open_clientfd {socket} {connect}

description#

클라이언트가 서버에게 메시지를 전달하기 위해 필요한 과정 중 socket, connect 부분을 래핑했다. getaddrinfo(3)의 hint 인자에 어떤 플래그를 다는지 위주로 읽어보자.

/*
 * open_clientfd - Open connection to server at <hostname, port> and
 *     return a socket descriptor ready for reading and writing. This
 *     function is reentrant and protocol-independent.
 *
 *     On error, returns:
 *       -2 for getaddrinfo error
 *       -1 with errno set for other errors.
 */
int open_clientfd(char *hostname, char *port) {
  int clientfd, rc;
  struct addrinfo hints, *listp, *p;

  /* Get a list of potential server addresses */
  memset(&hints, 0, sizeof(struct addrinfo));
  hints.ai_socktype = SOCK_STREAM; /* Open a connection */
  hints.ai_flags = AI_NUMERICSERV; /* ... using a numeric port arg. */
  hints.ai_flags |= AI_ADDRCONFIG; /* Recommended for connections */
  if ((rc = getaddrinfo(hostname, port, &hints, &listp)) != 0) {
    fprintf(stderr, "getaddrinfo failed (%s:%s): %s\n", hostname, port,
            gai_strerror(rc));
    return -2;
  }

  /* Walk the list for one that we can successfully connect to */
  for (p = listp; p; p = p->ai_next) {
    /* Create a socket descriptor */
    if ((clientfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) < 0)
      continue; /* Socket failed, try the next */

    /* Connect to the server */
    if (connect(clientfd, p->ai_addr, p->ai_addrlen) != -1) break; /* Success */
    if (close(clientfd) < 0) {
      /* Connect failed, try another */  // line:netp:openclientfd:closefd
      fprintf(stderr, "open_clientfd: close failed: %s\n", strerror(errno));
      return -1;
    }
  }

  /* Clean up */
  freeaddrinfo(listp);
  if (!p) /* All connects failed */
    return -1;
  else /* The last connect succeeded */
    return clientfd;
}

flags

AI_NUMERICSERV: service 인자에 들어갈 포트가 10진수 숫자임을 알려주는 플래그.
AI_ADDRCONFIG: IPv4 addresses are returned in the list pointed to by res only if the local system has at least one IPv4 address configured, and IPv6 addresses are returned only if the local system has at least one IPv6 address configured.
- 쉽게 말해 IPv4 서비스만 보고싶은 경우 필터링을 해준다고 한다.