diff --git a/ChangeLog b/ChangeLog
index 205da06..28d9012 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,23 @@
+2016-08-05  Adhemerval Zanella  <adhemerval.zanella@linaro.org>
+
+	[BZ #18241]
+	[BZ #20181]
+	* libio/Makefile (test): Add tst-memstream3 and tst-wmemstream3.
+	* libio/memstream.c (_IO_mem_sync): Only append a null byte if
+	write position is at the end the buffer.
+	* libio/wmemstream.c (_IO_wmem_sync): Likewise.
+	* libio/strops.c (_IO_str_switch_to_get_mode): New function.
+	(_IO_str_seekoff): Set correct offset from negative displacement and
+	set EINVAL for invalid ones.
+	* libio/wstrops.c (enlarge_userbuf): Use correct function to calculate
+	buffer length.
+	(_IO_wstr_switch_to_get_mode): New function.
+	(_IO_wstr_seekoff): Set correct offset from negative displacement and
+	set EINVAL for invalid ones.
+	* libio/tst-memstream3.c: New file.
+	* libio/tst-wmemstream3.c: Likewise.
+	* manual/examples/memstrm.c: Remove warning when priting size_t.
+
 2016-08-05  Torvald Riegel  <triegel@redhat.com>
 
 	* include/atomic.h (atomic_exchange_relaxed): New.
diff --git a/libio/Makefile b/libio/Makefile
index 12589f2..0c7751c 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -56,8 +56,8 @@ 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-wmemstream1 tst-wmemstream2 \
+	tst-memstream1 tst-memstream2 tst-memstream3 \
+	tst-wmemstream1 tst-wmemstream2 tst-wmemstream3 \
 	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/memstream.c b/libio/memstream.c
index e20b9c2..f1e8d58 100644
--- a/libio/memstream.c
+++ b/libio/memstream.c
@@ -112,8 +112,6 @@ _IO_mem_sync (_IO_FILE *fp)
       _IO_str_overflow (fp, '\0');
       --fp->_IO_write_ptr;
     }
-  else
-    *fp->_IO_write_ptr = '\0';
 
   *mp->bufloc = fp->_IO_write_base;
   *mp->sizeloc = fp->_IO_write_ptr - fp->_IO_write_base;
diff --git a/libio/strops.c b/libio/strops.c
index 2ba3704..1bb8a77 100644
--- a/libio/strops.c
+++ b/libio/strops.c
@@ -230,6 +230,21 @@ enlarge_userbuf (_IO_FILE *fp, _IO_off64_t offset, int reading)
   return 0;
 }
 
+static void
+_IO_str_switch_to_get_mode (_IO_FILE *fp)
+{
+  if (_IO_in_backup (fp))
+    fp->_IO_read_base = fp->_IO_backup_base;
+  else
+    {
+      fp->_IO_read_base = fp->_IO_buf_base;
+      if (fp->_IO_write_ptr > fp->_IO_read_end)
+        fp->_IO_read_end = fp->_IO_write_ptr;
+    }
+  fp->_IO_read_ptr = fp->_IO_read_end = fp->_IO_write_ptr;
+
+  fp->_flags &= ~_IO_CURRENTLY_PUTTING;
+}
 
 _IO_off64_t
 _IO_str_seekoff (_IO_FILE *fp, _IO_off64_t offset, int dir, int mode)
@@ -239,14 +254,14 @@ _IO_str_seekoff (_IO_FILE *fp, _IO_off64_t offset, int dir, int mode)
   if (mode == 0 && (fp->_flags & _IO_TIED_PUT_GET))
     mode = (fp->_flags & _IO_CURRENTLY_PUTTING ? _IOS_OUTPUT : _IOS_INPUT);
 
+  bool was_writing = (fp->_IO_write_ptr > fp->_IO_write_base
+		     || _IO_in_put_mode (fp));
+  if (was_writing)
+    _IO_str_switch_to_get_mode (fp);
+
   if (mode == 0)
     {
-      /* Don't move any pointers. But there is no clear indication what
-	 mode FP is in. Let's guess. */
-      if (fp->_IO_file_flags & _IO_NO_WRITES)
-        new_pos = fp->_IO_read_ptr - fp->_IO_read_base;
-      else
-        new_pos = fp->_IO_write_ptr - fp->_IO_write_base;
+      new_pos = fp->_IO_read_ptr - fp->_IO_read_base;
     }
   else
     {
@@ -256,48 +271,62 @@ _IO_str_seekoff (_IO_FILE *fp, _IO_off64_t offset, int dir, int mode)
       /* Move the get pointer, if requested. */
       if (mode & _IOS_INPUT)
 	{
+	  _IO_ssize_t base;
 	  switch (dir)
 	    {
-	    case _IO_seek_end:
-	      offset += cur_size;
+	    case _IO_seek_set:
+	      base = 0;
 	      break;
 	    case _IO_seek_cur:
-	      offset += fp->_IO_read_ptr - fp->_IO_read_base;
+	      base = fp->_IO_read_ptr - fp->_IO_read_base;
 	      break;
-	    default: /* case _IO_seek_set: */
+	    default: /* case _IO_seek_end: */
+	      base = cur_size;
 	      break;
 	    }
-	  if (offset < 0)
-	    return EOF;
-	  if ((_IO_ssize_t) offset > cur_size
-	      && enlarge_userbuf (fp, offset, 1) != 0)
+	  _IO_ssize_t maxval = SSIZE_MAX - base;
+	  if (offset < -base || offset > maxval)
+	    {
+	      __set_errno (EINVAL);
+	      return EOF;
+	    }
+	  base += offset;
+	  if (base > cur_size
+	      && enlarge_userbuf (fp, base, 1) != 0)
 	    return EOF;
-	  fp->_IO_read_ptr = fp->_IO_read_base + offset;
+	  fp->_IO_read_ptr = fp->_IO_read_base + base;
 	  fp->_IO_read_end = fp->_IO_read_base + cur_size;
-	  new_pos = offset;
+	  new_pos = base;
 	}
 
       /* Move the put pointer, if requested. */
       if (mode & _IOS_OUTPUT)
 	{
+	  _IO_ssize_t base;
 	  switch (dir)
 	    {
-	    case _IO_seek_end:
-	      offset += cur_size;
+	    case _IO_seek_set:
+	      base = 0;
 	      break;
 	    case _IO_seek_cur:
-	      offset += fp->_IO_write_ptr - fp->_IO_write_base;
+	      base = fp->_IO_write_ptr - fp->_IO_write_base;
 	      break;
-	    default: /* case _IO_seek_set: */
+	    default: /* case _IO_seek_end: */
+	      base = cur_size;
 	      break;
 	    }
-	  if (offset < 0)
-	    return EOF;
-	  if ((_IO_ssize_t) offset > cur_size
-	      && enlarge_userbuf (fp, offset, 0) != 0)
+	  _IO_ssize_t maxval = SSIZE_MAX - base;
+	  if (offset < -base || offset > maxval)
+	    {
+	      __set_errno (EINVAL);
+	      return EOF;
+	    }
+	  base += offset;
+	  if (base > cur_size
+	      && enlarge_userbuf (fp, base, 0) != 0)
 	    return EOF;
-	  fp->_IO_write_ptr = fp->_IO_write_base + offset;
-	  new_pos = offset;
+	  fp->_IO_write_ptr = fp->_IO_write_base + base;
+	  new_pos = base;
 	}
     }
   return new_pos;
diff --git a/libio/tst-memstream3.c b/libio/tst-memstream3.c
new file mode 100644
index 0000000..34b04e5
--- /dev/null
+++ b/libio/tst-memstream3.c
@@ -0,0 +1,165 @@
+/* Test for open_memstream implementation.
+   Copyright (C) 2016 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 <stdarg.h>
+#include <errno.h>
+
+
+#ifndef CHAR_T
+# define CHAR_T char
+# define W(o) o
+# define OPEN_MEMSTREAM open_memstream
+# define PRINTF printf
+# define FWRITE fwrite
+# define FPUTC fputc
+# define STRCMP strcmp
+#endif
+
+#define S(s) S1 (s)
+#define S1(s) #s
+
+static void
+mcheck_abort (enum mcheck_status ev)
+{
+  printf ("mecheck failed with status %d\n", (int) ev);
+  exit (1);
+}
+
+static void
+error_printf (int line, const char *fmt, ...)
+{
+  va_list ap;
+
+  printf ("error: %s:%i: ", __FILE__, line);
+  va_start (ap, fmt);
+  vprintf (fmt, ap);
+  va_end (ap);
+}
+
+#define ERROR_RET1(...) \
+  { error_printf(__LINE__, __VA_ARGS__); return 1; }
+
+static int
+do_test_bz18241 (void)
+{
+  CHAR_T *buf;
+  size_t size;
+
+  FILE *fp = OPEN_MEMSTREAM (&buf, &size);
+  if (fp == NULL)
+    ERROR_RET1 ("%s failed\n", S(OPEN_MEMSTREAM));
+
+  if (FPUTC (W('a'), fp) != W('a'))
+    ERROR_RET1 ("%s failed (errno = %d)\n", S(FPUTC), errno);
+  if (fflush (fp) != 0)
+    ERROR_RET1 ("fflush failed (errno = %d)\n", errno);
+  if (fseek (fp, -2, SEEK_SET) != -1)
+    ERROR_RET1 ("fseek failed (errno = %d)\n", errno);
+  if (errno != EINVAL)
+    ERROR_RET1 ("errno != EINVAL\n");
+  if (ftell (fp) != 1)
+    ERROR_RET1 ("ftell failed (errno = %d)\n", errno);
+  if (ferror (fp) != 0)
+    ERROR_RET1 ("ferror != 0\n");
+
+  if (fseek (fp, -1, SEEK_CUR) == -1)
+    ERROR_RET1 ("fseek failed (errno = %d)\n", errno);
+  if (ftell (fp) != 0)
+    ERROR_RET1 ("ftell failed (errno = %d)\n", errno);
+  if (ferror (fp) != 0)
+    ERROR_RET1 ("ferror != 0\n");
+  if (FPUTC (W('b'), fp) != W('b'))
+    ERROR_RET1 ("%s failed (errno = %d)\n", S(FPUTC), errno);
+  if (fflush (fp) != 0)
+    ERROR_RET1 ("fflush failed (errno = %d)\n", errno);
+
+  if (fclose (fp) != 0)
+    ERROR_RET1 ("fclose failed (errno = %d\n", errno);
+
+  if (STRCMP (buf, W("b")) != 0)
+    ERROR_RET1 ("%s failed\n", S(STRCMP));
+
+  free (buf);
+
+  return 0;
+}
+
+static int
+do_test_bz20181 (void)
+{
+  CHAR_T *buf;
+  size_t size;
+  size_t ret;
+
+  FILE *fp = OPEN_MEMSTREAM (&buf, &size);
+  if (fp == NULL)
+    ERROR_RET1 ("%s failed\n", S(OPEN_MEMSTREAM));
+
+  if ((ret = FWRITE (W("abc"), 1, 3, fp)) != 3)
+    ERROR_RET1 ("%s failed (errno = %d)\n", S(FWRITE), errno);
+
+  if (fseek (fp, 0, SEEK_SET) != 0)
+    ERROR_RET1 ("fseek failed (errno = %d)\n", errno);
+
+  if (FWRITE (W("z"), 1, 1, fp) != 1)
+    ERROR_RET1 ("%s failed (errno = %d)\n", S(FWRITE), errno);
+
+  if (fflush (fp) != 0)
+    ERROR_RET1 ("fflush failed (errno = %d)\n", errno);
+
+  /* Avoid truncating the buffer on close.  */
+  if (fseek (fp, 3, SEEK_SET) != 0)
+    ERROR_RET1 ("fseek failed (errno = %d)\n", errno);
+
+  if (fclose (fp) != 0)
+    ERROR_RET1 ("fclose failed (errno = %d\n", errno);
+
+  if (size != 3)
+    ERROR_RET1 ("size != 3\n");
+
+  if (buf[0] != W('z')
+      || buf[1] != W('b')
+      || buf[2] != W('c'))
+    {
+      PRINTF (W("error: buf {%c,%c,%c} != {z,b,c}\n"),
+	      buf[0], buf[1], buf[2]);
+      return 1;
+    }
+    
+  free (buf);
+
+  return 0;
+}
+
+static int
+do_test (void)
+{
+  int ret = 0;
+
+  mcheck_pedantic (mcheck_abort);
+
+  ret += do_test_bz18241 ();
+  ret += do_test_bz20181 ();
+
+  return ret;
+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
diff --git a/libio/tst-wmemstream3.c b/libio/tst-wmemstream3.c
new file mode 100644
index 0000000..190283a
--- /dev/null
+++ b/libio/tst-wmemstream3.c
@@ -0,0 +1,44 @@
+/* Test for open_memstream implementation.
+   Copyright (C) 2016 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 <wchar.h>
+
+/* Straighforward implementation so tst-memstream3 could use check
+   fwrite on open_memstream.  */
+static size_t
+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 STRCMP wcscmp
+
+#include "tst-memstream3.c"
diff --git a/libio/wmemstream.c b/libio/wmemstream.c
index bf2a50b..fd01be0 100644
--- a/libio/wmemstream.c
+++ b/libio/wmemstream.c
@@ -112,8 +112,6 @@ _IO_wmem_sync (_IO_FILE *fp)
       _IO_wstr_overflow (fp, '\0');
       --fp->_wide_data->_IO_write_ptr;
     }
-  else
-    *fp->_wide_data->_IO_write_ptr = '\0';
 
   *mp->bufloc = fp->_wide_data->_IO_write_base;
   *mp->sizeloc = (fp->_wide_data->_IO_write_ptr
diff --git a/libio/wstrops.c b/libio/wstrops.c
index 09fa543..0b2bec3 100644
--- a/libio/wstrops.c
+++ b/libio/wstrops.c
@@ -169,7 +169,7 @@ _IO_wstr_count (_IO_FILE *fp)
 static int
 enlarge_userbuf (_IO_FILE *fp, _IO_off64_t offset, int reading)
 {
-  if ((_IO_ssize_t) offset <= _IO_blen (fp))
+  if ((_IO_ssize_t) offset <= _IO_wblen (fp))
     return 0;
 
   struct _IO_wide_data *wd = fp->_wide_data;
@@ -235,6 +235,22 @@ enlarge_userbuf (_IO_FILE *fp, _IO_off64_t offset, int reading)
   return 0;
 }
 
+static void
+_IO_wstr_switch_to_get_mode (_IO_FILE *fp)
+{
+  if (_IO_in_backup (fp))
+    fp->_wide_data->_IO_read_base = fp->_wide_data->_IO_backup_base;
+  else
+    {
+      fp->_wide_data->_IO_read_base = fp->_wide_data->_IO_buf_base;
+      if (fp->_wide_data->_IO_write_ptr > fp->_wide_data->_IO_read_end)
+        fp->_wide_data->_IO_read_end = fp->_wide_data->_IO_write_ptr;
+    }
+  fp->_wide_data->_IO_read_ptr = fp->_wide_data->_IO_write_ptr;
+  fp->_wide_data->_IO_read_end = fp->_wide_data->_IO_write_ptr;
+
+  fp->_flags &= ~_IO_CURRENTLY_PUTTING;
+}
 
 _IO_off64_t
 _IO_wstr_seekoff (_IO_FILE *fp, _IO_off64_t offset, int dir, int mode)
@@ -244,15 +260,16 @@ _IO_wstr_seekoff (_IO_FILE *fp, _IO_off64_t offset, int dir, int mode)
   if (mode == 0 && (fp->_flags & _IO_TIED_PUT_GET))
     mode = (fp->_flags & _IO_CURRENTLY_PUTTING ? _IOS_OUTPUT : _IOS_INPUT);
 
+  bool was_writing = (fp->_wide_data->_IO_write_ptr >
+			fp->_wide_data->_IO_write_base
+		     || _IO_in_put_mode (fp));
+  if (was_writing)
+    _IO_wstr_switch_to_get_mode (fp);
+
   if (mode == 0)
     {
-      /* Don't move any pointers. But there is no clear indication what
-	 mode FP is in. Let's guess. */
-      if (fp->_IO_file_flags & _IO_NO_WRITES)
-        new_pos = fp->_wide_data->_IO_read_ptr - fp->_wide_data->_IO_read_base;
-      else
-        new_pos = (fp->_wide_data->_IO_write_ptr
-		   - fp->_wide_data->_IO_write_base);
+      new_pos = (fp->_wide_data->_IO_write_ptr
+		 - fp->_wide_data->_IO_write_base);
     }
   else
     {
@@ -262,25 +279,32 @@ _IO_wstr_seekoff (_IO_FILE *fp, _IO_off64_t offset, int dir, int mode)
       /* Move the get pointer, if requested. */
       if (mode & _IOS_INPUT)
 	{
+	  _IO_ssize_t base;
 	  switch (dir)
 	    {
-	    case _IO_seek_end:
-	      offset += cur_size;
+	    case _IO_seek_set:
+	      base = 0;
 	      break;
 	    case _IO_seek_cur:
-	      offset += (fp->_wide_data->_IO_read_ptr
-			 - fp->_wide_data->_IO_read_base);
+	      base = (fp->_wide_data->_IO_read_ptr
+		     - fp->_wide_data->_IO_read_base);
 	      break;
-	    default: /* case _IO_seek_set: */
+	    default: /* case _IO_seek_end: */
+	      base = cur_size;
 	      break;
 	    }
-	  if (offset < 0)
-	    return EOF;
-	  if ((_IO_ssize_t) offset > cur_size
-	      && enlarge_userbuf (fp, offset, 1) != 0)
+	  _IO_ssize_t maxval = SSIZE_MAX/sizeof (wchar_t) - base;
+	  if (offset < -base || offset > maxval)
+	    {
+	      __set_errno (EINVAL);
+	      return EOF;
+	    }
+	  base += offset;
+	  if (base > cur_size
+	      && enlarge_userbuf (fp, base, 1) != 0)
 	    return EOF;
 	  fp->_wide_data->_IO_read_ptr = (fp->_wide_data->_IO_read_base
-					  + offset);
+					  + base);
 	  fp->_wide_data->_IO_read_end = (fp->_wide_data->_IO_read_base
 					  + cur_size);
 	  new_pos = offset;
@@ -289,26 +313,33 @@ _IO_wstr_seekoff (_IO_FILE *fp, _IO_off64_t offset, int dir, int mode)
       /* Move the put pointer, if requested. */
       if (mode & _IOS_OUTPUT)
 	{
+	  _IO_ssize_t base;
 	  switch (dir)
 	    {
-	    case _IO_seek_end:
-	      offset += cur_size;
+	    case _IO_seek_set:
+	      base = 0;
 	      break;
 	    case _IO_seek_cur:
-	      offset += (fp->_wide_data->_IO_write_ptr
-			 - fp->_wide_data->_IO_write_base);
+	      base = (fp->_wide_data->_IO_write_ptr
+		     - fp->_wide_data->_IO_write_base);
 	      break;
-	    default: /* case _IO_seek_set: */
+	    default: /* case _IO_seek_end: */
+	      base = cur_size;
 	      break;
 	    }
-	  if (offset < 0)
-	    return EOF;
-	  if ((_IO_ssize_t) offset > cur_size
-	      && enlarge_userbuf (fp, offset, 0) != 0)
+	  _IO_ssize_t maxval = SSIZE_MAX/sizeof (wchar_t) - base;
+	  if (offset < -base || offset > maxval)
+	    {
+	      __set_errno (EINVAL);
+	      return EOF;
+	    }
+	  base += offset;
+	  if (base > cur_size
+	      && enlarge_userbuf (fp, base, 0) != 0)
 	    return EOF;
 	  fp->_wide_data->_IO_write_ptr = (fp->_wide_data->_IO_write_base
-					   + offset);
-	  new_pos = offset;
+					   + base);
+	  new_pos = base;
 	}
     }
   return new_pos;
diff --git a/manual/examples/memstrm.c b/manual/examples/memstrm.c
index 0d443b1..5701ba1 100644
--- a/manual/examples/memstrm.c
+++ b/manual/examples/memstrm.c
@@ -27,10 +27,10 @@ main (void)
   stream = open_memstream (&bp, &size);
   fprintf (stream, "hello");
   fflush (stream);
-  printf ("buf = `%s', size = %d\n", bp, size);
+  printf ("buf = `%s', size = %zu\n", bp, size);
   fprintf (stream, ", world");
   fclose (stream);
-  printf ("buf = `%s', size = %d\n", bp, size);
+  printf ("buf = `%s', size = %zu\n", bp, size);
 
   return 0;
 }
