Question

我正在检查由stackoverflow成员 atanos 编写的关于在2010年将多个客户端连接到单个服务器的程序，我修改了他的程序以接受来自命令行的连接细节，下面是服务器和客户代码。

Server.c

int main(int argc, char **argv)
{
  fd_set fds, readfds;
  int i, clientaddrlen, portno;
  int clientsock[2], rc, numsocks = 0, maxsocks = 2;

  if (argc < 2){
      fprintf(stderr,"ERROR, no port provided\n");
      exit(1);
  }

  int serversock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

  if (serversock == -1) perror("Socket");

  portno = atoi(argv[1]);
  struct sockaddr_in serveraddr, clientaddr;
  bzero(&serveraddr, sizeof(struct sockaddr_in));
  serveraddr.sin_family = AF_INET;
  serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
  serveraddr.sin_port = htons(portno);

  if (-1 == bind(serversock, (struct sockaddr *)&serveraddr,sizeof(struct sockaddr_in))) perror("Bind");

  if (-1 == listen(serversock, SOMAXCONN)) perror("Listen");

  FD_ZERO(&fds);
  FD_SET(serversock, &fds);

  while(1) {
    readfds = fds;
    rc = select(FD_SETSIZE, &readfds, NULL, NULL, NULL);

    if (rc == -1){
      perror("Select");
      break;
   }

    for (i = 0; i < FD_SETSIZE; i++)
    {
      if (FD_ISSET(i, &readfds)){
        if (i == serversock){
          if (numsocks < maxsocks){
            clientsock[numsocks] = accept(serversock,(struct sockaddr *) &clientaddr,
                (socklen_t *)&clientaddrlen);

            if (clientsock[numsocks] == -1) perror("Accept");
            else printf("Connection accepted\n");

            FD_SET(clientsock[numsocks], &fds);
             numsocks++;
          }
          else
            printf("Ran out of socket space.\n");
        }
        else
        {
          int messageLength = 10;
          char message[messageLength+1];
          int numOfChRead, index = 0, limit = messageLength+1;

          numOfChRead = recv(clientsock[i], message, messageLength,0);

          if(numOfChRead > 0)
              printf("Data Received !!! length: %d Message: %s", numOfChRead, message);
          else
              printf("Nothing read\n");
        }
      }
    }
  }
  close(serversock);
  return 0;
}

Client.c

int main(int argc, char **argv)
{
  struct sockaddr_in servaddr;
  struct hostent *server;
  int portno, bytesSent = 0;

  if (argc < 3){
      fprintf(stderr,"Usage %s hostname port\n", argv[0]);
      exit(0);
  }

  portno = atoi(argv[2]);
  int sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
  if (sock == -1) perror("Socket");

  server = gethostbyname(argv[1]);

  if (server == NULL){
      fprintf(stderr,"ERROR, no such host\n");
      exit(0);
  }

  bzero((void *) &servaddr, sizeof(servaddr));
  servaddr.sin_family = AF_INET;

  servaddr.sin_port = htons(portno);
  bcopy((char *)server->h_addr,(char *)&servaddr.sin_addr.s_addr,server->h_length);

  if (-1 == connect(sock, (struct sockaddr *)&servaddr, sizeof(servaddr))) perror("Connect");

  while(1) {
    char message[10];
    fgets(message, 10, stdin);
    message[10] = '\0';

    bytesSent = send(sock, message, strlen(message), 0);

    if(bytesSent == -1) printf("Sending failed with errno %d", errno);
    else printf("Sent %d characters", bytesSent);
  }
  close(sock);
}

我可以看到服务器接受多个连接但是当我从任何客户端发送消息时，服务器中的recv API不会读取任何内容。客户端的发送API成功，我可以看到所有字符都已发送。有人可以发表评论吗？

Answer 1

问题是，您实际上从未将客户端连接置于读取fd_set。

中

该行：

readfds = fds;

将readfds重置为仅包含服务器（侦听）套接字的状态。您需要在调用fd_set之前完全重新设置select中的每个客户端文件描述符，每次都在循环中。

可能是其他问题（需要经过很多代码），但这似乎是最有可能发生的问题。

（顺便说一句，我不确定作业是否合法。我不认为实施要求fd_set是简单的，可复制的结构。）

Answer 2

您的代码中存在多个错误。您没有正确使用选择呼叫。 select的第一个参数占用你正在监听的最多fds加1.而第二个参数包含要监视的fds列表。

我用fdmax替换了你的FD_SETSIZE。我更正了代码中的问题并粘贴在下面。

  8 int main(int argc, char **argv)
  9 {
 10   fd_set fds, readfds;
 11   int i, clientaddrlen, portno;
 12   int clientsock[2], rc, numsocks = 0, maxsocks = 2;
 13   int fdmax=0;
 14
 15   if (argc < 2){
 16       fprintf(stderr,"ERROR, no port provided\n");
 17       exit(1);
 18   }
 19
 20   int serversock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
 21
 22   if (serversock == -1) perror("Socket");
 23
 24   portno = atoi(argv[1]);
 25   struct sockaddr_in serveraddr, clientaddr;
 26   bzero(&serveraddr, sizeof(struct sockaddr_in));
 27   serveraddr.sin_family = AF_INET;
 28   serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
 29   serveraddr.sin_port = htons(portno);
 30
 31   if (-1 == bind(serversock, (struct sockaddr *)&serveraddr,sizeof(struct sockaddr_in))) perror("Bind");
 32
 33   if (-1 == listen(serversock, SOMAXCONN)) perror("Listen");
 34
 35   FD_ZERO(&fds);
 36   FD_SET(serversock, &fds);
 37   fdmax = serversock;
 38
 39   clientaddrlen = sizeof(clientaddr);
 40   while(1) {
 41     readfds = fds;
 42     rc = select(fdmax + 1, &readfds, NULL, NULL, NULL);
 43
 44     if (rc == -1){
 45       perror("Select");
 46       break;
 47    }
 48
 49     for (i = serversock; i <= fdmax; i++)
 50     {
 51       if (FD_ISSET(i, &readfds)){
 52         if (i == serversock){
 53           if (numsocks < maxsocks){
 54             clientsock[numsocks] = accept(serversock,(struct sockaddr *) &clientaddr,
 55                 (socklen_t *)&clientaddrlen);
 56
 57             if (clientsock[numsocks] == -1) perror("Accept");
 58             else printf("Connection accepted\n");
 59
 60             FD_SET(clientsock[numsocks], &fds);
 61             fdmax = clientsock[numsocks];
 62              numsocks++;
 63           }
 64           else
 65             printf("Ran out of socket space.\n");
 66         }
 67         else
 68         {
 69           int messageLength = 100;
 70           char message[messageLength+1];
 71           int numOfChRead, index = 0, limit = messageLength+1;
 72
 73           numOfChRead = recv(i, message, messageLength,0);
 74
 75           if(numOfChRead > 0)
 76               printf("Data Received !!! length: %d Message: %s", numOfChRead, message);
 77           else
 78               printf("Nothing read\n");
 79         }
 80       }
 81     }
 82   }
 83   close(serversock);
 84   return 0;
 85 }

即使连接已被接受，“recv”也无法正常工作

2 个答案: