clock_gettime() still not monotonic - alternatives?
c, clock, glibc, kernel, linux
Solution
This has nothing to do with the mythology of `clock_gettime`'s monotonic clock not actually being monotonic (which probably has a basis in reality, but which was never well documented and probably fixed a long time ago). It's just a bug in your program. `tv_nsec` is the nanoseconds portion of a time value that's stored as two fields:
- `tv_sec` - whole seconds
- `tv_nsec` - nanoseconds in the range 0 to 999999999
Of course `tv_nsec` is going to jump backwards from 999999999 to 0 when `tv_sec` increments. To compute differences of `timespec` structs, you need to take 1000000000 times the difference in seconds and add that to the difference in nanoseconds. Of course this could quickly overflow if you don't convert to a 64-bit type first.
Problem
As has been known for a while (see, e.g., this old question, and bug reports that pop when you google this), clock_gettime() doesn't appear to report back time monotonically. To rule out any silly error I might have overseen, here is the relevant code (excerpt from larger program): ``` <include time.h> long nano_1, nano_2; double delta; struct timespec tspec, *tspec_ptr; clock_gettime(CLOCK_MONOTONIC_RAW, tspec_ptr); nano_1 = tspec.tv_nsec; sort_selection(sorted_ptr, n); clock_gettime(CLOCK_MONOTONIC_RAW, tspec_ptr); nano_2 = tspec.tv_nsec; delta = (nano_2 - nano_1)/1000000.0; printf("\nSelection sort took %g micro seconds.\n", (double) delta); ``` Sorting small arrays (about 1,000 elements) reports plausible times. When I sort larger ones (10,000+) using 3 sort algorithms, 1-2 of the 3 report back negative sort time. I tried all clock types mentioned in the man page, not only CLOCK_MONOTONIC_RAW - no change. (1) Anything I overlooked in my code? (2) Is there an alternative to clock_gettime() that measures time in increments more accurate than seconds? I don't need nanonseconds, but seconds is too coarse to really help. System: - Ubuntu 12.04. - kernel 3.2.0-30 - gcc 4.6.3. - libc version 2.15 - compiled with -lrt