Using ctime to print current system time to millisecond precision?

Question

I have a data acquisition program running in an Ubuntu environment. When a data trigger is met, the program creates a new file containing the data, with the filename being a timestamp of when the trigger was met. Right now I'm generating a timestamp using ctime and the program works:

#include <time.h>

time_t rawtime; // Generating time stamp
time(&rawtime);
sprintf(buffer, "/usb/%s.txt", ctime(&rawtime));

A file is created named Fri_May_27_17_58_38_2022.txt

Is it possible to use the same method to get a more precise timestamp to milliseconds?

Answer 1

On most platforms you can take the second part of a struct timespec ( tv_sec ) gotten from timespec_get and use localtime or gmtime to break it down into its components, leaving only the nano second part.

#include <time.h>
#include <stdio.h>

int main() {
    struct timespec ts;
    timespec_get(&ts, TIME_UTC);
    time_t seconds = ts.tv_sec;

    printf("%s", ctime(&seconds)); // just for comparison

    struct tm *t = localtime(&seconds);

    printf("%04d-%02d-%02dT%02d:%02d:%02d.%09ld\n",
        t->tm_year+1900, t->tm_mon+1, t->tm_mday,
        t->tm_hour, t->tm_min, t->tm_sec,
        ts.tv_nsec
    );
}

Possible output:

Fri May 27 18:36:14 2022
2022-05-27T18:36:14.513916611

If you only need milliseconds:

    printf("%04d-%02d-%02dT%02d:%02d:%02d.%03ld\n",
        t->tm_year+1900, t->tm_mon+1, t->tm_mday,
        t->tm_hour, t->tm_min, t->tm_sec,
        ts.tv_nsec / 1000000
    );

Possible output:

Fri May 27 18:36:14 2022
2022-05-27T18:36:14.513