sock_programming/connect

connect.c

  int tcp_sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
  // AF_INET to create a IPv4 socket
  // SOCK_STREAM to use a stream-based protocol
  // IPPROTO_TCP to use the TCP protocol

  // if tcp_sock is -1, the socket failed to create
  if (tcp_sock < 0) {
    fputs("socket creation failed", stderr);
    exit(1);
  }

  // we create a struct to store ip address data;
  struct sockaddr_in address_info;

  // zero out the data in the struct
  memset(&address_info, 0, sizeof(address_info));

  // set the address family to be an internet address
  address_info.sin_family = AF_INET;

  char ip_address[] = "123.456.789.101";
  // `inet_pton` converts an ip address from a string representation into a
  // 32-bit binary representation and puts it in the ip_address_info struct
  int ip_convert_status = inet_pton(AF_INET, ip_address, &address_info.sin_addr.s_addr);
  if (ip_convert_status == 0) {
    fputs("invalid ip address string", stderr);
    exit(1);
  } else if (ip_convert_status < 0) {
    fputs("inet_pton() failed", stderr);
    exit(1);
  }

  in_port_t port = 42;
  // `htons` stands for "host to network short" and converts a port number to
  // the binary value format required by the socket api.
  address_info.sin_port = htons(port);

  // `connect` establishes a connection between a given socket and the ip address
  // and port specified by the given sockaddr_in struct. Note that we needed to
  // cast a sockaddr_in struct to sockaddr struct.
  if (connect(tcp_sock, (struct sockaddr*) &address_info, sizeof(address_info)) < 0) {
    fputs("connect() failed", stderr);
    exit(1);
  }

  char *data_str = "Here's some data";
  size_t data_str_len = strlen(data_str);
  // `send` sends the data to the ip address and port connected to the given socket.
  // It returns the number of bytes sent.
  ssize_t bytes_sent = send(tcp_sock, data_str, data_str_len, 0);
  if (bytes_sent < 0) {
    fputs("send() failed", stderr);
    exit(1);
  } else if (bytes_sent != data_str_len){
    fputs("send() sent unexpected number of bytes", stderr);
    exit(1);
  }

  // `close` lets the socket know that communication has ended, and deallocates
  // local resources of the socket.
  close(tcp_sock);