是clock_gettime（）调用原子的吗？

Question

i use timespec structure in multi-threaded code - one thread calls clock_gettime() to fill in the global timespec structure, the other - reads this structure. Question: is clock_gettime() call atomic or I have to use mutex?

Answer 1

I would be very surprised if it was, though I can't find any reference on it. Calling clock_gettime is nothing but calling a library function. So if I'm right, you definitely need a mutex to ensure that another thread is not reading the struct timespec while clock_gettime et reading it.

Answer 2

The clock_gettime man page - http://linux.die.net/man/3/clock_gettime - doesn't say it's atomic so you've no reasonable expectation of it being atomic, and the structure contains two long s so you've no particularly sound basis to even hope. Still, if your hardware supports atomic operations on 64-bit values then rather than fiddle with pointers as Remus suggests, or use a mutex, you could populate another timespec then atomically copy it over your global one, but if your hardware doesn't guarantee memory updates are visible across cores and CPUs (eg recent Intel/AMD hardware does, but at least 10 years ago when I cared the UltraSparc's didn't) then you'll need an explicit memory barrier (ie some machine code injected into your program - possibly using a compiler- or system-provider "builtin" function) to ensure visibility; a mutex implementation will already handle that for you.