diff --git a/ChangeLog b/ChangeLog
index b837538a5b..6a86de6f26 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 2016-04-29  Adhemerval Zanella  <adhemerval.zanella@linaro.org>
 
+	[BZ #20012]
+	* libio/fmemopen.c (fmemopen_read): Use buffer maximum position, not
+	length to calculate the buffer to read.
+	(fmemopen_write): Set the buffer position based on bytes written.
+	(fmemopen_seek): Return EINVAL for invalid whence modes.
+
 	[BZ #19957]
 	* sysdeps/unix/sysv/linux/Makefile [$(subdir) == nptl] (test): Remove
 	tst-getpid2.
diff --git a/libio/fmemopen.c b/libio/fmemopen.c
index 9264b724ee..0f655909e8 100644
--- a/libio/fmemopen.c
+++ b/libio/fmemopen.c
@@ -50,16 +50,14 @@ fmemopen_read (void *cookie, char *b, size_t s)
 
   if (c->pos + s > c->maxpos)
     {
-      if ((size_t) c->pos == c->maxpos)
-	return 0;
-      s = c->size - c->pos;
+      s = c->maxpos - c->pos;
+      if ((size_t) c->pos > c->maxpos)
+	s = 0;
     }
 
   memcpy (b, &(c->buffer[c->pos]), s);
 
   c->pos += s;
-  if ((size_t) c->pos > c->maxpos)
-    c->maxpos = c->pos;
 
   return s;
 }
@@ -70,28 +68,29 @@ fmemopen_write (void *cookie, const char *b, size_t s)
 {
   fmemopen_cookie_t *c = (fmemopen_cookie_t *) cookie;;
   _IO_off64_t pos = c->append ? c->maxpos : c->pos;
-  int addnullc;
+  int addnullc = (s == 0 || b[s - 1] != '\0');
 
-  addnullc = (s == 0 || b[s - 1] != '\0');
-
-  if (pos + s + addnullc > c->size)
+  if (pos + s > c->size)
     {
       if ((size_t) (c->pos + addnullc) >= c->size)
 	{
 	  __set_errno (ENOSPC);
 	  return 0;
 	}
-      s = c->size - pos - addnullc;
+      s = c->size - pos;
     }
 
   memcpy (&(c->buffer[pos]), b, s);
 
-  c->pos += s;
+  c->pos = pos + s;
   if ((size_t) c->pos > c->maxpos)
     {
       c->maxpos = c->pos;
-      if (addnullc)
+      if (c->maxpos < c->size && addnullc)
 	c->buffer[c->maxpos] = '\0';
+      /* A null byte is written in a stream open for update iff it fits.  */
+      else if (c->append == 0 && addnullc != 0)
+	c->buffer[c->size-1] = '\0';
     }
 
   return s;
@@ -123,7 +122,10 @@ fmemopen_seek (void *cookie, _IO_off64_t *p, int w)
     }
 
   if (np < 0 || (size_t) np > c->size)
-    return -1;
+    {
+      __set_errno (EINVAL);
+      return -1;
+    }
 
   *p = c->pos = np;
 
diff --git a/stdio-common/tst-fmemopen3.c b/stdio-common/tst-fmemopen3.c
index 250f9ecc75..054b0697b3 100644
--- a/stdio-common/tst-fmemopen3.c
+++ b/stdio-common/tst-fmemopen3.c
@@ -25,8 +25,13 @@ static void
 print_buffer (const char *s, size_t n)
 {
   size_t i;
+  printf ("{");
   for (i=0; i<n; ++i)
-    printf ("0x%02X (%c), ", s[i], s[i]);
+    {
+      printf ("0x%02X (%c)", s[i], s[i]);
+      if (i != n)
+	printf (", ");
+    }
 }
 
 /* This test check append mode initial position (a/a+) based on POSIX defition
@@ -185,6 +190,112 @@ do_test_read_seek_negative (void)
   return ret;
 }
 
+static int
+do_test_write_append_2 (const char *str)
+{
+  char buf[10];
+  size_t strn = strlen (str);
+  strcpy (buf, str);
+
+  FILE *fp = fmemopen (buf, sizeof (buf), "a+");
+  size_t r = ftell (fp);
+  size_t e = strlen (buf);
+  if (r != e)
+    {
+      printf ("%s: ftell returned %zu, expected %zu\n", __FUNCTION__, r, e);
+      return 1;
+    }
+
+  if (fseek (fp, 0, SEEK_SET) == -1)
+    {
+      printf ("%s: fseek returned -1\n", __FUNCTION__);
+      return 1;
+    }
+
+  int gr;
+  for (int i=0; i<strn; ++i)
+    {
+      if ((gr = getc (fp)) != str[i])
+	{
+	  printf ("%s: getc failed returned %d, expected %d\n", __FUNCTION__,
+		  gr, str[i]);
+	  return 1;
+        }
+    }
+  if ((gr = getc (fp)) != EOF)
+    {
+      printf ("%s: getc failed returned %d, expected EOF\n", __FUNCTION__,
+	      gr);
+      return 1;
+    }
+
+  if (fseek (fp, e+1, SEEK_SET) == -1)
+    {
+      printf ("%s: fseek returned -1\n", __FUNCTION__);
+      return 1;
+    }
+
+  if ((r = ftell (fp)) != e+1)
+    {
+      printf ("%s: ftell returned %zu, expected %zu\n", __FUNCTION__, r, e+1);
+      return 1;
+    }
+
+  if ((gr = getc (fp)) != EOF)
+    {
+      printf ("%s: getc failed returned %i\n", __FUNCTION__, gr);
+      return 1;
+    }
+
+  /* Check if internal position is not changed with a getc returning EOF.  */
+  if ((r = ftell (fp)) != e+1)
+    {
+      printf ("%s: ftell returned %zu, expected %zu\n", __FUNCTION__, r, e+1);
+      return 1;
+    }
+
+  if (fseek (fp, 0, SEEK_CUR) == -1)
+    {
+      printf ("%s: fseek returned -1\n", __FUNCTION__);
+      return 1;
+    }
+
+  /* This should be overwritten by fprintf + fflush.  */
+  buf[e+2] = 'X';
+
+  if ((r = fprintf (fp, "%d", 101)) != 3)
+    {
+      printf ("%s: fprintf returned %zu, expected %d\n", __FUNCTION__, r, 3);
+      return 1;
+    }
+
+  fflush (fp);
+
+  /* Check if internal position is changed by 3 (strlen of '101').  */
+  if ((r = ftell (fp)) != e+3)
+    {
+      printf ("%s: ftell returned %zu, expected %zu\n", __FUNCTION__, r, e+3);
+      return 1;
+    }
+
+  char exp[20];
+  sprintf (exp, "%s%d", str,  101);
+  if (memcmp (buf, exp, strlen (exp)) != 0)
+    {
+      printf ("%s: check failed:", __FUNCTION__);
+      printf ("\nexpected: ");
+      print_buffer (buf, sizeof (buf));
+      printf ("\nbuffer:   ");
+      print_buffer (exp, sizeof (exp));
+      printf ("\n");
+      return 1;
+    }
+
+  fclose(fp);
+
+  return 0;
+}
+
 static int
 do_test (void)
 {
@@ -199,6 +310,11 @@ do_test (void)
 
   ret += do_test_read_seek_negative ();
 
+  /* First test plus addend will fit in the define buffer of size 10.  */
+  ret += do_test_write_append_2 ("test");
+  /* The second test will also fit, but not the final '\0'.  */
+  ret += do_test_write_append_2 ("testing");
+
   return ret;
 }