Returns value of errno for the current thread.
On most systems, errno can be accessed directly and this function is not needed. However, on Windows, there are multiple implementations of the CRT library (single-threaded, multi-threaded, release, debug) and each of them has its own instance of errno. Thus, if nanomsg library and the application that uses it link with different versions of the CRT library, they don't share the same instance of errno. Consequently, error codes set by nanomsg cannot be accessed by the application. To overcome this problem, application can use nn_errno() function to retrieve nanomsg's value of errno.
rc = nn_send (s, "ABC", 3, 0); if (rc < 0) printf ("nn_send failed with error code %d\n", nn_errno ());