diff --git a/src/xrt/auxiliary/math/m_permutation.c b/src/xrt/auxiliary/math/m_permutation.c
index 184607fc2..fb2e54b99 100644
--- a/src/xrt/auxiliary/math/m_permutation.c
+++ b/src/xrt/auxiliary/math/m_permutation.c
@@ -130,7 +130,7 @@ printArray(const uint32_t *array, uint32_t num)
 }
 
 void
-m_do_the_thing()
+m_do_the_thing(void)
 {
 	struct m_permutator mp = {0};
 	uint32_t elements[7];
diff --git a/src/xrt/auxiliary/math/m_permutation.h b/src/xrt/auxiliary/math/m_permutation.h
index 3aa450b38..131f40868 100644
--- a/src/xrt/auxiliary/math/m_permutation.h
+++ b/src/xrt/auxiliary/math/m_permutation.h
@@ -40,7 +40,7 @@ m_permutator_reset(struct m_permutator *mp);
 
 
 void
-m_do_the_thing();
+m_do_the_thing(void);
 
 
 #ifdef __cplusplus
diff --git a/src/xrt/auxiliary/tracking/t_imu.h b/src/xrt/auxiliary/tracking/t_imu.h
index 890c8162a..c1a44eb66 100644
--- a/src/xrt/auxiliary/tracking/t_imu.h
+++ b/src/xrt/auxiliary/tracking/t_imu.h
@@ -26,7 +26,7 @@ struct imu_fusion;
  * @ingroup aux_tracking
  */
 struct imu_fusion *
-imu_fusion_create();
+imu_fusion_create(void);
 
 
 /*!
diff --git a/src/xrt/auxiliary/util/u_debug.h b/src/xrt/auxiliary/util/u_debug.h
index de5bf6af2..b036413d3 100644
--- a/src/xrt/auxiliary/util/u_debug.h
+++ b/src/xrt/auxiliary/util/u_debug.h
@@ -48,7 +48,7 @@ enum u_logging_level
 debug_get_log_option(const char *name, enum u_logging_level _default);
 
 #define DEBUG_GET_ONCE_OPTION(suffix, name, _default)                                                                  \
-	static const char *debug_get_option_##suffix()                                                                 \
+	static const char *debug_get_option_##suffix(void)                                                             \
 	{                                                                                                              \
 		static bool gotten = false;                                                                            \
 		static const char *stored;                                                                             \
@@ -60,7 +60,7 @@ debug_get_log_option(const char *name, enum u_logging_level _default);
 	}
 
 #define DEBUG_GET_ONCE_TRISTATE_OPTION(suffix, name)                                                                   \
-	static enum debug_tristate_option debug_get_tristate_option_##suffix()                                         \
+	static enum debug_tristate_option debug_get_tristate_option_##suffix(void)                                     \
 	{                                                                                                              \
 		static bool gotten = false;                                                                            \
 		static enum debug_tristate_option stored;                                                              \
@@ -72,7 +72,7 @@ debug_get_log_option(const char *name, enum u_logging_level _default);
 	}
 
 #define DEBUG_GET_ONCE_BOOL_OPTION(suffix, name, _default)                                                             \
-	static bool debug_get_bool_option_##suffix()                                                                   \
+	static bool debug_get_bool_option_##suffix(void)                                                               \
 	{                                                                                                              \
 		static bool gotten = false;                                                                            \
 		static bool stored;                                                                                    \
@@ -84,7 +84,7 @@ debug_get_log_option(const char *name, enum u_logging_level _default);
 	}
 
 #define DEBUG_GET_ONCE_NUM_OPTION(suffix, name, _default)                                                              \
-	static long debug_get_num_option_##suffix()                                                                    \
+	static long debug_get_num_option_##suffix(void)                                                                \
 	{                                                                                                              \
 		static long gotten = false;                                                                            \
 		static long stored;                                                                                    \
@@ -96,7 +96,7 @@ debug_get_log_option(const char *name, enum u_logging_level _default);
 	}
 
 #define DEBUG_GET_ONCE_FLOAT_OPTION(suffix, name, _default)                                                            \
-	static float debug_get_float_option_##suffix()                                                                 \
+	static float debug_get_float_option_##suffix(void)                                                             \
 	{                                                                                                              \
 		static long gotten = false;                                                                            \
 		static float stored;                                                                                   \
@@ -108,7 +108,7 @@ debug_get_log_option(const char *name, enum u_logging_level _default);
 	}
 
 #define DEBUG_GET_ONCE_LOG_OPTION(suffix, name, _default)                                                              \
-	static enum u_logging_level debug_get_log_option_##suffix()                                                    \
+	static enum u_logging_level debug_get_log_option_##suffix(void)                                                \
 	{                                                                                                              \
 		static long gotten = false;                                                                            \
 		static enum u_logging_level stored;                                                                    \
diff --git a/src/xrt/auxiliary/util/u_deque.h b/src/xrt/auxiliary/util/u_deque.h
index 44295413b..c98161a8a 100644
--- a/src/xrt/auxiliary/util/u_deque.h
+++ b/src/xrt/auxiliary/util/u_deque.h
@@ -21,7 +21,7 @@ extern "C" {
 	{                                                                                                              \
 		void *ptr;                                                                                             \
 	};                                                                                                             \
-	struct u_deque_##TYPE u_deque_##TYPE##_create();                                                               \
+	struct u_deque_##TYPE u_deque_##TYPE##_create(void);                                                           \
 	void u_deque_##TYPE##_push_back(struct u_deque_##TYPE ud, TYPE e);                                             \
 	bool u_deque_##TYPE##_pop_front(struct u_deque_##TYPE ud, TYPE *e);                                            \
 	TYPE u_deque_##TYPE##_at(struct u_deque_##TYPE ud, size_t i);                                                  \
diff --git a/src/xrt/auxiliary/util/u_metrics.c b/src/xrt/auxiliary/util/u_metrics.c
index d0a5163e9..4aa80c105 100644
--- a/src/xrt/auxiliary/util/u_metrics.c
+++ b/src/xrt/auxiliary/util/u_metrics.c
@@ -79,7 +79,7 @@ write_version(uint32_t major, uint32_t minor)
  */
 
 void
-u_metrics_init()
+u_metrics_init(void)
 {
 	const char *str = debug_get_option_metrics_file();
 	if (str == NULL) {
@@ -103,7 +103,7 @@ u_metrics_init()
 }
 
 void
-u_metrics_close()
+u_metrics_close(void)
 {
 	if (!g_metrics_initialized) {
 		return;
diff --git a/src/xrt/auxiliary/util/u_misc.c b/src/xrt/auxiliary/util/u_misc.c
index 06a68c326..7a1819f67 100644
--- a/src/xrt/auxiliary/util/u_misc.c
+++ b/src/xrt/auxiliary/util/u_misc.c
@@ -11,7 +11,7 @@
 
 
 int
-u_silence_pedantic_warning()
+u_silence_pedantic_warning(void)
 {
 	return 42;
 }
diff --git a/src/xrt/auxiliary/util/u_process.c b/src/xrt/auxiliary/util/u_process.c
index 2e0a4a595..baadc43e1 100644
--- a/src/xrt/auxiliary/util/u_process.c
+++ b/src/xrt/auxiliary/util/u_process.c
@@ -49,7 +49,7 @@ struct u_process
 };
 
 struct u_process *
-u_process_create_if_not_running()
+u_process_create_if_not_running(void)
 {
 #ifdef XRT_HAVE_LIBBSD
 
diff --git a/src/xrt/auxiliary/util/u_process.h b/src/xrt/auxiliary/util/u_process.h
index 4d0e33231..87475f844 100644
--- a/src/xrt/auxiliary/util/u_process.h
+++ b/src/xrt/auxiliary/util/u_process.h
@@ -27,7 +27,7 @@ struct u_process;
  * @ingroup aux_util
  */
 struct u_process *
-u_process_create_if_not_running();
+u_process_create_if_not_running(void);
 
 /*!
  * Releases the unique handle of the operating system user.
diff --git a/src/xrt/compositor/client/comp_gl_xlib_client.c b/src/xrt/compositor/client/comp_gl_xlib_client.c
index f7b996d7c..cd85de67d 100644
--- a/src/xrt/compositor/client/comp_gl_xlib_client.c
+++ b/src/xrt/compositor/client/comp_gl_xlib_client.c
@@ -118,7 +118,7 @@ client_gl_context_end_locked(struct xrt_compositor *xc)
 	}
 }
 
-typedef void (*void_ptr_func)();
+typedef void (*void_ptr_func)(void);
 
 #ifdef __cplusplus
 extern "C"
diff --git a/src/xrt/compositor/main/comp_compositor.c b/src/xrt/compositor/main/comp_compositor.c
index 1509e188c..bc2e425af 100644
--- a/src/xrt/compositor/main/comp_compositor.c
+++ b/src/xrt/compositor/main/comp_compositor.c
@@ -97,7 +97,7 @@ ns_to_ms(int64_t ns)
 }
 
 static double
-ts_ms()
+ts_ms(void)
 {
 	int64_t monotonic = os_monotonic_get_ns();
 	return ns_to_ms(monotonic);
diff --git a/src/xrt/drivers/arduino/arduino_interface.h b/src/xrt/drivers/arduino/arduino_interface.h
index 6c8bd32d2..1226e4191 100644
--- a/src/xrt/drivers/arduino/arduino_interface.h
+++ b/src/xrt/drivers/arduino/arduino_interface.h
@@ -30,7 +30,7 @@ struct os_ble_device;
  * @ingroup drv_arduino
  */
 struct xrt_auto_prober *
-arduino_create_auto_prober();
+arduino_create_auto_prober(void);
 
 /*!
  * Create a arduino device from a ble notify.
diff --git a/src/xrt/drivers/arduino/arduino_prober.c b/src/xrt/drivers/arduino/arduino_prober.c
index fd8fdfc89..fe50d09b1 100644
--- a/src/xrt/drivers/arduino/arduino_prober.c
+++ b/src/xrt/drivers/arduino/arduino_prober.c
@@ -99,7 +99,7 @@ arduino_prober_autoprobe(struct xrt_auto_prober *xap,
  */
 
 struct xrt_auto_prober *
-arduino_create_auto_prober()
+arduino_create_auto_prober(void)
 {
 	struct arduino_prober *ap = U_TYPED_CALLOC(struct arduino_prober);
 	ap->base.name = "Arduino";
diff --git a/src/xrt/drivers/daydream/daydream_interface.h b/src/xrt/drivers/daydream/daydream_interface.h
index 2319afe4a..d2640a244 100644
--- a/src/xrt/drivers/daydream/daydream_interface.h
+++ b/src/xrt/drivers/daydream/daydream_interface.h
@@ -26,7 +26,7 @@ extern "C" {
  * @ingroup drv_daydream
  */
 struct xrt_auto_prober *
-daydream_create_auto_prober();
+daydream_create_auto_prober(void);
 
 
 /*!
diff --git a/src/xrt/drivers/daydream/daydream_prober.c b/src/xrt/drivers/daydream/daydream_prober.c
index 989d18ea4..2c3f50d38 100644
--- a/src/xrt/drivers/daydream/daydream_prober.c
+++ b/src/xrt/drivers/daydream/daydream_prober.c
@@ -101,7 +101,7 @@ daydream_prober_autoprobe(struct xrt_auto_prober *xap,
  */
 
 struct xrt_auto_prober *
-daydream_create_auto_prober()
+daydream_create_auto_prober(void)
 {
 	struct daydream_prober *pdaydream = U_TYPED_CALLOC(struct daydream_prober);
 	pdaydream->base.name = "DayDream";
diff --git a/src/xrt/drivers/euroc/euroc_device.c b/src/xrt/drivers/euroc/euroc_device.c
index 76aeae78e..8defa610d 100644
--- a/src/xrt/drivers/euroc/euroc_device.c
+++ b/src/xrt/drivers/euroc/euroc_device.c
@@ -72,7 +72,7 @@ euroc_prober_autoprobe(struct xrt_auto_prober *xap,
 }
 
 struct xrt_auto_prober *
-euroc_create_auto_prober()
+euroc_create_auto_prober(void)
 {
 	// `ep` var name used for euroc_player, let's use `epr` instead
 	struct euroc_prober *epr = U_TYPED_CALLOC(struct euroc_prober);
diff --git a/src/xrt/drivers/qwerty/qwerty_prober.c b/src/xrt/drivers/qwerty/qwerty_prober.c
index 35f26f3dc..d3df0b130 100644
--- a/src/xrt/drivers/qwerty/qwerty_prober.c
+++ b/src/xrt/drivers/qwerty/qwerty_prober.c
@@ -73,7 +73,7 @@ qwerty_prober_autoprobe(struct xrt_auto_prober *xap,
 }
 
 struct xrt_auto_prober *
-qwerty_create_auto_prober()
+qwerty_create_auto_prober(void)
 {
 	struct qwerty_prober *qp = U_TYPED_CALLOC(struct qwerty_prober);
 	qp->base.name = "Qwerty";
diff --git a/src/xrt/drivers/realsense/rs_interface.h b/src/xrt/drivers/realsense/rs_interface.h
index 95f253205..753ccb623 100644
--- a/src/xrt/drivers/realsense/rs_interface.h
+++ b/src/xrt/drivers/realsense/rs_interface.h
@@ -59,7 +59,7 @@ rs_source_create(struct xrt_frame_context *xfctx, int device_idx);
  * @return An xrt_device that you can call get_tracked_pose on with XRT_INPUT_GENERIC_TRACKER_POSE
  */
 struct xrt_device *
-rs_create_tracked_device_internal_slam();
+rs_create_tracked_device_internal_slam(void);
 
 /*!
  * @dir drivers/realsense
diff --git a/src/xrt/drivers/rift_s/rift_s_tracker.c b/src/xrt/drivers/rift_s/rift_s_tracker.c
index 93f7366b3..1eb173cff 100644
--- a/src/xrt/drivers/rift_s/rift_s_tracker.c
+++ b/src/xrt/drivers/rift_s/rift_s_tracker.c
@@ -94,7 +94,7 @@ rift_s_tracker_switch_method_cb(void *t_ptr)
 }
 
 XRT_MAYBE_UNUSED struct t_imu_calibration
-rift_s_create_slam_imu_calib()
+rift_s_create_slam_imu_calib(void)
 {
 	/* FIXME: Validate these hard coded standard deviations against
 	 * some actual at-rest IMU measurements */
diff --git a/src/xrt/drivers/sample/sample_prober.c b/src/xrt/drivers/sample/sample_prober.c
index 199344163..027030a24 100644
--- a/src/xrt/drivers/sample/sample_prober.c
+++ b/src/xrt/drivers/sample/sample_prober.c
@@ -60,7 +60,7 @@ sample_auto_prober_autoprobe(struct xrt_auto_prober *xap,
 }
 
 struct xrt_auto_prober *
-sample_create_auto_prober()
+sample_create_auto_prober(void)
 {
 	struct sample_auto_prober *sap = U_TYPED_CALLOC(struct sample_auto_prober);
 	sap->base.name = "Sample";
diff --git a/src/xrt/drivers/simulated/simulated_prober.c b/src/xrt/drivers/simulated/simulated_prober.c
index 7681b86fc..984d3867d 100644
--- a/src/xrt/drivers/simulated/simulated_prober.c
+++ b/src/xrt/drivers/simulated/simulated_prober.c
@@ -73,7 +73,7 @@ simulated_prober_autoprobe(struct xrt_auto_prober *xap,
 }
 
 struct xrt_auto_prober *
-simulated_create_auto_prober()
+simulated_create_auto_prober(void)
 {
 	struct simulated_prober *dp = U_TYPED_CALLOC(struct simulated_prober);
 	dp->base.name = "Simulated";
diff --git a/src/xrt/include/xrt/xrt_prober.h b/src/xrt/include/xrt/xrt_prober.h
index 04ab01b33..53ca68526 100644
--- a/src/xrt/include/xrt/xrt_prober.h
+++ b/src/xrt/include/xrt/xrt_prober.h
@@ -722,7 +722,7 @@ struct xrt_prober_entry
  *
  * @ingroup xrt_iface
  */
-typedef struct xrt_auto_prober *(*xrt_auto_prober_create_func_t)();
+typedef struct xrt_auto_prober *(*xrt_auto_prober_create_func_t)(void);
 
 /*!
  * @interface xrt_auto_prober
diff --git a/src/xrt/targets/sdl_test/sdl_internal.h b/src/xrt/targets/sdl_test/sdl_internal.h
index 43c7ff610..d6b1d6713 100644
--- a/src/xrt/targets/sdl_test/sdl_internal.h
+++ b/src/xrt/targets/sdl_test/sdl_internal.h
@@ -386,7 +386,7 @@ sdl_instance_init(struct sdl_program *sp);
  * @ingroup sdl_test
  */
 struct sdl_program *
-sdl_program_plus_create();
+sdl_program_plus_create(void);
 
 /*!
  * Render a frame, called by the compositor when layers have been committed.