diff --git a/dbus/dbus-message-util.c b/dbus/dbus-message-util.c
index 0fa010c..562190a 100644
--- a/dbus/dbus-message-util.c
+++ b/dbus/dbus-message-util.c
@@ -1226,6 +1226,7 @@ _dbus_message_test (const char *test_data_dir)
       DBusError error = DBUS_ERROR_INIT;
       char *marshalled = NULL;
       int len = 0;
+      char garbage_header[DBUS_MINIMUM_HEADER_SIZE] = "xxx";
 
       if (!dbus_message_marshal (message, &marshalled, &len))
         _dbus_assert_not_reached ("failed to marshal message");
@@ -1233,6 +1234,7 @@ _dbus_message_test (const char *test_data_dir)
       _dbus_assert (len != 0);
       _dbus_assert (marshalled != NULL);
 
+      _dbus_assert (dbus_message_demarshal_bytes_needed (marshalled, len) == len);
       message2 = dbus_message_demarshal (marshalled, len, &error);
 
       _dbus_assert (message2 != NULL);
@@ -1255,6 +1257,14 @@ _dbus_message_test (const char *test_data_dir)
       _dbus_assert (message2 == NULL);
       _dbus_assert (dbus_error_is_set (&error));
       dbus_error_free (&error);
+
+      /* Bytes needed to demarshal empty message: 0 (more) */
+
+      _dbus_assert (dbus_message_demarshal_bytes_needed ("", 0) == 0);
+      
+      /* Bytes needed to demarshal invalid message: -1 (error). */
+
+      _dbus_assert (dbus_message_demarshal_bytes_needed (garbage_header, DBUS_MINIMUM_HEADER_SIZE) == -1);
     }
 
   dbus_message_unref (message);
diff --git a/dbus/dbus-message.c b/dbus/dbus-message.c
index cd44798..2871e5f 100644
--- a/dbus/dbus-message.c
+++ b/dbus/dbus-message.c
@@ -3941,6 +3941,9 @@ dbus_message_marshal (DBusMessage  *msg,
   _dbus_return_val_if_fail (msg != NULL, FALSE);
   _dbus_return_val_if_fail (marshalled_data_p != NULL, FALSE);
   _dbus_return_val_if_fail (len_p != NULL, FALSE);
+  
+  /* Lock the message, if it isn't already. */
+  _dbus_message_lock (msg);
 
   if (!_dbus_string_init (&tmp))
     return FALSE;
@@ -4023,6 +4026,62 @@ dbus_message_demarshal (const char *str,
   return NULL;
 }
 
+/**
+ * Returns the number of bytes required to be in the buffer to demarshal a
+ * D-Bus message.
+ *
+ * Generally, this function is only useful for encapsulating D-Bus messages in
+ * a different protocol.
+ *
+ * @param str data to be marshalled
+ * @param len the length of str
+ * @param error the location to save errors to
+ * @returns -1 if there was an error, 0 if there wasn't enough data to determine how much should be demarshalled. Otherwise returns the number of bytes to be demarshalled
+ * 
+ */
+int 
+dbus_message_demarshal_bytes_needed(const char *str, 
+                                    int         len)
+{
+  DBusString strtmp;
+  int byte_order, fields_array_len, header_len, body_len;
+  DBusValidity validity = DBUS_VALID;
+  int have_message;
+
+  if (!str || len < DBUS_MINIMUM_HEADER_SIZE)
+    return 0;
+
+  if (len > DBUS_MAXIMUM_MESSAGE_LENGTH)
+    len = DBUS_MAXIMUM_MESSAGE_LENGTH;
+
+  if (!_dbus_string_init (&strtmp))
+    return -1;
+
+  _dbus_string_append_len (&strtmp, str, len);
+  
+  validity = DBUS_VALID;
+  have_message
+    = _dbus_header_have_message_untrusted(DBUS_MAXIMUM_MESSAGE_LENGTH,
+                                          &validity, &byte_order,
+                                          &fields_array_len,
+                                          &header_len,
+                                          &body_len,
+                                          &strtmp, 0,
+                                          len);
+  _dbus_string_free (&strtmp);
+
+  if (have_message || validity == DBUS_VALID)
+    return header_len + body_len;
+  else
+    {
+      return -1; /* broken! */
+    }
+}
+
 /** @} */
 
 /* tests in dbus-message-util.c */
diff --git a/dbus/dbus-message.h b/dbus/dbus-message.h
index 1bf7d0c..d4e0799 100644
--- a/dbus/dbus-message.h
+++ b/dbus/dbus-message.h
@@ -220,6 +220,8 @@ DBusMessage* dbus_message_demarshal (const char *str,
                                      int         len,
                                      DBusError  *error);
 
+int          dbus_message_demarshal_bytes_needed (const char *str, int len);
+
 /** @} */
 
 DBUS_END_DECLS