platforms: fix clock build for macOS (yet again)

platforms/posix/src/px4_layer/drv_hrt.cpp

@@ -594,20 +594,29 @@ void abstime_to_ts(struct timespec *ts, hrt_abstime abstime)
 	ts->tv_nsec = abstime * 1000;
 }
 
-#if defined(ENABLE_LOCKSTEP_SCHEDULER)
 int px4_clock_gettime(clockid_t clk_id, struct timespec *tp)
 {
 	if (clk_id == CLOCK_MONOTONIC) {
-
+#if defined(ENABLE_LOCKSTEP_SCHEDULER)
 		const uint64_t abstime = lockstep_scheduler.get_absolute_time();
 		abstime_to_ts(tp, abstime - px4_timestart_monotonic);
 		return 0;
+#else // defined(ENABLE_LOCKSTEP_SCHEDULER)
+#if defined(__PX4_DARWIN)
+		// We don't have CLOCK_MONOTONIC on macOS, so we just have to
+		// resort back to CLOCK_REALTIME here.
+		return system_clock_gettime(CLOCK_REALTIME, tp);
+#else // defined(__PX4_DARWIN)
+		return system_clock_gettime(clk_id, tp);
+#endif // defined(__PX4_DARWIN)
+#endif // defined(ENABLE_LOCKSTEP_SCHEDULER)
 
 	} else {
 		return system_clock_gettime(clk_id, tp);
 	}
 }
 
+#if defined(ENABLE_LOCKSTEP_SCHEDULER)
 int px4_clock_settime(clockid_t clk_id, const struct timespec *ts)
 {
 	if (clk_id == CLOCK_REALTIME) {

platforms/posix/src/px4_layer/px4_sem.cpp

@@ -58,11 +58,14 @@ int px4_sem_init(px4_sem_t *s, int pshared, unsigned value)
 	pthread_cond_init(&(s->wait), nullptr);
 	pthread_mutex_init(&(s->lock), nullptr);
 
-	// We want to use CLOCK_MONOTONIC if possible.
+#if !defined(__PX4_DARWIN)
+	// We want to use CLOCK_MONOTONIC if possible but we can't on macOS
+	// because it's not available.
 	pthread_condattr_t attr;
 	pthread_condattr_init(&attr);
 	pthread_condattr_setclock(&attr, CLOCK_MONOTONIC);
 	pthread_cond_init(&(s->wait), &attr);
+#endif
 
 	return 0;
 }

src/lib/cdev/posix/cdev_platform.cpp

@@ -379,8 +379,8 @@ extern "C" {
 
 				// Get the current time
 				struct timespec ts;
-				// px4_sem_timedwait is implemented using CLOCK_MONOTONIC,
+				// Note, we can't actually use CLOCK_MONOTONIC on macOS
-				// at least for lockstep, on Qurt and on Linux.
+				// but that's hidden and implemented in px4_clock_gettime.
 				px4_clock_gettime(CLOCK_MONOTONIC, &ts);
 
 				// Calculate an absolute time in the future

src/platforms/px4_time.h

@@ -9,10 +9,11 @@
 #define clockid_t int
 #endif
 
+__EXPORT int px4_clock_gettime(clockid_t clk_id, struct timespec *tp);
+
 #if defined(ENABLE_LOCKSTEP_SCHEDULER) || defined(__PX4_QURT)
 
 __BEGIN_DECLS
-__EXPORT int px4_clock_gettime(clockid_t clk_id, struct timespec *tp);
 __EXPORT int px4_clock_settime(clockid_t clk_id, const struct timespec *tp);
 
 __EXPORT int px4_usleep(useconds_t usec);
@@ -24,7 +25,6 @@ __END_DECLS
 
 #else
 
-#define px4_clock_gettime system_clock_gettime
 #define px4_clock_settime system_clock_settime
 #define px4_usleep system_usleep
 #define px4_sleep system_sleep