[PATCH 3/6] Make fflush (NULL) flush input files (bug 32369)

[DJ Delorie]

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.