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Make fflush (NULL) flush input files (bug 32369)

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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.
This commit is contained in:
Joseph Myers 2025-01-28 21:53:49 +00:00
parent be6818be31
commit 94251ae99e
3 changed files with 102 additions and 0 deletions
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7
libio/genops.c
View file

@ -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;

1
stdio-common/Makefile
View file

@ -238,6 +238,7 @@ tests := \
tst-fdopen \
tst-fdopen2 \
tst-ferror \
tst-fflush-all-input \
tst-fgets \
tst-fgets2 \
tst-fileno \

94
stdio-common/tst-fflush-all-input.c Normal file
View file

@ -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>