sntp client(socket)

gistfile1.txt

#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define M_HOSTNAME_NTP      "ntp.nict.jp"
#define M_LOCAL_PORT        (123)


//https://www.ietf.org/rfc/rfc2030.txt
typedef struct {
    uint8_t options;                //b7-6:LI, b5-3:VN, b2-0:Mode
    uint8_t stratum;
    uint8_t poll;
    uint8_t precision;
    uint32_t root_delay;
    uint32_t root_dispersion;
    uint32_t ref_identifier;
    uint32_t ref_timestamp[2];
    uint32_t orig_timestamp[2];
    uint32_t recv_timestamp[2];
    uint32_t trans_timestamp[2];
    //uint8_t key_identifier[4];      //optional
    //uint8_t msg_digest[16];         //optional
} sntp_t;


int main(void)
{
    struct hostent *hostinfo;
    ssize_t sz;

    hostinfo = gethostbyname(M_HOSTNAME_NTP);
    if (!hostinfo || (hostinfo->h_addrtype != AF_INET)) {
        return;
    }

    int fd = socket(PF_INET, SOCK_DGRAM, 0);
    if (fd == -1) {
        printf("fail: sock\n");
        return;
    }

    struct sockaddr_in client_addr;
    memset(&client_addr, 0, sizeof(client_addr));
    client_addr.sin_family = AF_INET;
    client_addr.sin_addr.s_addr = INADDR_ANY;
    client_addr.sin_port = htons(M_LOCAL_PORT);
    if (bind(fd, (struct sockaddr *)&client_addr, sizeof(client_addr)) < 0) {
        perror("fail: bind\n");
        return;
    }
    printf("bind ok\n");

    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    memcpy(&server_addr.sin_addr,  hostinfo->h_addr, hostinfo->h_length);
    server_addr.sin_port = htons(M_LOCAL_PORT);
    printf("IP: %s(%d)\n", inet_ntoa(server_addr.sin_addr), M_LOCAL_PORT);

    sntp_t sntp = {0};
    // LI=no warning(0), VN=4, Mode=client(3)
    sntp.options = (0 << 6) | (4 << 3) | 3;

    sz = sendto(fd, &sntp, sizeof(sntp), 0, (struct sockaddr *)&server_addr, sizeof(server_addr));
    printf("sendto=%d\n", (int)sz);
    if (sz != sizeof(sntp)) {
        perror("fail: sendto\n");
        return;
    }

    socklen_t len = sizeof(client_addr);
    sz = recvfrom(fd, &sntp, sizeof(sntp), 0, (struct sockaddr *)&client_addr, &len);
    printf("recvfrom=%d\n", (int)sz);
    if (sz < 0) {
        perror("fail: recvfrom\n");
        return;
    }
    uint32_t timestamp = htonl(sntp.trans_timestamp[0]) - 2208988800UL;
    printf("ntp : %u\n", timestamp);
    close(fd);

    return 0;
}