diff --git a/platforms/posix/src/px4_layer/CMakeLists.txt b/platforms/posix/src/px4_layer/CMakeLists.txt
index 2dbf47c4bb..7ce430ae0e 100644
--- a/platforms/posix/src/px4_layer/CMakeLists.txt
+++ b/platforms/posix/src/px4_layer/CMakeLists.txt
@@ -55,9 +55,11 @@ add_library(px4_layer
 target_compile_definitions(px4_layer PRIVATE MODULE_NAME="px4")
 target_link_libraries(px4_layer PRIVATE work_queue)
 target_link_libraries(px4_layer PRIVATE px4_daemon)
-target_link_libraries(px4_layer PRIVATE lockstep_scheduler)
 
-include_directories(${PX4_SOURCE_DIR}/src/platforms/posix/lockstep_scheduler/include)
+if(LOCKSTEP_SCHEDULER_NEEDED)
+	target_link_libraries(px4_layer PRIVATE lockstep_scheduler)
+	include_directories(${PX4_SOURCE_DIR}/src/platforms/posix/lockstep_scheduler/include)
+endif()
 
 
 if (EXTRA_DEPENDS)
diff --git a/platforms/posix/src/px4_layer/drv_hrt.cpp b/platforms/posix/src/px4_layer/drv_hrt.cpp
index cdf6c30a54..fe6d35b6fe 100644
--- a/platforms/posix/src/px4_layer/drv_hrt.cpp
+++ b/platforms/posix/src/px4_layer/drv_hrt.cpp
@@ -1,6 +1,6 @@
 /****************************************************************************
  *
- *   Copyright (c) 2012, 2013 PX4 Development Team. All rights reserved.
+ *   Copyright (c) 2012 - 2018 PX4 Development Team. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -32,7 +32,7 @@
  ****************************************************************************/
 
 /**
- * @file drv_hrt.c
+ * @file drv_hrt.cpp
  *
  * High-resolution timer with callouts and timekeeping.
  */
@@ -43,49 +43,53 @@
 #include <px4_workqueue.h>
 #include <px4_tasks.h>
 #include <drivers/drv_hrt.h>
-#include <lockstep_scheduler/lockstep_scheduler.h>
 #include <semaphore.h>
 #include <time.h>
 #include <string.h>
 #include <errno.h>
 #include "hrt_work.h"
 
-
-static struct sq_queue_s	callout_queue;
-
-
-static void		hrt_call_reschedule(void);
+#if defined(__PX4_POSIX_SITL)
+#include <lockstep_scheduler/lockstep_scheduler.h>
+#endif
 
 // Intervals in usec
-#define HRT_INTERVAL_MIN	50
-#define HRT_INTERVAL_MAX	50000000
+static constexpr unsigned HRT_INTERVAL_MIN = 50;
+static constexpr unsigned HRT_INTERVAL_MAX = 50000000;
 
+static struct sq_queue_s	callout_queue;
 static px4_sem_t 	_hrt_lock;
 static struct work_s	_hrt_work;
+
 #ifndef __PX4_QURT
 static hrt_abstime px4_timestart_monotonic = 0;
 #else
 static int32_t dsp_offset = 0;
 #endif
+
 static hrt_abstime _start_delay_time = 0;
 static hrt_abstime _delay_interval = 0;
 static hrt_abstime max_time = 0;
-pthread_mutex_t _hrt_mutex = PTHREAD_MUTEX_INITIALIZER;
+static pthread_mutex_t _hrt_mutex = PTHREAD_MUTEX_INITIALIZER;
 
+#if defined(__PX4_POSIX_SITL)
 static LockstepScheduler lockstep_scheduler;
+#endif
 
-static void
-hrt_call_invoke(void);
-
-static hrt_abstime
-_hrt_absolute_time_internal(void);
 
+hrt_abstime hrt_absolute_time_offset(void);
+static void hrt_call_reschedule(void);
+static void hrt_call_invoke(void);
+static hrt_abstime _hrt_absolute_time_internal(void);
 __EXPORT hrt_abstime hrt_reset(void);
 
-
 hrt_abstime hrt_absolute_time_offset(void)
 {
+#ifndef __PX4_QURT
 	return px4_timestart_monotonic;
+#else
+	return 0;
+#endif
 }
 
 static void hrt_lock(void)
@@ -590,18 +594,9 @@ void abstime_to_ts(struct timespec *ts, hrt_abstime abstime)
 	ts->tv_nsec = abstime * 1000;
 }
 
+#if defined(__PX4_POSIX_SITL)
 int px4_clock_gettime(clockid_t clk_id, struct timespec *tp)
 {
-#if defined(__PX4_QURT)
-	// Don't use the timestart on the DSP on Snapdragon because we manually
-	// set the px4_timestart using the hrt_set_absolute_time_offset().
-	return clock_gettime(clk_id, &tp);
-
-#elif defined(__PX4_POSIX_EAGLE) || defined(__PX4_POSIX_EXCELSIOR)
-	// Don't do any offseting on the Linux side on the Snapdragon.
-	return clock_gettime(clk_id, &tp);
-#else
-
 	if (clk_id == CLOCK_MONOTONIC) {
 
 		const uint64_t abstime = lockstep_scheduler.get_absolute_time();
@@ -611,8 +606,6 @@ int px4_clock_gettime(clockid_t clk_id, struct timespec *tp)
 	} else {
 		return system_clock_gettime(clk_id, tp);
 	}
-
-#endif
 }
 
 int px4_clock_settime(clockid_t clk_id, const struct timespec *ts)
@@ -660,3 +653,4 @@ int px4_pthread_cond_timedwait(pthread_cond_t *cond,
 	const uint64_t scheduled = time_us + px4_timestart_monotonic;
 	return lockstep_scheduler.cond_timedwait(cond, mutex, scheduled);
 }
+#endif
diff --git a/platforms/posix/src/px4_layer/px4_sem.cpp b/platforms/posix/src/px4_layer/px4_sem.cpp
index f3598a349b..bb02319bc2 100644
--- a/platforms/posix/src/px4_layer/px4_sem.cpp
+++ b/platforms/posix/src/px4_layer/px4_sem.cpp
@@ -46,7 +46,7 @@
 #include <pthread.h>
 #include <errno.h>
 
-#if defined(__PX4_DARWIN) || defined(__PX4_CYGWIN) || defined(__PX4_POSIX)
+#if (defined(__PX4_DARWIN) || defined(__PX4_CYGWIN) || defined(__PX4_POSIX)) && !defined(__PX4_QURT)
 
 #include <px4_posix.h>
 
diff --git a/platforms/qurt/src/px4_layer/CMakeLists.txt b/platforms/qurt/src/px4_layer/CMakeLists.txt
index 87affa5a40..c9a4dac893 100644
--- a/platforms/qurt/src/px4_layer/CMakeLists.txt
+++ b/platforms/qurt/src/px4_layer/CMakeLists.txt
@@ -40,7 +40,7 @@ set(QURT_LAYER_SRCS
 	px4_qurt_impl.cpp
 	px4_qurt_tasks.cpp
 	lib_crc32.c
-	../../../posix/src/px4_layer/drv_hrt.c
+	../../../posix/src/px4_layer/drv_hrt.cpp
 	qurt_stubs.c
 	main.cpp
 	shmem_qurt.cpp
diff --git a/platforms/qurt/src/px4_layer/drv_hrt.c b/platforms/qurt/src/px4_layer/drv_hrt.c
index 052a9b24e1..80e2c97a9e 100644
--- a/platforms/qurt/src/px4_layer/drv_hrt.c
+++ b/platforms/qurt/src/px4_layer/drv_hrt.c
@@ -39,7 +39,6 @@
 
 #include <px4_defines.h>
 #include <px4_workqueue.h>
-#include <perf_counter.h>
 #include <drivers/drv_hrt.h>
 #include <semaphore.h>
 #include <time.h>
diff --git a/src/drivers/gps/definitions.h b/src/drivers/gps/definitions.h
index 2d402f0c2d..7d483212c9 100644
--- a/src/drivers/gps/definitions.h
+++ b/src/drivers/gps/definitions.h
@@ -40,6 +40,7 @@
 #pragma once
 
 #include <px4_defines.h>
+#include <px4_time.h>
 
 #define GPS_INFO(...) PX4_INFO(__VA_ARGS__)
 #define GPS_WARN(...) PX4_WARN(__VA_ARGS__)
diff --git a/src/include/visibility.h b/src/include/visibility.h
index b5044e216f..0f8494fb53 100644
--- a/src/include/visibility.h
+++ b/src/include/visibility.h
@@ -1,6 +1,6 @@
 /****************************************************************************
  *
- *   Copyright (C) 2012 PX4 Development Team. All rights reserved.
+ *   Copyright (C) 2012-2018 PX4 Development Team. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -64,10 +64,12 @@
  * still needs to be used, thus we remap system_exit to exit.
  */
 #define system_exit exit
-#ifdef __PX4_POSIX
-#include <stdlib.h>
 
+#if defined(__PX4_POSIX_SITL)
+
+#include <stdlib.h>
 #include <unistd.h>
+
 #define system_usleep usleep
 #pragma GCC poison usleep
 #define system_sleep sleep
@@ -81,18 +83,13 @@
 // We can't poison pthread_cond_timedwait because it seems to be used in the
 // <string> include.
 
-
 #ifdef __cplusplus
 #include <cstdlib>
 #endif
 #pragma GCC poison exit
-#endif
 
-#ifdef __PX4_NUTTX
-/* On NuttX we call clearenv() so we cannot use getenv() and others (see px4_task_spawn_cmd() in px4_nuttx_tasks.c).
- * We need to include the headers declaring getenv() before the pragma, otherwise it will trigger a poison error.
- */
 #include <stdlib.h>
+
 #ifdef __cplusplus
 #include <cstdlib>
 #endif
@@ -102,11 +99,25 @@
 #include <unistd.h>
 #include <time.h>
 
+#else // defined(__PX4_POSIX_SITL)
+
 #define system_usleep usleep
 #define system_sleep sleep
-
 #define system_clock_gettime clock_gettime
 #define system_clock_settime clock_settime
+#define system_pthread_cond_timedwait pthread_cond_timedwait
 
+#endif
+
+
+#if defined(__PX4_NUTTX)
+/* On NuttX we call clearenv() so we cannot use getenv() and others (see
+ * px4_task_spawn_cmd() in px4_nuttx_tasks.c).
+ * We need to include the headers declaring getenv() before the pragma,
+ * otherwise it will trigger a poison error.  */
+#include <stdlib.h>
+#ifdef __cplusplus
+#include <cstdlib>
+#endif
 #pragma GCC poison getenv setenv putenv
-#endif /* __PX4_NUTTX */
+#endif // defined(__PX4_NUTTX)
diff --git a/src/platforms/common/work_queue/hrt_thread.c b/src/platforms/common/work_queue/hrt_thread.c
index e17b90ddba..df1f0bd75e 100644
--- a/src/platforms/common/work_queue/hrt_thread.c
+++ b/src/platforms/common/work_queue/hrt_thread.c
@@ -42,6 +42,7 @@
 #include <px4_defines.h>
 #include <px4_posix.h>
 #include <px4_tasks.h>
+#include <px4_time.h>
 #include <stdint.h>
 #include <signal.h>
 #include <stdio.h>
diff --git a/src/platforms/px4_sem.h b/src/platforms/px4_sem.h
index 661fb97953..970a8985fa 100644
--- a/src/platforms/px4_sem.h
+++ b/src/platforms/px4_sem.h
@@ -50,7 +50,7 @@
 #define sem_setprotocol(s,p)
 #endif
 
-#if defined(__PX4_DARWIN) || defined(__PX4_CYGWIN) || defined(__PX4_POSIX)
+#if (defined(__PX4_DARWIN) || defined(__PX4_CYGWIN) || defined(__PX4_POSIX)) && !defined(__PX4_QURT)
 
 __BEGIN_DECLS
 
@@ -71,24 +71,36 @@ __EXPORT int		px4_sem_destroy(px4_sem_t *s);
 
 __END_DECLS
 
-#else
+//#elif defined(__PX4_QURT)
 
-__BEGIN_DECLS
+//typedef sem_t px4_sem_t;
+
+//#define px4_sem_init		sem_init
+//#define px4_sem_setprotocol sem_setprotocol
+//#define px4_sem_wait		sem_wait
+//#define px4_sem_trywait	sem_trywait
+//#define px4_sem_post		sem_post
+//#define px4_sem_getvalue	sem_getvalue
+//#define px4_sem_destroy		sem_destroy
+
+#else
 
 typedef sem_t px4_sem_t;
 
+__BEGIN_DECLS
+
 #define px4_sem_init		sem_init
-#define px4_sem_setprotocol sem_setprotocol
+#define px4_sem_setprotocol	sem_setprotocol
 #define px4_sem_wait		sem_wait
-#define px4_sem_trywait	sem_trywait
+#define px4_sem_trywait		sem_trywait
 #define px4_sem_post		sem_post
 #define px4_sem_getvalue	sem_getvalue
 #define px4_sem_destroy		sem_destroy
 
-#ifdef __PX4_QURT
+#if defined(__PX4_QURT)
 __EXPORT int		px4_sem_timedwait(px4_sem_t *sem, const struct timespec *abstime);
 #else
-#define px4_sem_timedwait	 sem_timedwait
+#define px4_sem_timedwait	sem_timedwait
 #endif
 
 __END_DECLS
diff --git a/src/platforms/px4_time.h b/src/platforms/px4_time.h
index f4ab83acf9..f6d5e4c66d 100644
--- a/src/platforms/px4_time.h
+++ b/src/platforms/px4_time.h
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <unistd.h>
 #include <sys/types.h>
 #include <time.h>
 #include <pthread.h>
@@ -8,20 +9,7 @@
 #define clockid_t unsigned
 #endif
 
-#if defined(__PX4_QURT)
-
-#include <sys/timespec.h>
-
-__BEGIN_DECLS
-
-int px4_clock_gettime(clockid_t clk_id, struct timespec *tp);
-int px4_clock_settime(clockid_t clk_id, struct timespec *tp);
-
-__EXPORT unsigned int sleep(unsigned int sec);
-
-__END_DECLS
-
-#elif defined(__PX4_POSIX)
+#if defined(__PX4_POSIX_SITL) || defined(__PX4_QURT)
 
 __BEGIN_DECLS
 __EXPORT int px4_clock_gettime(clockid_t clk_id, struct timespec *tp);
@@ -34,11 +22,12 @@ __EXPORT int px4_pthread_cond_timedwait(pthread_cond_t *cond,
 					const struct timespec *abstime);
 __END_DECLS
 
-#elif defined(__PX4_NUTTX)
+#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
+#define px4_pthread_cond_timedwait system_pthread_cond_timedwait
 
 #endif