C程序文件传输，在客户端打印服务器端错误

Question

我正在创建一个程序，该程序从服务器请求文件并将文件发回，创建一个新文件并将该文件的内容写入新文件。除了服务器端错误消息（即文件不存在）应在客户端打印外，一切都运行良好。谁能给我一个有关如何完成此任务的线索？我是 C 的新手，但愿意学习更多。我的主要问题是，如果我直接发送消息，客户端不知道这是一个错误，并且将完成并创建一个文件并将消息写入该文件。我在想我需要发回一个-1，所以当客户端调用recv时，它会返回-1作为长度并打印一条消息。

这是我的客户：

  ssize_t recvx(int sockfd, void *buf, size_t len) {
    int var = recv(sockfd, buf, len, 0);
    if(var != -1)
    {
      return var;
    } else {
      printf("%s \n","Did not receive.");
      exit(1);
    }
  }

  int main(int argc, char *argv[])
  {
    char buf[MAX_LINE];
    struct addrinfo hints;
    struct addrinfo *rp, *result;
    int bytes_received;
    int s;
    char *server;
    char *port;
    char *file;
    int fd = -1; //file descriptor
    int bytes_written;

    if (argc==4)
    {
      server = argv[1];
      port = argv[2];
      file = argv[3];
    }
    else
    {
      fprintf(stderr, "invalid # of arguments\n");
      exit(1);
    }

    /* Translate host name into peer's IP address */
    memset(&hints, 0, sizeof(hints));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = 0;
    hints.ai_protocol = 0;

    if ((s = getaddrinfo(server, port, &hints, &result)) != 0 )
    {
      fprintf(stderr, "%s: getaddrinfo: %s\n", argv[0], gai_strerror(s));
      exit(1);
    }

    /* Iterate through the address list and try to connect */
    for (rp = result; rp != NULL; rp = rp->ai_next)
    {
      if ((s = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol)) == -1 )
      {
        continue;
      }
      if (connect(s, rp->ai_addr, rp->ai_addrlen) != -1)
 {
        break;
      }
      close(s);
    }

    if (rp == NULL)
    {
      perror("stream-talk-client: connect");
      exit(1);
    }
    freeaddrinfo(result); 
    /*send lines of text */ 
   send(s, file, sizeof(file), 0);


   while(bytes_received != 0)
    {
      bytes_received = recvx(s, buf, 20);
      if(bytes_received == -1)
      {
        fprintf(stderr, "Client Error: Error receiving file \n");
        exit(1);
      } else {
        if(fd == -1)
        {
          fd = open(file, O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
          if(fd == -1)
          {
           fprintf(stderr,"Client Error: Open failed \n");
           exit(1);
         }
         bytes_written = write(fd,buf,bytes_received);
         if(bytes_written == -1)
         {
           fprintf(stderr,"%s \n", "Client Error: Write error");
           exit(1);
         }
       } else {
         bytes_written = write(fd,buf,bytes_received);
         if(bytes_written == -1)
         {
           fprintf(stderr,"%s \n", "Client Error: Write error");
           exit(1);
         }
       }
     }
   }
   if(close(fd) != 0)
   {
     printf("%s \n", "Client Error: File did not close successfully");
     exit(1);
   }


   close(s);

   return 0;
 }  

这是我的服务器：

int main(int argc, char *argv[])
  {
    struct addrinfo hints;
    struct addrinfo *rp, *result;
    char filename[MAX_LINE];
    int s, new_s;
    int bytes_transferred;
    int fd; //file descriptor
    struct stat statBuffer; //to hold file info
    off_t offset = 0;

    /* Build address data structure */
    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;
    hints.ai_protocol = 0;
    hints.ai_canonname = NULL;
    hints.ai_addr = NULL;
    hints.ai_next = NULL;

    /* Get local address info */
    if ((s = getaddrinfo(NULL, argv[1], &hints, &result)) != 0 )
    {
      fprintf(stderr, "%s: getaddrinfo: %s\n", argv[0], gai_strerror(s));
      exit(1);
    }

    /* Iterate through the address list and try to perform passive open */
    for (rp = result; rp != NULL; rp = rp->ai_next)
    {
      if ((s = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol)) == -1 )
      {
        continue;
      }

      if (!bind(s, rp->ai_addr, rp->ai_addrlen))
      {
        break;
      }
      close(s);
    }
    if (rp == NULL)
    {
      perror("stream-talk-server: bind");
      exit(1);
    }
    if (listen(s, MAX_PENDING) == -1)
    {
      perror("stream-talk-server: listen");
      close(s);
      exit(1);
    }

    freeaddrinfo(result);

    /* Wait for connection, then receive and print text */
    while(1)
     { 
      if ((new_s = accept(s, rp->ai_addr, &(rp->ai_addrlen))) < 0)
      {
        perror("stream-talk-server: accept");
        close(s);
        exit(1);
      }
      while(bytes_transferred == recv(new_s,filename,sizeof(filename),0))
      {
        if(bytes_transferred == -1)
        {
          fprintf(stderr, "Server Error: Error receiving filename \n");
          exit(1);
        }
      }
      printf("%s \n", filename);
      fd =open(filename,O_RDONLY);

      if(fd < 0)
      {
        fprintf(stderr,"Server Error: file doesn't exist\n");
        exit(1);
     }
     else
     {
       printf("%s \n","file opened successfully");
     }
     /*get info from file descriptor (fd) and store it in statBuffer struct */
     fstat(fd, &statBuffer);
     bytes_transferred = sendfile(new_s,fd,&offset,statBuffer.st_size);
     if(bytes_transferred == -1)
     {
       fprintf(stderr, "Server Error: File not transferred successfully");
     }
     else
     {
       printf("%s \n", "File transferred successfully");
     }
     if(close(fd) != 0)
     {
       fprintf(stderr, "Server Error: File not close successfully");
     }
     else{
       break;
     }
   }
   close(new_s);

   return 0;
 }

Answer 1

基本上，您要实现的内容与 HTTP 和 FTP 服务器的功能类似。两者都有用于处理错误的特定但相当不同的协议步骤。 HTTP 案例很容易理解——服务器不只是发回文件：它发回一堆头文件，然后是文件。客户端可以通过检查 headers 来了解是否有任何文件数据要跟踪。

在一个非常简单的情况下，我怀疑您可以在代码中实现类似的东西而无需太多工作——只需让服务器在文件的其余部分之前发送一个单字节的响应代码。您可以使用 0（例如）表示后面跟着一个有效文件，而其他数字则表示特定的错误情况。客户端会像当前一样读取数据，但如果第一个字节不为零，它就知道后面的任何其他内容（如果有的话）都是有效的。

或者您可以安排，如果第一个字节不为零，则后面的文本不是预期的文件，而是一条错误消息，应该由客户端显示。

您可以随意扩展这种方案的复杂性，直到最终使用 HTTP :)