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 库来编写 C++ 程序。
2. 是否存在完整的教程，逐步说明如何在 linux 中使用套接字进行编程？ （我知道网络中有很多教程，但我正在寻找可以帮助我从 INTERNET 接收 udp 数据的东西，而不是来自同一设备上的另一个程序。）。

3. 我在终端（使用 tcpdump）中看到我的计算机接收到指定端口的 udp 包。我的程序有什么问题，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;
   }
   

   当我打电话给receiveData() 时，什么都没有发生 - 我认为程序正在等待一些 udp 包（我不知道他为什么没有收到它）。

4. 谁能解释一下 udp 服务器和客户端程序的区别？

非常感谢。

Answer 1

当调用recvfrom() 时，你传递给它一个指向this->addressLength 的指针。在输入时，长度需要是地址缓冲区的完整大小。输出时，返回实际地址长度。所以recvfrom() 可能会改变你的addressLength 成员，影响后续代码。改用局部变量：

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;
}

或者，使用单独的成员变量：

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;
}