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.

2 files changed, 95 insertions, 0 deletions

diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index f6bdc32..26c1b88 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 0000000..8e3fca3
--- /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>