What time function do I need to use with pthread_cond_timedwait?

Question

The pthread_cond_timedwait function needs an absolute time in a time timespec structure.

What time function I'm suppose to use to obtain the absolute time. I saw a lot of example on the web and I found almost all time function used. (ftime, clock, gettimeofday, clock_gettime (with all possible CLOCK_...).

The pthread_cond_timedwait uses an absolute time. Will this waiting time affected by changing the time of the machine? Also if I get the absolute time with one of the time function, if the time of the machine change between the get and the addition of the delta time this will affect the wait time? Is there a possibility to wait for an event with a relative time instead?

Answer 1

timespec is just a time in seconds (time_t) with additional nanoseconds. You can use relative time with cond_relative_timed_wait.

To obtain current system time in time_t form call time(NULL); from "time.h". More accurate would be to use

timespec ts;
clock_gettime(CLOCK_REALTIME, &ts);

which will return also nanoseconds.

Other functions can be used as well but the final result must be converted to number of seconds and number of nanoseconds.

Answer 2

I've used clock_gettime with CLOCK_REALTIME myself. That should work satisfactorily and allow you to easily create an absolute timespec to timeout at.

Indeed there's a possibility that the machine time will advance while you do the wait setup. In general if you're waiting for a small enough amount of time that this would matter, you won't be getting as close to your requested wakeup time as you hope anyway. If your requested time has already passed by the time the timedwait call is made, it should just wake up immediately.

Solaris provides pthread_cond_reltimedwait_np, (np to mean non-portable) but I'm not aware of any such function on Linux. I'd suggest just waiting on the absolute time and, if needed, implementing a relative wait yourself.