libio: Flush stream at freopen (BZ#21037)

Message ID 1499977587-28794-1-git-send-email-adhemerval.zanella@linaro.org
State New
Headers show

Commit Message

Adhemerval Zanella July 13, 2017, 8:26 p.m.
As POSIX states [1] a freopen call should first flush the stream as
if by a call fflush.  C99 (n1256) and C11 (n1570) only states the
function should first close any file associated with the specific
stream.  Although current implementation only follow C specification,
current BSD and other libc implementation (musl) are in sync with
POSIX and fflush the stream.

This patch change freopen{64} to fflush the stream before actually
reopening it (or returning if the stream does not support reopen).
It also changes the Linux implementation to avoid a dynamic
allocation on 'fd_to_filename'.

Checked on x86_64-linux-gnu.

From Florian's comment, this is intended for 2.27.  This patch should
be applied on top on 'libio: Fix open_memstream fflush (NULL) (BZ#21735)' [2].

	[BZ #21037]
	* libio/freopen.c (freopen): Sync stream before reopen and adjust to
	new fd_to_filename interface.
	* libio/freopen64.c (freopen64): Likewise.
	* sysdeps/generic/fd_to_filename.h (fd_to_filename): Change signature.
	* sysdeps/unix/sysv/linux/fd_to_filename.h (fd_to_filename): Likewise
	and remove internal dynamic allocation.
	* libio/Makefile (tests): Add tst-memstream5 and tst-wmemstream5.
	* libio/tst-memstream.h (FPUTS): New macro.
	* libio/tst-memstream4.c: New file.
	* libio/tst-wmemstream4.c: Likewise.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/
[2] https://sourceware.org/ml/libc-alpha/2017-07/msg00548.html
---
 ChangeLog                                | 14 ++++++++
 libio/Makefile                           |  3 +-
 libio/freopen.c                          | 33 ++++++++++-------
 libio/freopen64.c                        | 30 ++++++++++------
 libio/tst-memstream.h                    |  2 ++
 libio/tst-memstream5.c                   | 62 ++++++++++++++++++++++++++++++++
 libio/tst-wmemstream5.c                  | 20 +++++++++++
 sysdeps/generic/fd_to_filename.h         |  6 ++--
 sysdeps/unix/sysv/linux/fd_to_filename.h | 31 +++++-----------
 9 files changed, 151 insertions(+), 50 deletions(-)
 create mode 100644 libio/tst-memstream5.c
 create mode 100644 libio/tst-wmemstream5.c

-- 
2.7.4

Comments

Adhemerval Zanella Aug. 8, 2017, 1:52 p.m. | #1
Ping.

On 13/07/2017 17:26, Adhemerval Zanella wrote:
> As POSIX states [1] a freopen call should first flush the stream as

> if by a call fflush.  C99 (n1256) and C11 (n1570) only states the

> function should first close any file associated with the specific

> stream.  Although current implementation only follow C specification,

> current BSD and other libc implementation (musl) are in sync with

> POSIX and fflush the stream.

> 

> This patch change freopen{64} to fflush the stream before actually

> reopening it (or returning if the stream does not support reopen).

> It also changes the Linux implementation to avoid a dynamic

> allocation on 'fd_to_filename'.

> 

> Checked on x86_64-linux-gnu.

> 

> From Florian's comment, this is intended for 2.27.  This patch should

> be applied on top on 'libio: Fix open_memstream fflush (NULL) (BZ#21735)' [2].

> 

> 	[BZ #21037]

> 	* libio/freopen.c (freopen): Sync stream before reopen and adjust to

> 	new fd_to_filename interface.

> 	* libio/freopen64.c (freopen64): Likewise.

> 	* sysdeps/generic/fd_to_filename.h (fd_to_filename): Change signature.

> 	* sysdeps/unix/sysv/linux/fd_to_filename.h (fd_to_filename): Likewise

> 	and remove internal dynamic allocation.

> 	* libio/Makefile (tests): Add tst-memstream5 and tst-wmemstream5.

> 	* libio/tst-memstream.h (FPUTS): New macro.

> 	* libio/tst-memstream4.c: New file.

> 	* libio/tst-wmemstream4.c: Likewise.

> 

> [1] http://pubs.opengroup.org/onlinepubs/9699919799/

> [2] https://sourceware.org/ml/libc-alpha/2017-07/msg00548.html

> ---

>  ChangeLog                                | 14 ++++++++

>  libio/Makefile                           |  3 +-

>  libio/freopen.c                          | 33 ++++++++++-------

>  libio/freopen64.c                        | 30 ++++++++++------

>  libio/tst-memstream.h                    |  2 ++

>  libio/tst-memstream5.c                   | 62 ++++++++++++++++++++++++++++++++

>  libio/tst-wmemstream5.c                  | 20 +++++++++++

>  sysdeps/generic/fd_to_filename.h         |  6 ++--

>  sysdeps/unix/sysv/linux/fd_to_filename.h | 31 +++++-----------

>  9 files changed, 151 insertions(+), 50 deletions(-)

>  create mode 100644 libio/tst-memstream5.c

>  create mode 100644 libio/tst-wmemstream5.c

> 

> diff --git a/libio/Makefile b/libio/Makefile

> index 6ee809a..8adf2d8 100644

> --- a/libio/Makefile

> +++ b/libio/Makefile

> @@ -58,8 +58,9 @@ tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc   \

>  	tst-mmap2-eofsync tst-mmap-offend bug-fopena+ bug-wfflush \

>  	bug-ungetc2 bug-ftell bug-ungetc3 bug-ungetc4 tst-fopenloc2 \

>  	tst-memstream1 tst-memstream2 tst-memstream3 tst-memstream4 \

> +	tst-memstream5 \

>  	tst-wmemstream1 tst-wmemstream2 tst-wmemstream3 tst-wmemstream4 \

> -	bug-memstream1 bug-wmemstream1 \

> +	tst-wmemstream5 bug-memstream1 bug-wmemstream1 \

>  	tst-setvbuf1 tst-popen1 tst-fgetwc bug-wsetpos tst-fseek \

>  	tst-fwrite-error tst-ftell-partial-wide tst-ftell-active-handler \

>  	tst-ftell-append tst-fputws

> diff --git a/libio/freopen.c b/libio/freopen.c

> index 980523a..505e77d 100644

> --- a/libio/freopen.c

> +++ b/libio/freopen.c

> @@ -24,28 +24,38 @@

>     This exception applies to code released by its copyright holders

>     in files containing the exception.  */

>  

> -#include "libioP.h"

> -#include "stdio.h"

> +#include <stdio.h>

>  #include <fcntl.h>

>  #include <stdlib.h>

>  #include <unistd.h>

>  

> -#include <shlib-compat.h>

> +#include <libioP.h>

>  #include <fd_to_filename.h>

> -

> -#include <kernel-features.h>

> +#include <shlib-compat.h>

>  

>  FILE *

>  freopen (const char *filename, const char *mode, FILE *fp)

>  {

> -  FILE *result;

> +  FILE *result = NULL;

> +  char fdfilename[30];

> +

>    CHECK_FILE (fp, NULL);

> -  if (!(fp->_flags & _IO_IS_FILEBUF))

> -    return NULL;

> +

>    _IO_acquire_lock (fp);

> +  /* First flush the stream (failure should be ignored).  */

> +  _IO_SYNC (fp);

> +

> +  if (!(fp->_flags & _IO_IS_FILEBUF))

> +    goto end;

> +

>    int fd = _IO_fileno (fp);

> -  const char *gfilename = (filename == NULL && fd >= 0

> -			   ? fd_to_filename (fd) : filename);

> +  const char *gfilename;

> +  if (filename == NULL && fd >= 0)

> +    gfilename = fd_to_filename (fd, fdfilename, sizeof fdfilename)

> +		? fdfilename : NULL;

> +  else

> +    gfilename = filename;

> +

>    fp->_flags2 |= _IO_FLAGS2_NOCLOSE;

>  #if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)

>    if (&_IO_stdin_used == NULL)

> @@ -101,9 +111,6 @@ freopen (const char *filename, const char *mode, FILE *fp)

>      __close (fd);

>  

>  end:

> -  if (filename == NULL)

> -    free ((char *) gfilename);

> -

>    _IO_release_lock (fp);

>    return result;

>  }

> diff --git a/libio/freopen64.c b/libio/freopen64.c

> index 1e56de6..28203c9 100644

> --- a/libio/freopen64.c

> +++ b/libio/freopen64.c

> @@ -24,27 +24,37 @@

>     This exception applies to code released by its copyright holders

>     in files containing the exception.  */

>  

> -#include "libioP.h"

> -#include "stdio.h"

> +#include <stdio.h>

>  #include <fcntl.h>

>  #include <stdlib.h>

>  #include <unistd.h>

>  

> +#include <libioP.h>

>  #include <fd_to_filename.h>

>  

> -#include <kernel-features.h>

> -

>  FILE *

>  freopen64 (const char *filename, const char *mode, FILE *fp)

>  {

> -  FILE *result;

> +  FILE *result = NULL;

> +  char fdfilename[30];

> +

>    CHECK_FILE (fp, NULL);

> -  if (!(fp->_flags & _IO_IS_FILEBUF))

> -    return NULL;

> +

>    _IO_acquire_lock (fp);

> +  /* First flush the stream (failure should be ignored).  */

> +  _IO_SYNC (fp);

> +

> +  if (!(fp->_flags & _IO_IS_FILEBUF))

> +    goto end;

> +

>    int fd = _IO_fileno (fp);

> -  const char *gfilename = (filename == NULL && fd >= 0

> -			   ? fd_to_filename (fd) : filename);

> +  const char *gfilename;

> +  if (filename == NULL && fd >= 0)

> +    gfilename = fd_to_filename (fd, fdfilename, sizeof fdfilename)

> +		? fdfilename : NULL;

> +  else

> +    gfilename = filename;

> +

>    fp->_flags2 |= _IO_FLAGS2_NOCLOSE;

>    _IO_file_close_it (fp);

>    _IO_JUMPS_FILE_plus (fp) = &_IO_file_jumps;

> @@ -84,8 +94,6 @@ freopen64 (const char *filename, const char *mode, FILE *fp)

>      __close (fd);

>  

>  end:

> -  if (filename == NULL)

> -    free ((char *) gfilename);

>    _IO_release_lock (fp);

>    return result;

>  }

> diff --git a/libio/tst-memstream.h b/libio/tst-memstream.h

> index ffc2a65..86ea0d7 100644

> --- a/libio/tst-memstream.h

> +++ b/libio/tst-memstream.h

> @@ -49,6 +49,7 @@ fwwrite (const void *ptr, size_t size, size_t nmemb, FILE *arq)

>  # define PRINTF wprintf

>  # define FWRITE fwwrite

>  # define FPUTC  fputwc

> +# define FPUTS  fputws

>  # define STRCMP wcscmp

>  # define STRLEN wcslen

>  #else

> @@ -58,6 +59,7 @@ fwwrite (const void *ptr, size_t size, size_t nmemb, FILE *arq)

>  # define PRINTF printf

>  # define FWRITE fwrite

>  # define FPUTC fputc

> +# define FPUTS  fputs

>  # define STRCMP strcmp

>  # define STRLEN strlen

>  #endif

> diff --git a/libio/tst-memstream5.c b/libio/tst-memstream5.c

> new file mode 100644

> index 0000000..7c1f37c

> --- /dev/null

> +++ b/libio/tst-memstream5.c

> @@ -0,0 +1,62 @@

> +/* Test for open_memstream BZ #21037.

> +   Copyright (C) 2017 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

> +   <http://www.gnu.org/licenses/>.  */

> +

> +#include "tst-memstream.h"

> +

> +static void

> +mcheck_abort (enum mcheck_status ev)

> +{

> +  printf ("mecheck failed with status %d\n", (int) ev);

> +  exit (1);

> +}

> +

> +static int

> +do_test (void)

> +{

> +  mcheck_pedantic (mcheck_abort);

> +

> +  /* Check if freopen proper fflush the stream.  */

> +  {

> +    CHAR_T old[] = W("old");

> +    CHAR_T *buf = old;

> +    size_t size;

> +

> +    FILE *fp = OPEN_MEMSTREAM (&buf, &size);

> +    TEST_VERIFY_EXIT (fp != NULL);

> +

> +    FPUTS (W("new"), fp);

> +    /* The stream buffer pointer should be updated with only a fflush or

> +       fclose.  */

> +    TEST_VERIFY (STRCMP (buf, old) == 0);

> +

> +    /* The old stream should be fflush the stream, even for an invalid

> +       streams.  */

> +    FILE *nfp = freopen ("invalid-file", "r", fp);

> +    TEST_VERIFY (nfp == NULL);

> +

> +    TEST_VERIFY (STRCMP (buf, W("new")) == 0);

> +

> +    TEST_VERIFY (fclose (fp) == 0);

> +

> +    free (buf);

> +  }

> +

> +  return 0;

> +}

> +

> +#include <support/test-driver.c>

> diff --git a/libio/tst-wmemstream5.c b/libio/tst-wmemstream5.c

> new file mode 100644

> index 0000000..4e87289

> --- /dev/null

> +++ b/libio/tst-wmemstream5.c

> @@ -0,0 +1,20 @@

> +/* Test for open_wmemstream BZ #21037.

> +   Copyright (C) 2017 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

> +   <http://www.gnu.org/licenses/>.  */

> +

> +#define TEST_WCHAR

> +#include <libio/tst-memstream5.c>

> diff --git a/sysdeps/generic/fd_to_filename.h b/sysdeps/generic/fd_to_filename.h

> index fe15f5d..db594f2 100644

> --- a/sysdeps/generic/fd_to_filename.h

> +++ b/sysdeps/generic/fd_to_filename.h

> @@ -18,8 +18,8 @@

>  

>  /* In general there is no generic way to query filename for an open

>     file descriptor.  */

> -static inline const char *

> -fd_to_filename (int fd)

> +static inline bool

> +fd_to_filename (int fd, char *buf, size_t len)

>  {

> -  return NULL;

> +  return false;

>  }

> diff --git a/sysdeps/unix/sysv/linux/fd_to_filename.h b/sysdeps/unix/sysv/linux/fd_to_filename.h

> index 414a30f..6cd0bdd 100644

> --- a/sysdeps/unix/sysv/linux/fd_to_filename.h

> +++ b/sysdeps/unix/sysv/linux/fd_to_filename.h

> @@ -16,30 +16,17 @@

>     License along with the GNU C Library; if not, see

>     <http://www.gnu.org/licenses/>.  */

>  

> -#include <stdlib.h>

> -#include <string.h>

> +#include <stdio.h>

>  #include <sys/stat.h>

> -#include <_itoa.h>

>  

> -static inline const char *

> -fd_to_filename (int fd)

> +static inline bool

> +fd_to_filename (int fd, char *buf, size_t len)

>  {

> -  char *ret = malloc (30);

> +  __snprintf (buf, len, "/proc/self/fd/%d", fd);

>  

> -  if (ret != NULL)

> -    {

> -      struct stat64 st;

> -

> -      *_fitoa_word (fd, __stpcpy (ret, "/proc/self/fd/"), 10, 0) = '\0';

> -

> -      /* We must make sure the file exists.  */

> -      if (__lxstat64 (_STAT_VER, ret, &st) < 0)

> -	{

> -	  /* /proc is not mounted or something else happened.  Don't

> -	     return the file name.  */

> -	  free (ret);

> -	  ret = NULL;

> -	}

> -    }

> -  return ret;

> +  /* We must make sure the file exists.  */

> +  if (__lxstat64 (_STAT_VER, buf, & (struct stat64) {}) < 0)

> +    /* /proc is not mounted or something else happened.  */

> +    return false;

> +  return true;

>  }

>

Patch hide | download patch | download mbox

diff --git a/libio/Makefile b/libio/Makefile
index 6ee809a..8adf2d8 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -58,8 +58,9 @@  tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc   \
 	tst-mmap2-eofsync tst-mmap-offend bug-fopena+ bug-wfflush \
 	bug-ungetc2 bug-ftell bug-ungetc3 bug-ungetc4 tst-fopenloc2 \
 	tst-memstream1 tst-memstream2 tst-memstream3 tst-memstream4 \
+	tst-memstream5 \
 	tst-wmemstream1 tst-wmemstream2 tst-wmemstream3 tst-wmemstream4 \
-	bug-memstream1 bug-wmemstream1 \
+	tst-wmemstream5 bug-memstream1 bug-wmemstream1 \
 	tst-setvbuf1 tst-popen1 tst-fgetwc bug-wsetpos tst-fseek \
 	tst-fwrite-error tst-ftell-partial-wide tst-ftell-active-handler \
 	tst-ftell-append tst-fputws
diff --git a/libio/freopen.c b/libio/freopen.c
index 980523a..505e77d 100644
--- a/libio/freopen.c
+++ b/libio/freopen.c
@@ -24,28 +24,38 @@ 
    This exception applies to code released by its copyright holders
    in files containing the exception.  */
 
-#include "libioP.h"
-#include "stdio.h"
+#include <stdio.h>
 #include <fcntl.h>
 #include <stdlib.h>
 #include <unistd.h>
 
-#include <shlib-compat.h>
+#include <libioP.h>
 #include <fd_to_filename.h>
-
-#include <kernel-features.h>
+#include <shlib-compat.h>
 
 FILE *
 freopen (const char *filename, const char *mode, FILE *fp)
 {
-  FILE *result;
+  FILE *result = NULL;
+  char fdfilename[30];
+
   CHECK_FILE (fp, NULL);
-  if (!(fp->_flags & _IO_IS_FILEBUF))
-    return NULL;
+
   _IO_acquire_lock (fp);
+  /* First flush the stream (failure should be ignored).  */
+  _IO_SYNC (fp);
+
+  if (!(fp->_flags & _IO_IS_FILEBUF))
+    goto end;
+
   int fd = _IO_fileno (fp);
-  const char *gfilename = (filename == NULL && fd >= 0
-			   ? fd_to_filename (fd) : filename);
+  const char *gfilename;
+  if (filename == NULL && fd >= 0)
+    gfilename = fd_to_filename (fd, fdfilename, sizeof fdfilename)
+		? fdfilename : NULL;
+  else
+    gfilename = filename;
+
   fp->_flags2 |= _IO_FLAGS2_NOCLOSE;
 #if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
   if (&_IO_stdin_used == NULL)
@@ -101,9 +111,6 @@  freopen (const char *filename, const char *mode, FILE *fp)
     __close (fd);
 
 end:
-  if (filename == NULL)
-    free ((char *) gfilename);
-
   _IO_release_lock (fp);
   return result;
 }
diff --git a/libio/freopen64.c b/libio/freopen64.c
index 1e56de6..28203c9 100644
--- a/libio/freopen64.c
+++ b/libio/freopen64.c
@@ -24,27 +24,37 @@ 
    This exception applies to code released by its copyright holders
    in files containing the exception.  */
 
-#include "libioP.h"
-#include "stdio.h"
+#include <stdio.h>
 #include <fcntl.h>
 #include <stdlib.h>
 #include <unistd.h>
 
+#include <libioP.h>
 #include <fd_to_filename.h>
 
-#include <kernel-features.h>
-
 FILE *
 freopen64 (const char *filename, const char *mode, FILE *fp)
 {
-  FILE *result;
+  FILE *result = NULL;
+  char fdfilename[30];
+
   CHECK_FILE (fp, NULL);
-  if (!(fp->_flags & _IO_IS_FILEBUF))
-    return NULL;
+
   _IO_acquire_lock (fp);
+  /* First flush the stream (failure should be ignored).  */
+  _IO_SYNC (fp);
+
+  if (!(fp->_flags & _IO_IS_FILEBUF))
+    goto end;
+
   int fd = _IO_fileno (fp);
-  const char *gfilename = (filename == NULL && fd >= 0
-			   ? fd_to_filename (fd) : filename);
+  const char *gfilename;
+  if (filename == NULL && fd >= 0)
+    gfilename = fd_to_filename (fd, fdfilename, sizeof fdfilename)
+		? fdfilename : NULL;
+  else
+    gfilename = filename;
+
   fp->_flags2 |= _IO_FLAGS2_NOCLOSE;
   _IO_file_close_it (fp);
   _IO_JUMPS_FILE_plus (fp) = &_IO_file_jumps;
@@ -84,8 +94,6 @@  freopen64 (const char *filename, const char *mode, FILE *fp)
     __close (fd);
 
 end:
-  if (filename == NULL)
-    free ((char *) gfilename);
   _IO_release_lock (fp);
   return result;
 }
diff --git a/libio/tst-memstream.h b/libio/tst-memstream.h
index ffc2a65..86ea0d7 100644
--- a/libio/tst-memstream.h
+++ b/libio/tst-memstream.h
@@ -49,6 +49,7 @@  fwwrite (const void *ptr, size_t size, size_t nmemb, FILE *arq)
 # define PRINTF wprintf
 # define FWRITE fwwrite
 # define FPUTC  fputwc
+# define FPUTS  fputws
 # define STRCMP wcscmp
 # define STRLEN wcslen
 #else
@@ -58,6 +59,7 @@  fwwrite (const void *ptr, size_t size, size_t nmemb, FILE *arq)
 # define PRINTF printf
 # define FWRITE fwrite
 # define FPUTC fputc
+# define FPUTS  fputs
 # define STRCMP strcmp
 # define STRLEN strlen
 #endif
diff --git a/libio/tst-memstream5.c b/libio/tst-memstream5.c
new file mode 100644
index 0000000..7c1f37c
--- /dev/null
+++ b/libio/tst-memstream5.c
@@ -0,0 +1,62 @@ 
+/* Test for open_memstream BZ #21037.
+   Copyright (C) 2017 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include "tst-memstream.h"
+
+static void
+mcheck_abort (enum mcheck_status ev)
+{
+  printf ("mecheck failed with status %d\n", (int) ev);
+  exit (1);
+}
+
+static int
+do_test (void)
+{
+  mcheck_pedantic (mcheck_abort);
+
+  /* Check if freopen proper fflush the stream.  */
+  {
+    CHAR_T old[] = W("old");
+    CHAR_T *buf = old;
+    size_t size;
+
+    FILE *fp = OPEN_MEMSTREAM (&buf, &size);
+    TEST_VERIFY_EXIT (fp != NULL);
+
+    FPUTS (W("new"), fp);
+    /* The stream buffer pointer should be updated with only a fflush or
+       fclose.  */
+    TEST_VERIFY (STRCMP (buf, old) == 0);
+
+    /* The old stream should be fflush the stream, even for an invalid
+       streams.  */
+    FILE *nfp = freopen ("invalid-file", "r", fp);
+    TEST_VERIFY (nfp == NULL);
+
+    TEST_VERIFY (STRCMP (buf, W("new")) == 0);
+
+    TEST_VERIFY (fclose (fp) == 0);
+
+    free (buf);
+  }
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/libio/tst-wmemstream5.c b/libio/tst-wmemstream5.c
new file mode 100644
index 0000000..4e87289
--- /dev/null
+++ b/libio/tst-wmemstream5.c
@@ -0,0 +1,20 @@ 
+/* Test for open_wmemstream BZ #21037.
+   Copyright (C) 2017 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
+   <http://www.gnu.org/licenses/>.  */
+
+#define TEST_WCHAR
+#include <libio/tst-memstream5.c>
diff --git a/sysdeps/generic/fd_to_filename.h b/sysdeps/generic/fd_to_filename.h
index fe15f5d..db594f2 100644
--- a/sysdeps/generic/fd_to_filename.h
+++ b/sysdeps/generic/fd_to_filename.h
@@ -18,8 +18,8 @@ 
 
 /* In general there is no generic way to query filename for an open
    file descriptor.  */
-static inline const char *
-fd_to_filename (int fd)
+static inline bool
+fd_to_filename (int fd, char *buf, size_t len)
 {
-  return NULL;
+  return false;
 }
diff --git a/sysdeps/unix/sysv/linux/fd_to_filename.h b/sysdeps/unix/sysv/linux/fd_to_filename.h
index 414a30f..6cd0bdd 100644
--- a/sysdeps/unix/sysv/linux/fd_to_filename.h
+++ b/sysdeps/unix/sysv/linux/fd_to_filename.h
@@ -16,30 +16,17 @@ 
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
-#include <stdlib.h>
-#include <string.h>
+#include <stdio.h>
 #include <sys/stat.h>
-#include <_itoa.h>
 
-static inline const char *
-fd_to_filename (int fd)
+static inline bool
+fd_to_filename (int fd, char *buf, size_t len)
 {
-  char *ret = malloc (30);
+  __snprintf (buf, len, "/proc/self/fd/%d", fd);
 
-  if (ret != NULL)
-    {
-      struct stat64 st;
-
-      *_fitoa_word (fd, __stpcpy (ret, "/proc/self/fd/"), 10, 0) = '\0';
-
-      /* We must make sure the file exists.  */
-      if (__lxstat64 (_STAT_VER, ret, &st) < 0)
-	{
-	  /* /proc is not mounted or something else happened.  Don't
-	     return the file name.  */
-	  free (ret);
-	  ret = NULL;
-	}
-    }
-  return ret;
+  /* We must make sure the file exists.  */
+  if (__lxstat64 (_STAT_VER, buf, & (struct stat64) {}) < 0)
+    /* /proc is not mounted or something else happened.  */
+    return false;
+  return true;
 }