diff --git a/udev_device.c b/udev_device.c
index b52c9bb..5d9a86b 100644
--- a/udev_device.c
+++ b/udev_device.c
@@ -145,7 +145,7 @@ struct udev_device *udev_device_get_parent_with_subsystem_devtype(struct udev_de
     return NULL;
 }
 
-int udev_device_get_is_initialized(struct udev_device *udev_device)
+/* XXX NOT IMPLEMENTED */ int udev_device_get_is_initialized(struct udev_device *udev_device)
 {
     return 1;
 }
@@ -155,15 +155,13 @@ const char *udev_device_get_action(struct udev_device *udev_device)
     return udev_device_get_property_value(udev_device, "ACTION");
 }
 
-int udev_device_has_tag(struct udev_device *udev_device, const char *tag)
+/* XXX NOT IMPLEMENTED */ int udev_device_has_tag(struct udev_device *udev_device, const char *tag)
 {
-    // XXX NOT IMPLEMENTED
     return 0;
 }
 
-struct udev_list_entry *udev_device_get_devlinks_list_entry(struct udev_device *udev_device)
+/* XXX NOT IMPLEMENTED */ struct udev_list_entry *udev_device_get_devlinks_list_entry(struct udev_device *udev_device)
 {
-    // XXX NOT IMPLEMENTED
     return NULL;
 }
 
@@ -172,9 +170,8 @@ struct udev_list_entry *udev_device_get_properties_list_entry(struct udev_device
     return udev_device ? udev_list_entry_get_next(&udev_device->properties) : NULL;
 }
 
-struct udev_list_entry *udev_device_get_tags_list_entry(struct udev_device *udev_device)
+/* XXX NOT IMPLEMENTED */ struct udev_list_entry *udev_device_get_tags_list_entry(struct udev_device *udev_device)
 {
-    // XXX NOT IMPLEMENTED
     return NULL;
 }
 
@@ -657,15 +654,13 @@ struct udev_device *udev_device_new_from_file(struct udev *udev, const char *pat
     return udev_device;
 }
 
-struct udev_device *udev_device_new_from_device_id(struct udev *udev, const char *id)
+/* XXX NOT IMPLEMENTED */ struct udev_device *udev_device_new_from_device_id(struct udev *udev, const char *id)
 {
-    // XXX NOT IMPLEMENTED
     return NULL;
 }
 
-struct udev_device *udev_device_new_from_environment(struct udev *udev)
+/* XXX NOT IMPLEMENTED */ struct udev_device *udev_device_new_from_environment(struct udev *udev)
 {
-    // XXX NOT IMPLEMENTED
     return NULL;
 }
 
diff --git a/udev_enumerate.c b/udev_enumerate.c
index 558f288..44d10e0 100644
--- a/udev_enumerate.c
+++ b/udev_enumerate.c
@@ -83,19 +83,17 @@ int udev_enumerate_add_match_sysname(struct udev_enumerate *udev_enumerate, cons
     return udev_list_entry_add(&udev_enumerate->sysname_match, sysname, NULL, 0) ? 0 : -1;
 }
 
-int udev_enumerate_add_match_tag(struct udev_enumerate *udev_enumerate, const char *tag)
+/* XXX NOT IMPLEMENTED */ int udev_enumerate_add_match_tag(struct udev_enumerate *udev_enumerate, const char *tag)
 {
-    // XXX NOT IMPLEMENTED
     return 0;
 }
 
-int udev_enumerate_add_match_parent(struct udev_enumerate *udev_enumerate, struct udev_device *parent)
+/* XXX NOT IMPLEMENTED */ int udev_enumerate_add_match_parent(struct udev_enumerate *udev_enumerate, struct udev_device *parent)
 {
-    // XXX NOT IMPLEMENTED
     return 0;
 }
 
-int udev_enumerate_add_match_is_initialized(struct udev_enumerate *udev_enumerate)
+/* XXX NOT IMPLEMENTED */ int udev_enumerate_add_match_is_initialized(struct udev_enumerate *udev_enumerate)
 {
     return 0;
 }
@@ -342,9 +340,8 @@ int udev_enumerate_scan_devices(struct udev_enumerate *udev_enumerate)
     return 0;
 }
 
-int udev_enumerate_scan_subsystems(struct udev_enumerate *udev_enumerate)
+/* XXX NOT IMPLEMENTED */ int udev_enumerate_scan_subsystems(struct udev_enumerate *udev_enumerate)
 {
-    // XXX NOT IMPLEMENTED
     return 0;
 }
 
@@ -353,9 +350,8 @@ struct udev_list_entry *udev_enumerate_get_list_entry(struct udev_enumerate *ude
     return udev_enumerate ? udev_list_entry_get_next(&udev_enumerate->devices) : NULL;
 }
 
-int udev_enumerate_add_syspath(struct udev_enumerate *udev_enumerate, const char *syspath)
+/* XXX NOT IMPLEMENTED */ int udev_enumerate_add_syspath(struct udev_enumerate *udev_enumerate, const char *syspath)
 {
-    // XXX NOT IMPLEMENTED
     return 0;
 }
 
diff --git a/udev_hwdb.c b/udev_hwdb.c
index c76f479..ac69d99 100644
--- a/udev_hwdb.c
+++ b/udev_hwdb.c
@@ -2,22 +2,22 @@
 
 #include "udev.h"
 
-struct udev_hwdb *udev_hwdb_new(struct udev *udev)
+/* XXX NOT IMPLEMENTED */ struct udev_hwdb *udev_hwdb_new(struct udev *udev)
 {
     return NULL;
 }
 
-struct udev_hwdb *udev_hwdb_ref(struct udev_hwdb *hwdb)
+/* XXX NOT IMPLEMENTED */ struct udev_hwdb *udev_hwdb_ref(struct udev_hwdb *hwdb)
 {
     return NULL;
 }
 
-struct udev_hwdb *udev_hwdb_unref(struct udev_hwdb *hwdb)
+/* XXX NOT IMPLEMENTED */ struct udev_hwdb *udev_hwdb_unref(struct udev_hwdb *hwdb)
 {
     return NULL;
 }
 
-struct udev_list_entry *udev_hwdb_get_properties_list_entry(struct udev_hwdb *hwdb, const char *modalias, unsigned int flags)
+/* XXX NOT IMPLEMENTED */ struct udev_list_entry *udev_hwdb_get_properties_list_entry(struct udev_hwdb *hwdb, const char *modalias, unsigned int flags)
 {
     return NULL;
 }
diff --git a/udev_monitor.c b/udev_monitor.c
index f835a5a..81a803c 100644
--- a/udev_monitor.c
+++ b/udev_monitor.c
@@ -166,9 +166,8 @@ int udev_monitor_enable_receiving(struct udev_monitor *udev_monitor)
     return 0;
 }
 
-int udev_monitor_set_receive_buffer_size(struct udev_monitor *udev_monitor, int size)
+/* XXX NOT IMPLEMENTED */ int udev_monitor_set_receive_buffer_size(struct udev_monitor *udev_monitor, int size)
 {
-    // XXX NOT IMPLEMENTED
     return 0;
 }
 
@@ -182,15 +181,13 @@ struct udev *udev_monitor_get_udev(struct udev_monitor *udev_monitor)
     return udev_monitor ? udev_monitor->udev : NULL;
 }
 
-int udev_monitor_filter_update(struct udev_monitor *udev_monitor)
+/* XXX NOT IMPLEMENTED */ int udev_monitor_filter_update(struct udev_monitor *udev_monitor)
 {
-    // XXX NOT IMPLEMENTED
     return 0;
 }
 
-int udev_monitor_filter_remove(struct udev_monitor *udev_monitor)
+/* XXX NOT IMPLEMENTED */ int udev_monitor_filter_remove(struct udev_monitor *udev_monitor)
 {
-    // XXX NOT IMPLEMENTED
     return 0;
 }
 
@@ -209,9 +206,8 @@ int udev_monitor_filter_add_match_subsystem_devtype(struct udev_monitor *udev_mo
     return 0;
 }
 
-int udev_monitor_filter_add_match_tag(struct udev_monitor *udev_monitor, const char *tag)
+/* XXX NOT IMPLEMENTED */ int udev_monitor_filter_add_match_tag(struct udev_monitor *udev_monitor, const char *tag)
 {
-    // XXX NOT IMPLEMENTED
     return 0;
 }