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Question

The UDP server receives packets with a system call select. And I want to get the last packet from every UDP client. (I also want to listen to some UDP client packets).

The codes for my simple UDP server are:

int main(void) {
    int fd;
    int port = 5678;
    char buffer[1024];
    fd_set readfs;
    socklen_t client_length;
    struct timeval timeout_interval;
    struct sockaddr_in6 server_addr;
    struct sockaddr_in6 client_addr;
    int result;
    int recv;
    char client_addr_ipv6[100];

    fd = socket(PF_INET6, SOCK_DGRAM, 0);
    printf(" \e[1m \e[34m ---------------------------------------- \n--------------------  UDP SERVER --------------------\n   \e[39m \e[0m \n");
    printf("Process: \e[34m %d \e[49m Port ..\n", port);

    if (fd < 0) {
        printf("ERR: fd < 0");
    } else {
        memset(&server_addr, 0, sizeof(server_addr));
        server_addr.sin6_family = AF_INET6;
        server_addr.sin6_addr = in6addr_any;
        server_addr.sin6_port = htons(port);
        memset(&client_addr, 0, sizeof(client_addr));
        client_addr.sin6_family = AF_INET6;
        client_addr.sin6_addr = in6addr_any;
        client_addr.sin6_port = htons(port);

        if (bind(fd, (struct sockaddr *) &server_addr, sizeof(server_addr))
                >= 0) {
            printf("\e[1m INFO: \e[0m \e[34m Bind success.. \e[39m\n");

        } else {
            printf("Bind.");
            return -1;
        }
        for (;;) {
            FD_ZERO(&readfs);
            FD_SET(fd, &readfs);
            int max_fd = MAX(0, fd);
            timeout_interval.tv_sec = 3;
            timeout_interval.tv_usec = 50000000;

            result = select(max_fd + 1, &readfs, NULL, NULL, &timeout_interval);
            //printf("\n %d \t %d \n", result, fd);
            if (result < 0) {
                printf("ERR\n");
            } else if (result == 0) {
                printf("\nTimeout\n");
            } else {
                if (FD_ISSET(fd, &readfs)) {
                    client_length = sizeof(client_addr);
                    if ((recv = recvfrom(fd, buffer, sizeof(buffer), 0,
                                    (struct sockaddr *) &client_addr, &client_length))
                            < 0) {
                        printf("Recv-ERR!");
                        break;
                    }
                    inet_ntop(AF_INET6, &(client_addr.sin6_addr), client_addr_ipv6, 100);
                    //printf("Client IP/Port : %s  ",client_addr_ipv6);

                    printf("\n ------------------------------------------ \n");
                    printf("\e[1m Data: \e[0m  \e[32m %.*s \n Client IP/Port : \e[34m %s / %d \n\e[39m", recv, buffer,client_addr_ipv6,ntohs(client_addr.sin6_port));
                }
            }
        }
    }
}

Answer 1

The best way to handle this is for the sender to put a sequence number in each packet. For each packet sent, the sequence number is incremented by 1.

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