I needed a function to use as a simple seed for srand(). Unixtime in nanoseconds changes very frequently and serves as a semi-decent random seed.

#include <time.h> // for clock_gettime()

// Nanoseconds since Unix epoch
uint64_t nanos() {
    struct timespec ts;

    // int8_t ok = clock_gettime(CLOCK_MONOTONIC, &ts); // Uptime
    int8_t ok = clock_gettime(CLOCK_REALTIME, &ts);  // Since epoch

    if (ok != 0) {
        return 0; // Return 0 on failure (you can handle this differently)
    }

    // Calculate nanoseconds
    uint64_t ret = (uint64_t)ts.tv_sec * 1000000000ULL + (uint64_t)ts.tv_nsec;

    return ret;
}

See also: Microseconds since epoch in Perl