// plik: ntpc.c
    // Zobacz:
    // https://en.wikipedia.org/wiki/Network_Time_Protocol
    //
    // https://github.com/SanketDG/c-projects/blob/master/ntp-client.c
    // https://www.cbk.poznan.pl/
    //
    // Kompilacja: cygwin
    // gcc ntpc.c -o ntpc
    // $ ./ntpc
    // Usage: ./ntpc <server> [port]
    //  
    //  $ ./ntpc 150.254.183.15
    //  Time: Sat Mar 30 09:28:12 2019
    //
    // ./ntpc vega.cbk.poznan.pl
    // 
    // ./ntpc 150.254.183.15 123
    //---------------------------------------------------------------
     
    #include <stdio.h>
    #include <stdlib.h>
    #include <stdint.h>
    #include <string.h>
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netdb.h>
    #include <arpa/inet.h>
    #include <time.h>
    #include <unistd.h>
     
    // Difference between Jan 1, 1900 and Jan 1, 1970
    #define UNIX_OFFSET 2208988800L
     
    #define NTP_DEFAULT_PORT "123"
     
    // Flags 00|100|011 for li=0, vn=4, mode=3
    #define NTP_FLAGS 0x23
     
    typedef struct {
     
        uint8_t flags;
     
        uint8_t stratum;
        uint8_t poll;
     
        uint8_t precision;
     
        uint32_t root_delay;
        uint32_t root_dispersion;
     
        uint8_t referenceID[4];
     
        uint32_t ref_ts_secs;
        uint32_t ref_ts_frac;
     
        uint32_t origin_ts_secs;
        uint32_t origin_ts_frac;
     
        uint32_t recv_ts_secs; // This is what we need mostly to get current time.
        uint32_t recv_ts_fracs;
     
        uint32_t transmit_ts_secs;
        uint32_t transmit_ts_frac;
     
    } ntp_packet;
     
     
    int main(int argc, char *argv[]) {
     
        char *server;
        char *port = NTP_DEFAULT_PORT;
     
        int server_sock, status;
     
        struct addrinfo hints, *servinfo, *ap;
     
        socklen_t addrlen = sizeof(struct sockaddr_storage);
     
        if(argc < 2) {
            printf("Usage: %s <server> [port]\n", argv[0]);
            exit(1);
        }
     
        server = argv[1]; // if it is supplied as argument
     
        if(argc == 3) {
            // Port is also passed in as argument
            port = argv[2];
        }
     
        ntp_packet packet = { .flags = NTP_FLAGS }; // populate the struct
     
        hints = (struct addrinfo) { .ai_family = AF_UNSPEC, .ai_socktype = SOCK_DGRAM };
     
        // Get the info of the NTP server
     
        if((status = getaddrinfo(server, port, &hints, &servinfo)) != 0) {
            fprintf(stderr, "getaddrinfo() error: %s\n", gai_strerror(status));
            exit(2);
        }
     
        // Initalize the socket
     
        for(ap = servinfo; ap != NULL; ap = ap->ai_next) {
            server_sock = socket(ap->ai_family, ap->ai_socktype, ap->ai_protocol);
            if(server_sock == -1)
                continue;
            break;
        }
     
        if (ap == NULL) {
            fprintf(stderr, "socket() error\n");
            exit(2);
        }
     
        // Send the structure to the server
     
        if((status = sendto(server_sock, &packet, sizeof(packet), 0, ap->ai_addr, addrlen)) == -1) {
            perror("sendto() error");
            exit(2);
        }
     
        // Recive the structure from the server
     
        if((status = recvfrom(server_sock, &packet, sizeof(packet), 0, ap->ai_addr, &addrlen)) == -1) {
            perror("recvfrom() error");
            exit(2);
        }
     
        freeaddrinfo(servinfo);
        close(server_sock);
     
        // Convert from network's to host's endian order
     
        packet.recv_ts_secs = ntohl(packet.recv_ts_secs);
     
        // The number of seconds we get back from the server is equal to the no. of
        // seconds since Jan 1, 1900. Since Unix time starts from Jan 1, 1970, we
        // subtract 70 years worth of seconds from Jan 1, 1990.
        
        time_t time_struct = (time_t) packet.recv_ts_secs - UNIX_OFFSET;
     
        printf("Time s: %s", ctime(&time_struct));

        time_t rawtime;
        struct tm * timeinfo;
        time (&rawtime);
        timeinfo = localtime (&rawtime);
        printf("Time l: %s", asctime(timeinfo));
    }