OSX connectx sample from https://github.com/shadowsocks/shadowsocks/issues/399#issuecomment-126280454

main.c

#include <errno.h>
#include <stdio.h>
#include <unistd.h>
#include <netinet/tcp.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/socket.h>

int main(int argc, char **argv) {
  int s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
  if (s < 0)
  {
    perror("socket");
    return 1;
  }
  struct sockaddr_in addr;
  addr.sin_family = AF_INET;
  addr.sin_port = htons(80);
  addr.sin_len = sizeof(addr);
  inet_aton("127.0.0.1", &addr.sin_addr);
  sa_endpoints_t endpoints;
  endpoints.sae_srcif = 0;
  endpoints.sae_srcaddr = NULL;
  endpoints.sae_srcaddrlen = 0;
  endpoints.sae_dstaddr = (struct sockaddr *)&addr;
  endpoints.sae_dstaddrlen = sizeof(addr);
  int rc = connectx(s, &endpoints, SAE_ASSOCID_ANY, CONNECT_RESUME_ON_READ_WRITE | CONNECT_DATA_IDEMPOTENT, NULL, 0, NULL, NULL);
  if (rc < 0)
  {
    perror("connectx");
    return 1;
  }
  char buf[1024];
  rc = write(s, buf, 1024);
  if (rc < 0)
  {
    perror("write");
    return 1;
  }
  close(s);
  return 0;
}