Problem: size of zmq_msg_t is not known to FFI wrappers

Solution: add a ZMQ_MSG_T_SIZE context read-only option so that
wrappers can call zmq_ctx_get (ctx, ZMQ_MSG_T_SIZE) to get the
size at runtime.

include/zmq.h

@@ -206,6 +206,9 @@ ZMQ_EXPORT void zmq_version (int *major, int *minor, int *patch);
 #define ZMQ_THREAD_PRIORITY 3
 #define ZMQ_THREAD_SCHED_POLICY 4
 #define ZMQ_MAX_MSGSZ 5
+//  All options after this is for version 4.3 and still *draft*
+//  Subject to arbitrary change without notice
+#define ZMQ_MSG_T_SIZE 6
 
 /*  Default for new contexts                                                  */
 #define ZMQ_IO_THREADS_DFLT  1

src/ctx.cpp

@@ -301,6 +301,9 @@ int zmq::ctx_t::get (int option_)
     else
     if (option_ == ZMQ_MAX_MSGSZ)
         rc = max_msgsz;
+    else
+    if (option_ == ZMQ_MSG_T_SIZE)
+        rc = sizeof (zmq_msg_t);
     else {
         errno = EINVAL;
         rc = -1;

tests/test_ctx_options.cpp

@@ -48,6 +48,7 @@ int main (void)
 #endif
     assert (zmq_ctx_get (ctx, ZMQ_IO_THREADS) == ZMQ_IO_THREADS_DFLT);
     assert (zmq_ctx_get (ctx, ZMQ_IPV6) == 0);
+    assert (zmq_ctx_get (ctx, ZMQ_MSG_T_SIZE) == sizeof (zmq_msg_t));
     
     rc = zmq_ctx_set (ctx, ZMQ_IPV6, true);
     assert (zmq_ctx_get (ctx, ZMQ_IPV6) == 1);