What is wrong with clock_gettime with CLOCK_REALTIME?

Consider the following code:

struct timespec ts;
uint64_t start_time;
uint64_t stop_time;

if (clock_gettime(CLOCK_REALTIME, &ts) != 0) {
    abort();
}
 
start_time = ts.tv_sec * UINT64_C(1000000000) + ts.tv_nsec;
 
/* some computation... */
 
if (clock_gettime(CLOCK_REALTIME, &ts) != 0) {
    abort();
}
 
stop_time = ts.tv_sec * UINT64_C(1000000000) + ts.tv_nsec;
 
printf("%" PRIu64 "\n", (stop_time - start_time + 500000000) / 1000000000);

In the vast majority of cases, the code works as I expected, i.e., prints the number of seconds that took the computation. Very rarely, however, one anomaly occurs. The program reports the number of seconds like 18446743875, 18446743877, 18446743962, etc. I figured this number roughly matched 2⁶⁴ nanoseconds (~584 years). So I got the suspicion that ts.tv_nsec is sometimes equal to −1.

So my question is: What's wrong with my code? Where and why does adding 2⁶⁴ nanoseconds happen?

I don't see anything wrong with your code. I suspect your OS is occasionally delivering an anomalous value for CLOCK_REALTIME — although I'm surprised, and I can't quite imagine what it might be.

I suggest rewriting your code like this:

struct timespec start_ts, stop_ts;
uint64_t start_time;
uint64_t stop_time;

if (clock_gettime(CLOCK_REALTIME, &start_ts) != 0) {
    abort();
}
 
start_time = start_ts.tv_sec * UINT64_C(1000000000) + start_ts.tv_nsec;
 
/* some computation... */
 
if (clock_gettime(CLOCK_REALTIME, &stop_ts) != 0) {
    abort();
}
 
stop_time = stop_ts.tv_sec * UINT64_C(1000000000) + stop_ts.tv_nsec;

uint64_t elapsed = (stop_time - start_time + 500000000) / 1000000000;
printf("%" PRIu64 "\n", elapsed);

if(elapsed > 365 * 86400 * UINT64_C(1000000000)) {
    printf("ANOMALY:\n");
    printf("start_ts = %lu %lu\n", start_ts.tv_sec, start_ts.tv_nsec);
    printf("stop_ts = %lu %lu\n", stop_ts.tv_sec, stop_ts.tv_nsec);
}

Then, if/when it happens again, you'll have more information to go on.