无法使用udp协议从网络接收数据

Question

我的目标是编写使用udp协议从网络接收数据的C++程序。操作系统 - Linux。我有几个问题。

1. 我想确保我可以使用linux C库编写C++程序。
2. 是否有任何地方存在完整的教程，一步步展示如何使用Linux中的套接字进行编程？ （我知道在网络中有很多教程，但我正在寻找一些帮助我从INTERNET接收udp数据的东西，而不是来自同一设备上的其他程序）。

3. 我在终端（使用tcpdump）看到我的计算机接收到指定端口的udp包。这有什么错我的程序，它看起来像这样

   class Connection 
   { 
   private: 
       char * buffer; 
       int bufferSize; 
       int sock; 
       struct sockaddr_in server; 
       struct sockaddr_in other; 
       socklen_t addressLength; 
   
   public: 
       Connection(string address, int port, int bufferSize); 
       ~Connection(); 
       int reciveData(); 
   }; 
   
   Connection::Connection(string address, int port, int bufferSize) 
   { 
       this->addressLength = sizeof(this->server); 
   
       this->sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP); 
   
       if(this->sock == -1) 
       { 
        cout << "Socket error!" << endl; 
        exit(1); 
       } 
   
       memset(&(this->server), 0, sizeof(this->server)); 
       this->server.sin_family = AF_INET; 
       this->server.sin_port = htons(port); 
       this->server.sin_addr.s_addr = htonl(INADDR_ANY); 
   
       if(bind(this->sock, (sockaddr *) &(this->server), this->addressLength) == -1) 
       { 
        cout << "Bind socket error!" << endl; 
        exit(1); 
       } 
   
       this->bufferSize = bufferSize; 
       this->buffer = new char[bufferSize]; 
   
       cout << "Socket created!" << endl; 
   } 
   
   int Connection::receiveData() 
   { 
       int returned; 
       fflush(stdout); 
   
       if((returned =recvfrom(this->sock, this->buffer, this->bufferSize, 0, (sockaddr *) &(this->other), &this->addressLength)) == -1) 
       { 
        cout << "Receiving error!" << endl; 
        exit(1); 
       } 
   
       return returned; 
   } 
   

   当我打电话receiveData()什么也没有发生 - 我认为程序等待一些UDP包（我不知道他为什么不接受）。

4. 有人能解释我udp服务器和客户端程序之间的区别吗？

非常感谢。

Answer 1

当致电recvfrom()时，您传递一个指向this->addressLength的指针。输入时，长度需要是地址缓冲区的完整大小。输出时，返回实际的地址长度。所以recvfrom()可能会改变你的addressLength成员，影响后续的代码。使用本地变量，而不是：

int Connection::receiveData() 
{ 
    int returned; 
    int addrlen = sizeof(this->other); 

    fflush(stdout); 

    returned = recvfrom(this->sock, this->buffer, this->bufferSize, 0, (sockaddr *) &(this->other), &addrLen); 
    if(returned == -1) 
    { 
     cout << "Receiving error!" << endl; 
     exit(1); 
    } 

    return returned; 
} 

另外，使用单独的成员变量，而不是：

class Connection 
{ 
private: 
    char * buffer; 
    int bufferSize; 
    int sock; 
    struct sockaddr_in server; 
    struct sockaddr_in other; 
    socklen_t serverAddressLength; 
    socklen_t otherAddressLength; 

public: 
    Connection(string address, int port, int bufferSize); 
    ~Connection(); 
    int reciveData(); 
}; 

Connection::Connection(string address, int port, int bufferSize) 
{ 
    this->serverAddressLength = sizeof(this->server); 

    this->sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP); 

    if(this->sock == -1) 
    { 
     cout << "Socket error!" << endl; 
     exit(1); 
    } 

    if(this->sock == -1) 
    { 
     cout << "Socket error!" << endl; 
     exit(1); 
    } 

    memset(&(this->server), 0, sizeof(this->server)); 
    this->server.sin_family = AF_INET; 
    this->server.sin_port = htons(port); 
    this->server.sin_addr.s_addr = htonl(INADDR_ANY); 

    if(bind(this->sock, (sockaddr *) &(this->server), this->serverAddressLength) == -1) 
    { 
     cout << "Bind socket error!" << endl; 
     exit(1); 
    } 

    this->bufferSize = bufferSize; 
    this->buffer = new char[bufferSize]; 

    cout << "Socket created!" << endl; 
} 

int Connection::receiveData() 
{ 
    int returned; 
    fflush(stdout); 

    this->otherAddressLength = sizeof(this->other); 
    if((returned =recvfrom(this->sock, this->buffer, this->bufferSize, 0, (sockaddr *) &(this->other), &this->otherAddressLength)) == -1) 
    { 
     cout << "Receiving error!" << endl; 
     exit(1); 
    } 

    return returned; 
}