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I'm trying to use the POSIX clock functions in the kernel but the compiler keeps giving me the error: error: implicit declaration of function ‘clock_gettime’

long __timer_end(struct timespec start_time) { struct timespec end_time; clock_gettime(CLOCK_REALTIME_COARSE, &end_time); return(end_time.tv_nsec - start_time.tv_nsec); } struct timespec __timer_start(void) { struct timespec start_time; clock_gettime(CLOCK_REALTIME_COARSE, &start_time); return start_time; }

The functions are defined in <linux/posix_clock.h> as part of structure called posix_clock_operations and there are a pair of functions, posix_clock_register() and posix_clock_unregister(). The comments lead one to believe that these functions will populate the posix_clock_operations structure. I've implemented both in my init and exit functions hoping that their presence would magically make the forward declarations for clock_gettime() appear, but it doesn't.

Does anyone know what I need to do to make this one function work? Do I really need to define all my own functions an populate posix clock_operations?

Thanks in advance,

Pete

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  • clock_gettime() is defined in time.h. Are you referring to this, or to the actual clock_get_time() (web.mit.edu/darwin/src/modules/xnu/osfmk/man/…) ? Commented Mar 22, 2014 at 14:54
  • Hi @John. In the kernel header <linux/time.h> the functions are not declared, just the structures and IDs etc. Commented Mar 22, 2014 at 15:04
  • I was referring to time.h, not linux/time.h. The former actually defines extern int clock_gettime (clockid_t __clock_id, struct timespec *__tp). Commented Mar 22, 2014 at 16:05
  • Right, I understand that, but time.h is not in the kernel tree. I think I've figured it out now though. There is no such function in the kernel. There is however current_kernel_time() which is a nsec granularity timer. Commented Mar 22, 2014 at 16:14
  • Related: How can I print the current time in the kernel? Commented May 3 at 13:51

3 Answers 3

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It seems there is no clock_gettime() in the kernel however there is a nsec resolution clock called current_kernel_time(). So rewriting my timer looks like this:

long timer_end(struct timespec start_time) { struct timespec end_time = current_kernel_time(); return(end_time.tv_nsec - start_time.tv_nsec); } struct timespec timer_start(void) { return current_kernel_time(); }

It seems to work fine, but a higher performance version of the same suitable for ns granular performance testing looks like this:

long timer_end(struct timespec start_time) { struct timespec end_time; getrawmonotonic(&end_time); return(end_time.tv_nsec - start_time.tv_nsec); } struct timespec timer_start(void) { struct timespec start_time; getrawmonotonic(&start_time); return start_time; }

Thanks for the comments and pointers.

Pete

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As pointed out in answer by pjenney58, you can use current_kernel_time(), but do keep in mind that it gives you the "wallclock" time (in HZ accuracy), which is not recommended if you want to profile time taken by some kernel code - simply because it'll also consider time spent in suspended mode. To profile timing in kernel code there is a kernel equivalent of userland clock_gettime(CLOCK_MONOTONIC,...) viz. do_posix_clock_monotonic_gettime() which is defined as a macro in timekeeping.h. It takes pointer to struct timespec as an argument in which monotonic clock time is returned in sec and nsec accuracy.

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Since 2018, current_kernel_time(), getrawmonotonic() etc. are deprecated, according to ktime accessors docs.

They say to use ktime_t ktime_get(void) as a default choice. There are examples all over the Kernel, let's take arch/mips/kernel/spinlock_test.c:

static int ss_get(void *data, u64 *val) { ktime_t start, finish; [...] start = ktime_get(); /* Loop to measure performance */ [...] finish = ktime_get(); *val = ktime_us_delta(finish, start); return 0; }

Personally, I prefer u64 ktime_get_raw_ns(void) for doing measurements of nanoseconds, because I think it more readable and CLOCK_MONOTONIC_RAW is unaffected by NTP clock adjustments.

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