问如何在不知道产品中使用的NTP客户端的情况下，检查系统时间是否同步到ntp服务器？

EN

After=time-sync.target
Requires=time-sync.target

       The  buf.status  field  is a bit mask that is used to set and/or retrieve status bits associated with the NTP implementation.  Some bits in the mask
       are both readable and settable, while others are read-only.
...
       STA_UNSYNC (read-write)
              Clock unsynchronized.

RETURN VALUE
       On success, adjtimex() and ntp_adjtime() return the clock state; that is, one of the following values:
...
       TIME_ERROR  The system clock is not synchronized to a reliable server.  This value is returned when any of the following holds true:

                   *  Either STA_UNSYNC or STA_CLOCKERR is set.

                   *  STA_PPSSIGNAL is clear and either STA_PPSFREQ or STA_PPSTIME is set.

                   *  STA_PPSTIME and STA_PPSJITTER are both set.

                   *  STA_PPSFREQ is set and either STA_PPSWANDER or STA_PPSJITTER is set.

                   The symbolic name TIME_BAD is a synonym for TIME_ERROR, provided for backward compatibility.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

#include <sys/timex.h>

/* Written for https://serverfault.com/questions/1077601/how-to-check-whether-the-system-time-is-synchronised-to-ntp-server-without-knowi */

void test_status(
    int st) 
{
  if (st & STA_PLL)
    printf("Phase locked loop\n");
  if (st & STA_PPSFREQ)
    printf("Pulse per second frequency discipline\n");
  if (st & STA_FLL)
    printf("PPS Time discipline\n");
  if (st & STA_INS)
    printf("Insert leap second and end-of-day\n");
  if (st & STA_DEL)
    printf("Delete leap second and end-of-day\n");
  if (st & STA_UNSYNC)
    printf("Clock is not syncronized\n");
  if (st & STA_FREQHOLD)
    printf("Hold frequency\n");
  if (st & STA_PPSSIGNAL)
    printf("Valid PPS signal is present\n");
  if (st & STA_PPSJITTER)
    printf("PPS signal jitter exceeded\n");
  if (st & STA_PPSWANDER)
    printf("PPS Signal wander exceeded\n");
  if (st & STA_PPSERROR)
    printf("PPS signal calibration error\n");
  if (st & STA_CLOCKERR)
    printf("Clock hardware fault\n");

  if (st & STA_NANO)
    printf("Nanosecond resolution\n");
  else
    printf("Microsecond resolution\n");

  if (st & STA_MODE)
    printf("Frequency locked loop\n");
  else
    printf("Phase locked loop\n");
}

int main() {
  struct timex tx = {};
  tx.modes = ADJ_OFFSET_SS_READ;
  int err = adjtimex(&tx);

  switch(err) {
    case -1:
      printf("Time error: %s\n", strerror(errno));
    break;

    case TIME_WAIT:
      printf("Leap second insert/delete completed\n");
    break;

    case TIME_INS:
      printf("Leap second to be added next UTC day\n");
    break;

    case TIME_DEL:
      printf("Leap second to be deleted next UTC day\n");
    break;

    case TIME_OOP:
      printf("Leap second insertion in progress\n");
    break;

    case TIME_ERROR:
      printf("Error getting time\n");
    break;

    case TIME_OK:
      printf("Time OK\n");
    break;

    default:
      printf("Time default: %x (%d)\n", err, err);
    break;
  }

  test_status(tx.status);
  exit(0);
}

$ ./timex 
Error getting time
Clock is not syncronized
Microsecond resolution
Phase locked loop

# podman run -v /tmp/timex/timex:/timex  docker.io/gammabytehosting/rockylinux /timex
Error getting time
Clock is not syncronized
Microsecond resolution
Phase locked loop

# systemctl start chronyd
# chronyc sources
210 Number of sources = 9
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
^* _gateway            2   6     7     1  +5568ns[ -720ms] +/-   32ms
# ./timex 
Time OK
Microsecond resolution
Phase locked loop

# podman run -v /tmp/timex/timex:/timex  docker.io/gammabytehosting/rockylinux /timex
Time OK
Microsecond resolution
Phase locked loop