timestamp.c

#include <stdio.h>
#include <time.h>
#include <sys/time.h>
#include <unistd.h>
#include <assert.h>

static void set_time(void);
static void get_time(void);

int main(void)
{
    set_time();
    while (1) {
        get_time();
        sleep(0.01);
    }
}

// 02:37:01.086
char* asktime(const struct tm *timeptr)
{
  static char result[26];
  sprintf(result, "%.2d:%.2d:%.2d\n",
    timeptr->tm_hour,
    timeptr->tm_min, timeptr->tm_sec);
  return result;
}

static void set_time(void)
{
    // Prepare the broken-down time.
    // See https://en.cppreference.com/w/c/chrono/tm
    struct tm initial_time = {
        .tm_year = 2022 - 1900,
        .tm_mon = 3 - 1,
        .tm_mday = 17 - 1,
        .tm_hour = 21,
        .tm_min = 35,
        .tm_sec = 0
    };
    // Convert to Unix time.
    // See https://en.cppreference.com/w/c/chrono/mktime
    time_t initial_unix_time = mktime(&initial_time);
    // Convert to 'struct timeval'
    struct timeval initial_timeval = {
        .tv_sec = initial_unix_time,
        .tv_usec = 0
    };
    // Set system time.
    // See https://linux.die.net/man/2/settimeofday
    int err = settimeofday(&initial_timeval, NULL);
    // assert(err == 0);
    printf("Time set to: %s", asctime(&initial_time));
}

static void get_time(void)
{
    // Get current time as 'struct timeval'.
    // See https://linux.die.net/man/2/gettimeofday
    struct timeval new_timeval;
    int err = gettimeofday(&new_timeval, NULL);
    assert(err == 0);
    // Extract Unix time
    time_t new_unix_time = new_timeval.tv_sec;
    int iMilliSec = new_timeval.tv_usec / 1000;
    int iMicroSec = new_timeval.tv_usec;
    // Convert to broken-down time
    // See https://en.cppreference.com/w/c/chrono/localtime
    struct tm new_time;
    localtime_r(&new_unix_time, &new_time);
    // 'new_time' now contains the current time components
    
  static char result[26];
  sprintf(result, "%.2d:%.2d:%.2d.%.3d\n",
    new_time.tm_hour,
    new_time.tm_min, new_time.tm_sec,iMilliSec);
    printf("%s", result);
}