zmq_close(socket) doesn't seem to release inproc endpoint immediately

gistfile1.txt

#include <zmq.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <assert.h>

// The second bind will eventually fail with an 'Address already in use'
// unless we sleep long enough. I'd expect the zmq_close(sock) call to release
// the endpoint imeediately but that doesn't seem to be the case.

int main (void)
{
    //  Socket to talk to clients
    void *context = zmq_ctx_new ();
    void *sock = zmq_socket (context, ZMQ_PUSH);
    int rc = zmq_bind (sock, "inproc://in");
    assert (rc == 0);

    int i = 0;
    while (1) {
        printf("Iteration: %d\n", i);
        zmq_close (sock);
        sleep(0.1);

        sock = zmq_socket (context, ZMQ_PUSH);
        rc = zmq_bind (sock, "inproc://in");
        if (rc != 0) {
            printf ("bind failed: %s\n", strerror (errno));
            return -1;
        }
        i += 1;
    }
    return 0;
}