为什么套接字中的 recv() 函数不返回任何内容？

Question

I am beginner to socket programming and I am using C++ in eclipse. The program executes successfully but recv() function in server-side does not return anything. It keeps waiting without terminating the program.

recv() function of server program:

void Server2::readSocket(){

    if(recv(newsockfd, buffer, sizeof(buffer), 0) < 0){
        int err = WSAGetLastError();
        cout << "Read failed with error " << err << endl;
        perror("Reading failed");
        exit(EXIT_FAILURE);
    }

    cout << "MESSAGE: " << buffer << endl;
}

client program:

void Client2::clientSocket(){

    #ifdef WIN32
    // Initialize Winsock
    int iResult;
    WSADATA wsaData;
    iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
    if (iResult != 0) {
        std::cout << "WSAStartup failed: " << iResult << std::endl;

    }
    #endif

    sockfd = socket(AF_INET, SOCK_STREAM, 0);

    if(sockfd < 0){
        int err = WSAGetLastError();
        cout << "Client Socket failed with error " << err << endl;
        exit(EXIT_FAILURE);
    }

}

void Client2::clientBCopy(){

    server = gethostbyname("127.0.0.1");

    bzero((char *)&serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;

    bcopy((char *)server->h_addr, (char *)&serv_addr.sin_addr.s_addr, server->h_length);

    serv_addr.sin_port = htons(portno);

}

void Client2::clientConnect(){

    if(connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0){
        int err = WSAGetLastError();
        cout << "Client Connect failed with error " << err << endl;
        exit(EXIT_FAILURE);
    }

    cout << "Please enter message: ";
    bzero(buffer, 256);
    fgets(buffer, 255, stdin);

}

void Client2::clientWrite(){

    if(write(sockfd, buffer, strlen(buffer)) < 0){
        int err = WSAGetLastError();
        cout << "Client Write failed with error " << err << endl;
        perror("Write Failed");
        exit(EXIT_FAILURE);
    }

    bzero(buffer, 256);
}

void Client2::clientRead(){

    if(recv(sockfd, buffer, 255, 0) < 0){
        int err = WSAGetLastError();
        cout << "Client Read failed with error " << err << endl;
        exit(EXIT_FAILURE);
    }
}

How can I solve this issue? Please ask me to provide the rest of the code if this is not enough and thank you for helping me!

Answer 1

As per my understanding,

The recv() function shall return the length of the message written to the buffer pointed to by the buffer argument.

int fd_set_blocking(int fd, int blocking) {
/* Save the current flags */
int flags = fcntl(fd, F_GETFL, 0);
if (flags == -1)
    return 0;

if (blocking)`enter code here`
    flags &= ~O_NONBLOCK;
else
    flags |= O_NONBLOCK;
return fcntl(fd, F_SETFL, flags) != -1;

}

If no messages are available at the socket and O_NONBLOCK is not set on the socket's file descriptor, recv() shall block until a message arrives. might be buffer is not copied properly into the file descriptor.

Thanks & Regards, Gunasekaran J

Answer 2

Finally, I found my error.

    cout << "Please enter message: ";
    bzero(buffer, 256);
    fgets(buffer, 255, stdin);

This code line I used in clientConnect() method of client program cause the problem. Once I removed that part recv function executed successfully printing the the message sent from the client. I just found this code online and I have no idea what's happening from the above piece of code