# 6.2. 簡單的 Stream Client

Client 這傢伙比 server 簡單多了，client 所需要做的就是：連線到你在命令列所指定的主機 3490 port，接著，client 會收到 server 送回的字串。

Client 的程式碼 \[21]：

\[21] <http://beej.us/guide/bgnet/examples/client.c>

{% code lineNumbers="true" %}

```c
/*
/*
** client.c -- 一個 stream socket client 的 demo
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <arpa/inet.h>

#define PORT "3490" // Client 所要連線的 port
#define MAXDATASIZE 100 // 我們一次可以收到的最大位原組數（number of bytes）

// 取得 IPv4 或 IPv6 的 sockaddr：
void *get_in_addr(struct sockaddr *sa)
{
  if (sa->sa_family == AF_INET) {
    return &(((struct sockaddr_in*)sa)->sin_addr);
  }

  return &(((struct sockaddr_in6*)sa)->sin6_addr);
}

int main(int argc, char *argv[])
{
  int sockfd, numbytes;
  char buf[MAXDATASIZE];
  struct addrinfo hints, *servinfo, *p;
  int rv;
  char s[INET6_ADDRSTRLEN];

  if (argc != 2) {
    fprintf(stderr,"usage: client hostname\n");
    exit(1);
  }

  memset(&hints, 0, sizeof hints);
  hints.ai_family = AF_UNSPEC;
  hints.ai_socktype = SOCK_STREAM;

  if ((rv = getaddrinfo(argv[1], PORT, &hints, &servinfo)) != 0) {
    fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
    return 1;
  }

  // 用迴圈取得全部的結果，並先連線到能成功連線的
   for(p = servinfo; p != NULL; p = p->ai_next) {
    if ((sockfd = socket(p->ai_family, p->ai_socktype,
      p->ai_protocol)) == -1) {
      perror("client: socket");
      continue;
    }

    if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
      close(sockfd);
      perror("client: connect");
      continue;
    }

    break;
  }

  if (p == NULL) {
    fprintf(stderr, "client: failed to connect\n");
    return 2;
  }

  inet_ntop(p->ai_family, get_in_addr((struct sockaddr *)p->ai_addr), s, sizeof s);

  printf("client: connecting to %s\n", s);

  freeaddrinfo(servinfo); // 全部皆以這個 structure 完成

  if ((numbytes = recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) {
    perror("recv");
    exit(1);
  }

  buf[numbytes] = '\0';
  printf("client: received '%s'\n",buf);

  close(sockfd);
  return 0;
}
```

{% endcode %}

要注意的是，你如果沒有在執行 client 以前先啟動 server 的話，connect() 會傳回 "Connection refused"，這個訊息很有幫助。


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