Formatting struct timespec

One way to format it is:

printf("%lld.%.9ld", (long long)ts.tv_sec, ts.tv_nsec);

The following will return an ISO8601 and RFC3339-compliant UTC timestamp, including nanoseconds.

It uses strftime(), which works with struct timespec just as well as with struct timeval because all it cares about is the number of seconds, which both provide. Nanoseconds are then appended (careful to pad with zeros!) as well as the UTC suffix 'Z'.

Example output: 2021-01-19T04:50:01.435561072Z

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

int utc_system_timestamp(char[]);

int main(void) {
    char buf[31];
    utc_system_timestamp(buf);
    printf("%s\n", buf);
}

// Allocate exactly 31 bytes for buf
int utc_system_timestamp(char buf[]) {
    const int bufsize = 31;
    const int tmpsize = 21;
    struct timespec now;
    struct tm tm;
    int retval = clock_gettime(CLOCK_REALTIME, &now);
    gmtime_r(&now.tv_sec, &tm);
    strftime(buf, tmpsize, "%Y-%m-%dT%H:%M:%S.", &tm);
    sprintf(buf + tmpsize -1, "%09luZ", now.tv_nsec);
    return retval;
}

GCC command line example (note the -lrt):

gcc foo.c -o foo -lrt

I wanted to ask the same question. Here is my current solution to obtain a string like this: 2013-02-07 09:24:40.749355372 I am not sure if there is a more straight forward solution than this, but at least the string format is freely configurable with this approach.

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

#define NANO 1000000000L

// buf needs to store 30 characters
int timespec2str(char *buf, uint len, struct timespec *ts) {
    int ret;
    struct tm t;

    tzset();
    if (localtime_r(&(ts->tv_sec), &t) == NULL)
        return 1;

    ret = strftime(buf, len, "%F %T", &t);
    if (ret == 0)
        return 2;
    len -= ret - 1;

    ret = snprintf(&buf[strlen(buf)], len, ".%09ld", ts->tv_nsec);
    if (ret >= len)
        return 3;

    return 0;
}

int main(int argc, char **argv) {
    clockid_t clk_id = CLOCK_REALTIME;
    const uint TIME_FMT = strlen("2012-12-31 12:59:59.123456789") + 1;
    char timestr[TIME_FMT];

    struct timespec ts, res;
    clock_getres(clk_id, &res);
    clock_gettime(clk_id, &ts);

    if (timespec2str(timestr, sizeof(timestr), &ts) != 0) {
        printf("timespec2str failed!\n");
        return EXIT_FAILURE;
    } else {
        unsigned long resol = res.tv_sec * NANO + res.tv_nsec;
        printf("CLOCK_REALTIME: res=%ld ns, time=%s\n", resol, timestr);
        return EXIT_SUCCESS;
    }
}

output:

gcc mwe.c -lrt 
$ ./a.out 
CLOCK_REALTIME: res=1 ns, time=2013-02-07 13:41:17.994326501