[v2] libio: Flush stream at freopen (BZ#21037)

Message ID 1528925590-29895-1-git-send-email-adhemerval.zanella@linaro.org
State New
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  • [v2] libio: Flush stream at freopen (BZ#21037)
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Commit Message

Adhemerval Zanella June 13, 2018, 9:33 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.

	[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/
---
 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

Comments

Andreas Schwab June 14, 2018, 1:20 p.m. | #1
On Jun 13 2018, Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:

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

That file doesn't exist yet.  Where does it come from?

Andreas.
Adhemerval Zanella June 14, 2018, 3:01 p.m. | #2
On 14/06/2018 10:20, Andreas Schwab wrote:
> On Jun 13 2018, Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:
> 
>> 	* libio/tst-memstream.h (FPUTS): New macro.
> 
> That file doesn't exist yet.  Where does it come from?
> 
> Andreas.
> 

Bacause I rebased against a wrong branch.  Below it is an updated patch
(which I fixed some copyright dates as well).

---

	* libio/Makefile (tests): Add tst-memstream4 and tst-wmemstream4.
	* libio/freopen.c (freopen): Sync stream before reopen and adjust to
	new fd_to_filename interface.
	* libio/freopen64.c (freopen64): Likewise.
	* libio/tst-memstream.h: New file.
	* libio/tst-memstream4.c: Likewise.
	* libio/tst-wmemstream4.c: 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.

---

diff --git a/libio/Makefile b/libio/Makefile
index cbe14a8..077bd40 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -59,8 +59,8 @@ tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc   \
 	tst-mmap-eofsync tst-mmap-fflushsync bug-mmap-fflush \
 	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-wmemstream1 tst-wmemstream2 tst-wmemstream3 \
+	tst-memstream1 tst-memstream2 tst-memstream3 tst-memstream4 \
+	tst-wmemstream1 tst-wmemstream2 tst-wmemstream3 tst-wmemstream4 \
 	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 \
diff --git a/libio/freopen.c b/libio/freopen.c
index abf19e9..fa63429 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 ea7ebd3..1daeba6 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
new file mode 100644
index 0000000..b9b02bd
--- /dev/null
+++ b/libio/tst-memstream.h
@@ -0,0 +1,68 @@
+/* Common definitions for open_memstream tests.
+   Copyright (C) 2018 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 <mcheck.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdarg.h>
+#include <errno.h>
+
+#include <support/check.h>
+
+#ifdef TEST_WCHAR
+# include <wchar.h>
+
+/* Straighforward implementation so tst-memstream3 could use check
+   fwrite on open_memstream.  */
+static size_t __attribute__ ((used))
+fwwrite (const void *ptr, size_t size, size_t nmemb, FILE *arq)
+{
+  const wchar_t *wcs = (const wchar_t*) (ptr);
+  for (size_t s = 0; s < size; s++)
+    {
+      for (size_t n = 0; n < nmemb; n++)
+        if (fputwc (wcs[n], arq) == WEOF)
+          return n;
+    }
+  return size * nmemb;
+}
+
+# define CHAR_T wchar_t
+# define W(o) L##o
+# define OPEN_MEMSTREAM open_wmemstream
+# define PRINTF wprintf
+# define FWRITE fwwrite
+# define FPUTC  fputwc
+# define FPUTS  fputws
+# define STRCMP wcscmp
+# define STRLEN wcslen
+#else
+# define CHAR_T char
+# define W(o) o
+# define OPEN_MEMSTREAM open_memstream
+# define PRINTF printf
+# define FWRITE fwrite
+# define FPUTC fputc
+# define FPUTS  fputs
+# define STRCMP strcmp
+# define STRLEN strlen
+#endif
+
+#define S(s) S1 (s)
+#define S1(s) #s
diff --git a/libio/tst-memstream4.c b/libio/tst-memstream4.c
new file mode 100644
index 0000000..43082f1
--- /dev/null
+++ b/libio/tst-memstream4.c
@@ -0,0 +1,62 @@
+/* Test for open_memstream BZ #21037.
+   Copyright (C) 2018 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-wmemstream4.c b/libio/tst-wmemstream4.c
new file mode 100644
index 0000000..8ff146e
--- /dev/null
+++ b/libio/tst-wmemstream4.c
@@ -0,0 +1,20 @@
+/* Test for open_wmemstream BZ #21037.
+   Copyright (C) 2018 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-memstream4.c>
diff --git a/sysdeps/generic/fd_to_filename.h b/sysdeps/generic/fd_to_filename.h
index bacfe5b..c60be86 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 297716b..8381f6d 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;
 }
Paul Eggert June 14, 2018, 4:43 p.m. | #3
On 06/14/2018 08:01 AM, Adhemerval Zanella wrote:
> + char fdfilename[30]; 

The magic number 30 should be turned into a named constant defined in 
fd_to_filename.h, to help prevent future mistakes. Once that is done, 
you can change the signature of fd_to_filename to not pass the size, and 
to require the caller to pass an array of at least size 30, so that 
fd_to_filename need not check for buffer overflow (see below for more on 
this).

> +  const char *gfilename;
> +  if (filename == NULL && fd >= 0)
> +    gfilename = fd_to_filename (fd, fdfilename, sizeof fdfilename)
> +		? fdfilename : NULL;
> +  else
> +    gfilename = filename;

Cleaner would be:

   const char *gfilename
     = filename != NULL ? filename : fd_to_filename (fd, fdfilename);

That is, let fd_to_filename worry about what to do with negative fd, and 
have it return fdfilename or NULL, and don't pass the size (which should 
be that magic number regardless).


> -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;
>   }

The __snprintf would be quite wrong if the string did not fit. Again, I 
suggest simply requiring the buffer to be long enough and not checking 
its length, and sticking with stpcpy + _fitoa_word which should be more 
efficient than __snprintf anyway (or if you prefer simplicity to speed, 
just use sprintf).

The '& (struct stat64) {}' construct looks pretty but is less efficient 
as it makes the compiler zero out the structure unnecessarily, so the 
code should keep doing that struct the old-fashioned way.
Adhemerval Zanella June 14, 2018, 6:28 p.m. | #4
On 14/06/2018 13:43, Paul Eggert wrote:
> On 06/14/2018 08:01 AM, Adhemerval Zanella wrote:
>> + char fdfilename[30]; 
> 
> The magic number 30 should be turned into a named constant defined in fd_to_filename.h, to help prevent future mistakes. Once that is done, you can change the signature of fd_to_filename to not pass the size, and to require the caller to pass an array of at least size 30, so that fd_to_filename need not check for buffer overflow (see below for more on this).
> 
>> +  const char *gfilename;
>> +  if (filename == NULL && fd >= 0)
>> +    gfilename = fd_to_filename (fd, fdfilename, sizeof fdfilename)
>> +        ? fdfilename : NULL;
>> +  else
>> +    gfilename = filename;
> 
> Cleaner would be:
> 
>   const char *gfilename
>     = filename != NULL ? filename : fd_to_filename (fd, fdfilename);
> 
> That is, let fd_to_filename worry about what to do with negative fd, and have it return fdfilename or NULL, and don't pass the size (which should be that magic number regardless).
> 
> 
>> -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;
>>   }
> 
> The __snprintf would be quite wrong if the string did not fit. Again, I suggest simply requiring the buffer to be long enough and not checking its length, and sticking with stpcpy + _fitoa_word which should be more efficient than __snprintf anyway (or if you prefer simplicity to speed, just use sprintf).
> 
> The '& (struct stat64) {}' construct looks pretty but is less efficient as it makes the compiler zero out the structure unnecessarily, so the code should keep doing that struct the old-fashioned way.

Thanks, fixed based on your review:

---

	[BZ #21037]
	* libio/Makefile (tests): Add tst-memstream4 and tst-wmemstream4.
	* libio/freopen.c (freopen): Sync stream before reopen and adjust to
	new fd_to_filename interface.
	* libio/freopen64.c (freopen64): Likewise.
	* libio/tst-memstream.h: New file.
	* libio/tst-memstream4.c: Likewise.
	* libio/tst-wmemstream4.c: 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.

---

diff --git a/libio/Makefile b/libio/Makefile
index cbe14a8..077bd40 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -59,8 +59,8 @@ tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc   \
 	tst-mmap-eofsync tst-mmap-fflushsync bug-mmap-fflush \
 	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-wmemstream1 tst-wmemstream2 tst-wmemstream3 \
+	tst-memstream1 tst-memstream2 tst-memstream3 tst-memstream4 \
+	tst-wmemstream1 tst-wmemstream2 tst-wmemstream3 tst-wmemstream4 \
 	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 \
diff --git a/libio/freopen.c b/libio/freopen.c
index abf19e9..6ce74ae 100644
--- a/libio/freopen.c
+++ b/libio/freopen.c
@@ -24,28 +24,34 @@
    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[FD_TO_FILENAME_SIZE];
+
   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
+    = filename != NULL ? filename : fd_to_filename (fd, fdfilename);
+
   fp->_flags2 |= _IO_FLAGS2_NOCLOSE;
 #if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
   if (&_IO_stdin_used == NULL)
@@ -101,9 +107,6 @@ 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 ea7ebd3..d13c70f 100644
--- a/libio/freopen64.c
+++ b/libio/freopen64.c
@@ -24,27 +24,33 @@
    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[FD_TO_FILENAME_SIZE];
+
   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
+    = filename != NULL ? filename : fd_to_filename (fd, fdfilename);
+
   fp->_flags2 |= _IO_FLAGS2_NOCLOSE;
   _IO_file_close_it (fp);
   _IO_JUMPS_FILE_plus (fp) = &_IO_file_jumps;
@@ -84,8 +90,6 @@ 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
new file mode 100644
index 0000000..b9b02bd
--- /dev/null
+++ b/libio/tst-memstream.h
@@ -0,0 +1,68 @@
+/* Common definitions for open_memstream tests.
+   Copyright (C) 2018 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 <mcheck.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdarg.h>
+#include <errno.h>
+
+#include <support/check.h>
+
+#ifdef TEST_WCHAR
+# include <wchar.h>
+
+/* Straighforward implementation so tst-memstream3 could use check
+   fwrite on open_memstream.  */
+static size_t __attribute__ ((used))
+fwwrite (const void *ptr, size_t size, size_t nmemb, FILE *arq)
+{
+  const wchar_t *wcs = (const wchar_t*) (ptr);
+  for (size_t s = 0; s < size; s++)
+    {
+      for (size_t n = 0; n < nmemb; n++)
+        if (fputwc (wcs[n], arq) == WEOF)
+          return n;
+    }
+  return size * nmemb;
+}
+
+# define CHAR_T wchar_t
+# define W(o) L##o
+# define OPEN_MEMSTREAM open_wmemstream
+# define PRINTF wprintf
+# define FWRITE fwwrite
+# define FPUTC  fputwc
+# define FPUTS  fputws
+# define STRCMP wcscmp
+# define STRLEN wcslen
+#else
+# define CHAR_T char
+# define W(o) o
+# define OPEN_MEMSTREAM open_memstream
+# define PRINTF printf
+# define FWRITE fwrite
+# define FPUTC fputc
+# define FPUTS  fputs
+# define STRCMP strcmp
+# define STRLEN strlen
+#endif
+
+#define S(s) S1 (s)
+#define S1(s) #s
diff --git a/libio/tst-memstream4.c b/libio/tst-memstream4.c
new file mode 100644
index 0000000..43082f1
--- /dev/null
+++ b/libio/tst-memstream4.c
@@ -0,0 +1,62 @@
+/* Test for open_memstream BZ #21037.
+   Copyright (C) 2018 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-wmemstream4.c b/libio/tst-wmemstream4.c
new file mode 100644
index 0000000..8ff146e
--- /dev/null
+++ b/libio/tst-wmemstream4.c
@@ -0,0 +1,20 @@
+/* Test for open_wmemstream BZ #21037.
+   Copyright (C) 2018 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-memstream4.c>
diff --git a/sysdeps/generic/fd_to_filename.h b/sysdeps/generic/fd_to_filename.h
index bacfe5b..d41b345 100644
--- a/sysdeps/generic/fd_to_filename.h
+++ b/sysdeps/generic/fd_to_filename.h
@@ -16,10 +16,12 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+#define FD_TO_FILENAME_SIZE 0
+
 /* In general there is no generic way to query filename for an open
    file descriptor.  */
 static inline const char *
-fd_to_filename (int fd)
+fd_to_filename (int fd, char *buf)
 {
   return NULL;
 }
diff --git a/sysdeps/unix/sysv/linux/fd_to_filename.h b/sysdeps/unix/sysv/linux/fd_to_filename.h
index 297716b..ae88ce8 100644
--- a/sysdeps/unix/sysv/linux/fd_to_filename.h
+++ b/sysdeps/unix/sysv/linux/fd_to_filename.h
@@ -16,30 +16,21 @@
    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>
+
+#define FD_TO_FILENAME_SIZE ((sizeof ("/proc/self/fd/") - 1) \
+			     + (sizeof ("2147483647") - 1) + 1)
 
 static inline const char *
-fd_to_filename (int fd)
+fd_to_filename (int fd, char *buf)
 {
-  char *ret = malloc (30);
-
-  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;
+  *_fitoa_word (fd, __stpcpy (buf, "/proc/self/fd/"), 10, 0) = '\0'; 
+
+  /* We must make sure the file exists.  */
+  struct stat64 st;
+  if (__lxstat64 (_STAT_VER, buf, &st) < 0)
+    /* /proc is not mounted or something else happened.  */
+    return NULL;
+  return buf;
 }

Patch

diff --git a/libio/Makefile b/libio/Makefile
index 077bd40..5b2308e 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -60,8 +60,9 @@  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 tst-bz22415 tst-fgetc-after-eof
diff --git a/libio/freopen.c b/libio/freopen.c
index abf19e9..fa63429 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 ea7ebd3..1daeba6 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 bacfe5b..c60be86 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 297716b..8381f6d 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;
 }