C 客户端未从服务器接收

Question

I made a simple server client program with select() but the problem is that the client doesn't receive the welcome message. I did this several times earlier without select() and it worked, but all of a sudden it doesn't want to work when working with select() . What am I doing wrong?

Server:

#define MAX_CLIENTS 10

int main()
{
    system("clear");
    printf("***SERVER STARTED***\n");

    int master_socket = socket(AF_INET, SOCK_STREAM, 0);
    fd_set fdsForReading;

    struct sockaddr_in address;
    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;
    address.sin_port = htons(8888);
    bind(master_socket, (struct sockaddr *)&address, sizeof(address));
    listen(master_socket, 3);

    while (1)
    {
        FD_ZERO(&fdsForReading);               // Clearing the file descriptor set
        FD_SET(master_socket, &fdsForReading); // Add master_socket to file descriptor set

        printf("Monitoring for connections...\n");
        select(master_socket + 1, &fdsForReading, NULL, NULL, NULL);

        int accepted_socket = accept(master_socket, (struct sockaddr *)&address, sizeof(address));
        printf("[+] New connection accepted\n");

        char buffer[125] = "Welcome to my server";
        send(accepted_socket, buffer, sizeof(buffer), 0);

        printf("End of while loop\n");
    }
}

Client:

int main()
{
    system("clear");
    int clientSocket = socket(AF_INET, SOCK_STREAM, 0), x;

    struct sockaddr_in serverAddress;
    serverAddress.sin_family = AF_INET;
    serverAddress.sin_port = htons(8888);
    serverAddress.sin_addr.s_addr = INADDR_ANY;

    connect(clientSocket, (struct sockaddr *)&serverAddress, sizeof(serverAddress));

    char serverResponse[125];
    recv(clientSocket, &serverResponse, sizeof(serverResponse), 0);
    printf("test: %s\n", serverResponse);

    scanf("%d", &x);

    close(clientSocket);
}

Answer 1

• You don't check for errors. Always do!
• &serverResponse should be serverResponse
• recv may receive fewer characters than you sent so you should check how many characters you received and possibly concatenate what you receive until you have the full message.
• What you receive will usually not be null terminated so you need to add \0 if what you receive is to be used with functions like printf("%s", ... that require null terminated strings. In your case you actually send 125 char s even though the string is much shorter. You should probably not do that. This will send the string + the null terminating character:

   send(accepted_socket, buffer, strlen(buffer) + 1, 0);

• The third argument to accept should be a socklen_t * - not sizeof(address) . Example:

   socklen_t addrlen = sizeof address; int accepted_socket = accept(master_socket, (struct sockaddr *)&address, &addrlen);

With those changes, the client will receive the greeting fine given that no errors occurs - but when you've added error checking you will see where the problem is in that case.