see http://sgros.blogspot.com/2013/08/tcp-client-self-connect.html

tcp-self-connect.c

#include <netinet/ip.h>
#include <string.h>
int main(int argc, const char *const *argv) {
  struct sockaddr_in6 a =
    { .sin6_family = AF_INET6, .sin6_addr = IN6ADDR_LOOPBACK_INIT };
  int s = socket(AF_INET6, SOCK_STREAM, 0);
  setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &(int) {1}, sizeof(int));
  bind(s, (struct sockaddr *) &a, sizeof(a));
  getsockname(s, (struct sockaddr *) &a, &(socklen_t) {sizeof(a)});
  connect(s, (struct sockaddr *) &a, sizeof(a));
  send(s, argv[argc > 1], strlen(argv[argc > 1]), 0);
}