Problem: no tests for socket-related functions passing NULL as socket

Soluton: added tests

tests/test_socket_null.cpp

@@ -0,0 +1,82 @@
+/*
+    Copyright (c) 2007-2017 Contributors as noted in the AUTHORS file
+
+    This file is part of libzmq, the ZeroMQ core engine in C++.
+
+    libzmq is free software; you can redistribute it and/or modify it under
+    the terms of the GNU Lesser General Public License (LGPL) as published
+    by the Free Software Foundation; either version 3 of the License, or
+    (at your option) any later version.
+
+    As a special exception, the Contributors give you permission to link
+    this library with independent modules to produce an executable,
+    regardless of the license terms of these independent modules, and to
+    copy and distribute the resulting executable under terms of your choice,
+    provided that you also meet, for each linked independent module, the
+    terms and conditions of the license of that module. An independent
+    module is a module which is not derived from or based on this library.
+    If you modify this library, you must extend this exception to your
+    version of the library.
+
+    libzmq 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include "testutil.hpp"
+
+//  tests all socket-related functions with a NULL socket argument
+int main (void)
+{
+    void *s = zmq_socket (NULL, ZMQ_PAIR);
+    assert (s == NULL);
+    assert (errno == EFAULT); // TODO use EINVAL instead?
+
+    int rc = zmq_close (NULL);
+    assert (rc == -1);
+    assert (errno == ENOTSOCK); // TODO use EINVAL instead?
+
+    int hwm = 100;
+    size_t hwm_size = sizeof hwm;
+    rc = zmq_setsockopt (NULL, ZMQ_SNDHWM, &hwm, hwm_size);
+    assert (rc == -1);
+    assert (errno == ENOTSOCK); // TODO use EINVAL instead?
+
+    rc = zmq_getsockopt (NULL, ZMQ_SNDHWM, &hwm, &hwm_size);
+    assert (rc == -1);
+    assert (errno == ENOTSOCK); // TODO use EINVAL instead?
+
+    rc = zmq_socket_monitor (NULL, "inproc://monitor", ZMQ_EVENT_ALL);
+    assert (rc == -1);
+    assert (errno == ENOTSOCK); // TODO use EINVAL instead?
+
+#ifdef ZMQ_BUILD_DRAFT_API
+    rc = zmq_join (NULL, "group");
+    assert (rc == -1);
+    assert (errno == ENOTSOCK); // TODO use EINVAL instead?
+
+    rc = zmq_leave (NULL, "group");
+    assert (rc == -1);
+    assert (errno == ENOTSOCK); // TODO use EINVAL instead?
+#endif
+
+    rc = zmq_bind (NULL, "inproc://socket");
+    assert (rc == -1);
+    assert (errno == ENOTSOCK); // TODO use EINVAL instead?
+
+    rc = zmq_connect (NULL, "inproc://socket");
+    assert (rc == -1);
+    assert (errno == ENOTSOCK); // TODO use EINVAL instead?
+
+    rc = zmq_unbind (NULL, "inproc://socket");
+    assert (rc == -1);
+    assert (errno == ENOTSOCK); // TODO use EINVAL instead?
+
+    rc = zmq_disconnect (NULL, "inproc://socket");
+    assert (rc == -1);
+    assert (errno == ENOTSOCK); // TODO use EINVAL instead?
+}