c - UDP套接字: server sending file to client Address family not supported by protocol family

Question

我只是套接字编程的初学者，目前正在开发一个使用UDP处理文件传输的小程序。该程序是用 C 语言编写的。

这是我的问题:
UDP服务器将首先使用recvfrom()函数来捕获来自UDP客户端的消息，以便开始发送文件。我先发送了文件名，但无法通过，并显示:协议(protocol)族不支持地址族作为错误消息()。我检查了client_addr的sin_family，它是7。此外，在我尝试设置client_addr.sin_family = AF_INET之后；服务器工作正常，只是客户端无法接收任何消息。

我检查了一些来源，但碰巧没有那么有帮助，如果有人知道原因并愿意告诉的话，请告诉我。感谢您的所有帮助。

下面是服务器代码的一小部分:

int socketfd;
/* my address information */
struct sockaddr_in server_addr;
/* connector’s address information */
struct sockaddr_in client_addr;
socklen_t clientLength;
int numbytes;
char buffer[BUFLEN];
int portNum = atoi(port);
time_t timer;
char charfileSize[20];
int percent, count = 0;
struct tm *tm_info;
char timeBuf[30];
if((socketfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) {
  perror("Server-socket() sockfd error lol!");
  exit(1);
} else {
  printf("Server-socket() sockfd is OK...\n");
}

server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(portNum);
server_addr.sin_addr.s_addr = INADDR_ANY;
memset(&(server_addr.sin_zero), 0, 8);

// bind the socket to the server ip address
if(bind(socketfd, (struct sockaddr *)&server_addr, sizeof(struct sockaddr)) == -1) {
  perror("server socket bind to IP error lol!\n");
  exit(1);
} else {
  printf("successfully bind server ip with socket ...\n");
}
// client_addr.sin_port = htons(portNum);
client_addr.sin_family = AF_INET;

//*  for ensuring client connection  *//
int tempGet;
char tempBuf[BUFLEN];
//if( (tempGet = recvfrom(socketfd, tempBuf, BUFLEN, 0, (struct sockaddr *)&client_addr, &clientLength)) > 0 ) {
tempGet = recvfrom(socketfd, tempBuf, BUFLEN, 0, (struct sockaddr *)&client_addr, &clientLength);
if( strcmp( tempBuf, "send file" ) == 0) {
  printf("Can start transferring file...\n");
}
printf("sin family:%d\n", client_addr.sin_family);

FILE *fp = fopen(filename, "rb");
if ( !fp ) {
  perror("Error opening the file\n");
  exit(1);
} else {
  // successfully opened the file that's going to be transferred
  printf("file opened: %s\n", filename);

  // get file size
  int file_block_size = 0;
  bzero(buffer, BUFLEN);
  long file_size;
  fseek(fp, 0, SEEK_END);
  file_size = ftell(fp);
  // goes back to the beginning of file(fp)
  rewind(fp);
  printf("file name: %s\n", filename);
  printf("file size: %ld kb\n", file_size/1024);
  // get time
  time(&timer);
  clientLength = sizeof(client_addr);

  //client_addr.sin_family = AF_INET;
  int sendFileName;
  // length of file name
  if( (sendFileName = sendto(socketfd, filename, strlen(filename), 0, (struct sockaddr *)&client_addr, sizeof(struct sockaddr_in))) >= 0) {
    printf("file name sent.\n");
  } else {
    //printf("%d\n", sendFileName);
    perror("file name send error.\n");
    exit(1);
  }
}

Answer 1

recvfrom 的最后一个参数采用 socklen_t 的地址，该地址需要使用 sockaddr 参数的大小进行初始化。我没有看到 clientLength 在此调用之前初始化，因此当该函数返回时，client_addr 可能没有正确更新。这会导致后续调用 sendto 失败。

如果在调用 recvfrom 之前初始化 clientLength，问题就会得到解决。

clientLength = sizeof(client_addr);
tempGet = recvfrom(socketfd, tempBuf, BUFLEN, 0, (struct sockaddr *)&client_addr, &clientLength);