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Read System Clock for Monotonic Time

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This PR changes the `janet_gettime` implementation for OSX <10.12 to
read the system clock for `(os/clock :monotonic)`. As far as I was able
to find online this is _a_ monotonic clock, although it produces
different values from `clock_gettime(CLOCK_MONOTONIC, ...)` on the same
system. I can speculate that this is related to `SYSTEM_CLOCK` monotonic
time being implemented with `mach_absolute_time` which is documented to
_not advance during sleep_, and I suspect that
`clock_gettime(CLOCK_MONOTONIC, ...)` does.

**Resources**:
- `clock_get_time` implementation for the `SYSTEM_CLOCK`:
  <e3723e1f17/osfmk/kern/clock_oldops.c (L284-L296)>
  <2ff845c2e0/osfmk/arm/rtclock.c (L248-L260)>
- `mach_absolute_time` and `mach_continuous_time` definitions:
  <e3723e1f17/osfmk/mach/mach_time.h (L55-L68)>
- Stack overflow post for implementing `clock_gettime` on OS X before 10.12:
  <https://stackoverflow.com/questions/11680461/monotonic-clock-on-osx>
This commit is contained in:
Nicholas Rodrigues Lordello 2025-08-18 10:32:26 +02:00
parent 99abada2c2
commit ad8a5cb6c7
No known key found for this signature in database
GPG Key ID: 117BA330BEA207A2
1 changed files with 14 additions and 17 deletions
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31
src/core/util.c
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@ -931,27 +931,24 @@ int janet_gettime(struct timespec *spec, enum JanetTimeSource source) {
#include <mach/clock.h>
#include <mach/mach.h>
int janet_gettime(struct timespec *spec, enum JanetTimeSource source) {
if (source == JANET_TIME_REALTIME) {
clock_serv_t cclock;
mach_timespec_t mts;
host_get_clock_service(mach_host_self(), CALENDAR_CLOCK, &cclock);
clock_get_time(cclock, &mts);
mach_port_deallocate(mach_task_self(), cclock);
spec->tv_sec = mts.tv_sec;
spec->tv_nsec = mts.tv_nsec;
} else if (source == JANET_TIME_MONOTONIC) {
clock_serv_t cclock;
int nsecs;
mach_msg_type_number_t count;
host_get_clock_service(mach_host_self(), SYSTEM_CLOCK, &cclock);
clock_get_attributes(cclock, CLOCK_GET_TIME_RES, (clock_attr_t)&nsecs, &count);
mach_port_deallocate(mach_task_self(), cclock);
clock_getres(CLOCK_MONOTONIC, spec);
}
if (source == JANET_TIME_CPUTIME) {
clock_t tmp = clock();
spec->tv_sec = tmp / CLOCKS_PER_SEC;
spec->tv_nsec = ((tmp - (spec->tv_sec * CLOCKS_PER_SEC)) * 1000000000) / CLOCKS_PER_SEC;
} else {
clock_serv_t cclock;
mach_timespec_t mts;
clock_id_t cid = CALENDAR_CLOCK;
if (source == JANET_TIME_REALTIME) {
cid = CALENDAR_CLOCK;
} else if (source == JANET_TIME_MONOTONIC) {
cid = SYSTEM_CLOCK;
}
host_get_clock_service(mach_host_self(), cid, &cclock);
clock_get_time(cclock, &mts);
mach_port_deallocate(mach_task_self(), cclock);
spec->tv_sec = mts.tv_sec;
spec->tv_nsec = mts.tv_nsec;
}
return 0;
}