.c
> new file mode 100644
> index 0000000000..e9df3a0c08
> --- /dev/null
> +++ b/stdio-common/tst-fflush-all-input.c
> @@ -0,0 +1,53 @@
> +/* Test fflush (NULL) flushes input files (bug 32369).
> + 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 <unistd.h>
> +#include <wchar.h>
> +
> +#include <support/check.h>
> +#include <support/xstdio.h>
Ok.
> +int
> +do_test (void)
> +{
> + FILE *temp = tmpfile ();
> + TEST_VERIFY_EXIT (temp != NULL);
> + fprintf (temp, "abc");
> + TEST_COMPARE (fflush (temp), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
> + TEST_COMPARE (fgetc (temp), 'a');
> + TEST_COMPARE (fflush (NULL), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
> + xfclose (temp);
Ok.
> + /* Likewise, but in wide mode. */
> + temp = tmpfile ();
> + TEST_VERIFY_EXIT (temp != NULL);
> + fwprintf (temp, L"abc");
> + TEST_COMPARE (fflush (temp), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
> + TEST_COMPARE (fgetwc (temp), L'a');
> + TEST_COMPARE (fflush (NULL), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
Shouldn't this be sizeof(wchar_t), not 1 ? /me tests... no,
fwprintf(L"abc") writes three narrow chars to the file! Needs comment?
Needs specific locale? Are there any locales where we write this out as
a wide char and not a multi-byte char?
> + xfclose (temp);
> +
> + return 0;
> +}
> +
> +#include <support/test-driver.c>
Ok.
* Joseph Myers:
> diff --git a/libio/genops.c b/libio/genops.c
> index 2197bfe7a1..e4378ca48f 100644
> --- a/libio/genops.c
> +++ b/libio/genops.c
> @@ -730,6 +730,13 @@ _IO_flush_all (void)
> )
> && _IO_OVERFLOW (fp, EOF) == EOF)
> result = EOF;
> + if (_IO_fileno (fp) >= 0
> + && ((fp->_mode <= 0 && fp->_IO_read_ptr < fp->_IO_read_end)
> + || (_IO_vtable_offset (fp) == 0
> + && fp->_mode > 0 && (fp->_wide_data->_IO_read_ptr
> + < fp->_wide_data->_IO_read_end)))
> + && _IO_SYNC (fp) != 0)
> + result = EOF;
>
> _IO_funlockfile (fp);
> run_fp = NULL;
I was confused for a bit why exit would block indefinitely with a locked
input stream prior this change, but then I realized that we
unconditionally lock the stream before this check.
> + FILE *temp = tmpfile ();
> + TEST_VERIFY_EXIT (temp != NULL);
> + fprintf (temp, "abc");
> + TEST_COMPARE (fflush (temp), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
> + TEST_COMPARE (fgetc (temp), 'a');
> + TEST_COMPARE (fflush (NULL), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
> + xfclose (temp);
> +
> + /* Likewise, but in wide mode. */
> + temp = tmpfile ();
> + TEST_VERIFY_EXIT (temp != NULL);
> + fwprintf (temp, L"abc");
> + TEST_COMPARE (fflush (temp), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
> + TEST_COMPARE (fgetwc (temp), L'a');
> + TEST_COMPARE (fflush (NULL), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
> + xfclose (temp);
> +
> + return 0;
> +}
Could you add tests that do the flush via fork and exit?
Thanks,
Florian
On Wed, 15 Jan 2025, DJ Delorie wrote:
> I was surprised at the way fwprintf worked, perhaps a comment explaining
> that we expect the wide result to be the same as the narrow result
> because we expect the wide chars to be converted to multibyte chars?
That's simply how all the wide I/O functions work.
As remarked for patch 1, the file position indicator is unspecified after
ungetwc (hence no tests using ungetwc - tests in this series only use wide
wide I/O in cases where no pushback is involved - and no special checks
needed in general for the combination of wide I/O and pushback).
> Shouldn't this be sizeof(wchar_t), not 1 ? /me tests... no,
> fwprintf(L"abc") writes three narrow chars to the file! Needs comment?
> Needs specific locale? Are there any locales where we write this out as
> a wide char and not a multi-byte char?
This test is using the C locale; since it's only using ASCII characters,
no specific locale is needed. A null character can't be part of another
multibyte character, which means that UTF-16 and UTF-32 are not possible
locale multibyte encodings to write out to the file.
Joseph Myers <josmyers@redhat.com> writes:
> That's simply how all the wide I/O functions work.
I'm not doubting the test's correctness or now the functions run, I'm
just saying I was surprised that it did a wide->multi conversion. And
IMHO when someone reading code is surprised, that's a good spot for a
comment ;-)
> This test is using the C locale; since it's only using ASCII characters,
> no specific locale is needed. A null character can't be part of another
> multibyte character, which means that UTF-16 and UTF-32 are not possible
> locale multibyte encodings to write out to the file.
Do we have an API that can write UTF-16 or UTF-32 encodings to files?
Or do they devolve to using the regular fwrite et al?
Also, I read the POSIX spec for those functions and it wasn't obvious
from those that they did a wide->multi conversion either! I mean, you
can figure it out eventually if you know they're doing it, but a new
reader would miss it.
Sigh.
On Wed, 15 Jan 2025, DJ Delorie wrote:
> Joseph Myers <josmyers@redhat.com> writes:
> > That's simply how all the wide I/O functions work.
>
> I'm not doubting the test's correctness or now the functions run, I'm
> just saying I was surprised that it did a wide->multi conversion. And
> IMHO when someone reading code is surprised, that's a good spot for a
> comment ;-)
I'm thinking of this as something basic from the description of files in
the C standard (C23 7.23.3; two paragraphs "The wide character input
functions read multibyte characters from the stream and convert them to
wide characters as if ..." and "The wide character output functions
convert wide characters to multibyte characters and write them to the
stream as if ..."), so not worthy of remark in the context of an
individual test.
> > This test is using the C locale; since it's only using ASCII characters,
> > no specific locale is needed. A null character can't be part of another
> > multibyte character, which means that UTF-16 and UTF-32 are not possible
> > locale multibyte encodings to write out to the file.
>
> Do we have an API that can write UTF-16 or UTF-32 encodings to files?
> Or do they devolve to using the regular fwrite et al?
You'd need to use fwrite. Text files are made up of multibyte characters.
On Wed, 15 Jan 2025, Florian Weimer wrote:
> > + /* Likewise, but in wide mode. */
> > + temp = tmpfile ();
> > + TEST_VERIFY_EXIT (temp != NULL);
> > + fwprintf (temp, L"abc");
> > + TEST_COMPARE (fflush (temp), 0);
> > + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
> > + TEST_COMPARE (fgetwc (temp), L'a');
> > + TEST_COMPARE (fflush (NULL), 0);
> > + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
> > + xfclose (temp);
> > +
> > + return 0;
> > +}
>
> Could you add tests that do the flush via fork and exit?
I've added those in v2 of the patch.
Make fflush (NULL) flush input files (bug 32369)
As discussed in bug 32369 and required by POSIX, the POSIX feature
fflush (NULL) should flush input files, not just output files. The
POSIX requirement is that "fflush() shall perform this flushing action
on all streams for which the behavior is defined above", and the
definition for input files is for "a stream open for reading with an
underlying file description, if the file is not already at EOF, and
the file is one capable of seeking".
Implement this requirement in glibc. (The underlying flushing
implementation is what deals with avoiding errors for seeking on an
unseekable file.)
Tested for x86_64.
---
Changed in v2: also test the case where flushing of all input files is
achieved by a call to exit after forking rather than an explicit call
to fflush (NULL).
---
libio/genops.c | 7 +++
stdio-common/Makefile | 1 +
stdio-common/tst-fflush-all-input.c | 94 +++++++++++++++++++++++++++++
3 files changed, 102 insertions(+)
create mode 100644 stdio-common/tst-fflush-all-input.c
diff --git a/libio/genops.c b/libio/genops.c
index 2197bfe7a1..e4378ca48f 100644
--- a/libio/genops.c
+++ b/libio/genops.c
@@ -730,6 +730,13 @@ _IO_flush_all (void)
)
&& _IO_OVERFLOW (fp, EOF) == EOF)
result = EOF;
+ if (_IO_fileno (fp) >= 0
+ && ((fp->_mode <= 0 && fp->_IO_read_ptr < fp->_IO_read_end)
+ || (_IO_vtable_offset (fp) == 0
+ && fp->_mode > 0 && (fp->_wide_data->_IO_read_ptr
+ < fp->_wide_data->_IO_read_end)))
+ && _IO_SYNC (fp) != 0)
+ result = EOF;
_IO_funlockfile (fp);
run_fp = NULL;
diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index b5f78c365b..48fbf05a85 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -238,6 +238,7 @@ tests := \
tst-fdopen \
tst-fdopen2 \
tst-ferror \
+ tst-fflush-all-input \
tst-fgets \
tst-fgets2 \
tst-fileno \
diff --git a/stdio-common/tst-fflush-all-input.c b/stdio-common/tst-fflush-all-input.c
new file mode 100644
index 0000000000..8e3fca3a08
--- /dev/null
+++ b/stdio-common/tst-fflush-all-input.c
@@ -0,0 +1,94 @@
+/* Test fflush (NULL) flushes input files (bug 32369).
+ 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 <stdlib.h>
+#include <unistd.h>
+#include <wchar.h>
+
+#include <support/check.h>
+#include <support/xstdio.h>
+#include <support/xunistd.h>
+
+int
+do_test (void)
+{
+ FILE *temp = tmpfile ();
+ TEST_VERIFY_EXIT (temp != NULL);
+ fprintf (temp, "abc");
+ TEST_COMPARE (fflush (temp), 0);
+ TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
+ TEST_COMPARE (fgetc (temp), 'a');
+ TEST_COMPARE (fflush (NULL), 0);
+ TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
+ xfclose (temp);
+
+ /* Likewise, but in wide mode. */
+ temp = tmpfile ();
+ TEST_VERIFY_EXIT (temp != NULL);
+ fwprintf (temp, L"abc");
+ TEST_COMPARE (fflush (temp), 0);
+ TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
+ TEST_COMPARE (fgetwc (temp), L'a');
+ TEST_COMPARE (fflush (NULL), 0);
+ TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
+ xfclose (temp);
+
+ /* Similar tests, but with the flush implicitly occurring on exit
+ (in a forked subprocess). */
+
+ temp = tmpfile ();
+ TEST_VERIFY_EXIT (temp != NULL);
+ pid_t pid = xfork ();
+ if (pid == 0)
+ {
+ fprintf (temp, "abc");
+ TEST_COMPARE (fflush (temp), 0);
+ TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
+ TEST_COMPARE (fgetc (temp), 'a');
+ exit (EXIT_SUCCESS);
+ }
+ else
+ {
+ TEST_COMPARE (xwaitpid (pid, NULL, 0), pid);
+ TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
+ xfclose (temp);
+ }
+
+ temp = tmpfile ();
+ TEST_VERIFY_EXIT (temp != NULL);
+ pid = xfork ();
+ if (pid == 0)
+ {
+ fwprintf (temp, L"abc");
+ TEST_COMPARE (fflush (temp), 0);
+ TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
+ TEST_COMPARE (fgetwc (temp), L'a');
+ exit (EXIT_SUCCESS);
+ }
+ else
+ {
+ TEST_COMPARE (xwaitpid (pid, NULL, 0), pid);
+ TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
+ xfclose (temp);
+ }
+
+ return 0;
+}
+
+#include <support/test-driver.c>
I suspect trying to be more clever about sync-after-fork would lead to
lots of problems, like, if we ungetc, fork, exit in the child, does the
parent lose the ungetc? We would need to fflush(NULL) in fork() before
the actual fork syscall, and that might not catch everything anyway.
LGTM
Reviewed-by: DJ Delorie <dj@redhat.com>
FYI patchwork really wants "v2" type patches to be sent without the Re:
text and with a "v2" in the subject, in case you were looking for this
patch in patchwork ;-)
Joseph Myers <josmyers@redhat.com> writes:
> As discussed in bug 32369 and required by POSIX, the POSIX feature
> fflush (NULL) should flush input files, not just output files. The
> POSIX requirement is that "fflush() shall perform this flushing action
> on all streams for which the behavior is defined above", and the
> definition for input files is for "a stream open for reading with an
> underlying file description, if the file is not already at EOF, and
> the file is one capable of seeking".
>
> Implement this requirement in glibc. (The underlying flushing
> implementation is what deals with avoiding errors for seeking on an
> unseekable file.)
>
> Tested for x86_64.
>
> ---
>
> Changed in v2: also test the case where flushing of all input files is
> achieved by a call to exit after forking rather than an explicit call
> to fflush (NULL).
> diff --git a/libio/genops.c b/libio/genops.c
> index 2197bfe7a1..e4378ca48f 100644
> --- a/libio/genops.c
> +++ b/libio/genops.c
> @@ -730,6 +730,13 @@ _IO_flush_all (void)
> )
> && _IO_OVERFLOW (fp, EOF) == EOF)
> result = EOF;
> + if (_IO_fileno (fp) >= 0
> + && ((fp->_mode <= 0 && fp->_IO_read_ptr < fp->_IO_read_end)
> + || (_IO_vtable_offset (fp) == 0
> + && fp->_mode > 0 && (fp->_wide_data->_IO_read_ptr
> + < fp->_wide_data->_IO_read_end)))
> + && _IO_SYNC (fp) != 0)
> + result = EOF;
>
> _IO_funlockfile (fp);
> run_fp = NULL;
same as v1, ok.
> diff --git a/stdio-common/Makefile b/stdio-common/Makefile
> index b5f78c365b..48fbf05a85 100644
> --- a/stdio-common/Makefile
> +++ b/stdio-common/Makefile
> @@ -238,6 +238,7 @@ tests := \
> tst-fdopen \
> tst-fdopen2 \
> tst-ferror \
> + tst-fflush-all-input \
> tst-fgets \
> tst-fgets2 \
> tst-fileno \
same as v1, ok.
> diff --git a/stdio-common/tst-fflush-all-input.c b/stdio-common/tst-fflush-all-input.c
> new file mode 100644
> index 0000000000..8e3fca3a08
> --- /dev/null
> +++ b/stdio-common/tst-fflush-all-input.c
> @@ -0,0 +1,94 @@
> +/* Test fflush (NULL) flushes input files (bug 32369).
> + 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 <stdlib.h>
> +#include <unistd.h>
> +#include <wchar.h>
> +
> +#include <support/check.h>
> +#include <support/xstdio.h>
> +#include <support/xunistd.h>
Ok.
> +int
> +do_test (void)
> +{
> + FILE *temp = tmpfile ();
> + TEST_VERIFY_EXIT (temp != NULL);
> + fprintf (temp, "abc");
> + TEST_COMPARE (fflush (temp), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
> + TEST_COMPARE (fgetc (temp), 'a');
> + TEST_COMPARE (fflush (NULL), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
> + xfclose (temp);
> +
> + /* Likewise, but in wide mode. */
> + temp = tmpfile ();
> + TEST_VERIFY_EXIT (temp != NULL);
> + fwprintf (temp, L"abc");
> + TEST_COMPARE (fflush (temp), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
> + TEST_COMPARE (fgetwc (temp), L'a');
> + TEST_COMPARE (fflush (NULL), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
> + xfclose (temp);
Same as v1, ok.
> + /* Similar tests, but with the flush implicitly occurring on exit
> + (in a forked subprocess). */
> +
> + temp = tmpfile ();
> + TEST_VERIFY_EXIT (temp != NULL);
> + pid_t pid = xfork ();
> + if (pid == 0)
> + {
child, ok
> + fprintf (temp, "abc");
> + TEST_COMPARE (fflush (temp), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
> + TEST_COMPARE (fgetc (temp), 'a');
> + exit (EXIT_SUCCESS);
Ok; file left at position 1.
> + }
> + else
> + {
> + TEST_COMPARE (xwaitpid (pid, NULL, 0), pid);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
> + xfclose (temp);
Ok.
> + }
> +
> + temp = tmpfile ();
> + TEST_VERIFY_EXIT (temp != NULL);
> + pid = xfork ();
> + if (pid == 0)
> + {
> + fwprintf (temp, L"abc");
> + TEST_COMPARE (fflush (temp), 0);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
> + TEST_COMPARE (fgetwc (temp), L'a');
> + exit (EXIT_SUCCESS);
> + }
> + else
> + {
> + TEST_COMPARE (xwaitpid (pid, NULL, 0), pid);
> + TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
> + xfclose (temp);
> + }
Ok.
> + return 0;
> +}
> +
> +#include <support/test-driver.c>
Ok.
@@ -730,6 +730,13 @@ _IO_flush_all (void)
)
&& _IO_OVERFLOW (fp, EOF) == EOF)
result = EOF;
+ if (_IO_fileno (fp) >= 0
+ && ((fp->_mode <= 0 && fp->_IO_read_ptr < fp->_IO_read_end)
+ || (_IO_vtable_offset (fp) == 0
+ && fp->_mode > 0 && (fp->_wide_data->_IO_read_ptr
+ < fp->_wide_data->_IO_read_end)))
+ && _IO_SYNC (fp) != 0)
+ result = EOF;
_IO_funlockfile (fp);
run_fp = NULL;
@@ -238,6 +238,7 @@ tests := \
tst-fdopen \
tst-fdopen2 \
tst-ferror \
+ tst-fflush-all-input \
tst-fgets \
tst-fgets2 \
tst-fileno \
new file mode 100644
@@ -0,0 +1,53 @@
+/* Test fflush (NULL) flushes input files (bug 32369).
+ 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 <unistd.h>
+#include <wchar.h>
+
+#include <support/check.h>
+#include <support/xstdio.h>
+
+int
+do_test (void)
+{
+ FILE *temp = tmpfile ();
+ TEST_VERIFY_EXIT (temp != NULL);
+ fprintf (temp, "abc");
+ TEST_COMPARE (fflush (temp), 0);
+ TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
+ TEST_COMPARE (fgetc (temp), 'a');
+ TEST_COMPARE (fflush (NULL), 0);
+ TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
+ xfclose (temp);
+
+ /* Likewise, but in wide mode. */
+ temp = tmpfile ();
+ TEST_VERIFY_EXIT (temp != NULL);
+ fwprintf (temp, L"abc");
+ TEST_COMPARE (fflush (temp), 0);
+ TEST_COMPARE (lseek (fileno (temp), 0, SEEK_SET), 0);
+ TEST_COMPARE (fgetwc (temp), L'a');
+ TEST_COMPARE (fflush (NULL), 0);
+ TEST_COMPARE (lseek (fileno (temp), 0, SEEK_CUR), 1);
+ xfclose (temp);
+
+ return 0;
+}
+
+#include <support/test-driver.c>