如何在Linux用户空间应用程序中获得最精确的时钟？

Question

I have an embedded system and I would like to share in a kernel module/driver the most accurate clock with an userspace application which is very sensitive to the clock, and should be the most accurate.

1. These functions requires context switch and a huge overhead and the clock won't be accurate since the syscall.

2. I thought to increase a shared integer every jiffy from the kernel module, assuming the userspace application can access it directly, the problem is that I can't share an integer/long which is not aligned to a page size and declare a whole new page for only one long variable is a huge overhead.

What should I do?

Answer 1

You want to use clock_gettime() for obtaining the current time (since startup) with either CLOCK_MONOTONIC (monotonic but not steady as it is influenced by NTP) or CLOCK_MONOTONIC_RAW (monotonic and steady, but Linux specific and requires a kernel >= 2.6.28).

For waking up at exact intervals, use clock_nanosleep() and specify TIMER_ABSTIME . Unfortunately clock_nanosleep() only supports CLOCK_MONOTONIC and not CLOCK_MONOTONIC_RAW , so you cannot pass a wakeup time obtained with CLOCK_MONOTONIC_RAW because these clocks may differ. Don't forget to check the return code for EINTR .