Changed return type of zmq_msg_size to ssize_t to allow error return

include/zmq.h

@@ -167,14 +167,14 @@ ZMQ_EXPORT int zmq_msg_close (zmq_msg_t *msg);
 ZMQ_EXPORT int zmq_msg_move (zmq_msg_t *dest, zmq_msg_t *src);
 ZMQ_EXPORT int zmq_msg_copy (zmq_msg_t *dest, zmq_msg_t *src);
 ZMQ_EXPORT void *zmq_msg_data (zmq_msg_t *msg);
-ZMQ_EXPORT size_t zmq_msg_size (zmq_msg_t *msg);
+
+ZMQ_EXPORT ssize_t zmq_msg_size (zmq_msg_t *msg);
 ZMQ_EXPORT int zmq_msg_more (zmq_msg_t *msg);
 ZMQ_EXPORT int zmq_msg_get (zmq_msg_t *msg, int option, void *optval,
                             size_t *optvallen);
 ZMQ_EXPORT int zmq_msg_set (zmq_msg_t *msg, int option, const void *optval,
                             size_t *optvallen);
 
-
 /******************************************************************************/
 /*  0MQ infrastructure (a.k.a. context) initialisation & termination.         */
 /******************************************************************************/

src/zmq.cpp

@@ -576,7 +576,7 @@ void *zmq_msg_data (zmq_msg_t *msg_)
     return ((zmq::msg_t*) msg_)->data ();
 }
 
-size_t zmq_msg_size (zmq_msg_t *msg_)
+ssize_t zmq_msg_size (zmq_msg_t *msg_)
 {
     if (!msg_) {
         errno = EFAULT;