diff --git a/obt/display.c b/obt/display.c
index d72f8ad6..72c1b4d6 100644
--- a/obt/display.c
+++ b/obt/display.c
@@ -117,7 +117,7 @@ gboolean obt_display_open(const char *display_name)
     return obt_display != NULL;
 }
 
-void obt_display_close()
+void obt_display_close(void)
 {
     obt_keyboard_shutdown();
     if (obt_display) XCloseDisplay(obt_display);
diff --git a/obt/keyboard.c b/obt/keyboard.c
index 710a94c2..699fa8ba 100644
--- a/obt/keyboard.c
+++ b/obt/keyboard.c
@@ -49,7 +49,7 @@ static gboolean hyper_l = FALSE;
 
 static gboolean started = FALSE;
 
-void obt_keyboard_reload()
+void obt_keyboard_reload(void)
 {
     gint i, j, k;
 
@@ -97,7 +97,7 @@ void obt_keyboard_reload()
     modkeys_keys[OBT_KEYBOARD_MODKEY_CONTROL] = ControlMask;
 }
 
-void obt_keyboard_shutdown()
+void obt_keyboard_shutdown(void)
 {
     XFreeModifiermap(modmap);
     modmap = NULL;
diff --git a/obt/mainloop.c b/obt/mainloop.c
index 64972dd7..691c6875 100644
--- a/obt/mainloop.c
+++ b/obt/mainloop.c
@@ -140,7 +140,7 @@ struct _ObtMainLoopFdHandlerType
     GDestroyNotify destroy;
 };
 
-ObtMainLoop *obt_main_loop_new()
+ObtMainLoop *obt_main_loop_new(void)
 {
     ObtMainLoop *loop;
 
diff --git a/obt/prop.c b/obt/prop.c
index 5fe0f730..108c1184 100644
--- a/obt/prop.c
+++ b/obt/prop.c
@@ -33,7 +33,7 @@ gboolean prop_started = FALSE;
 #define CREATE(var) CREATE_NAME(var, #var)
 #define CREATE_(var) CREATE_NAME(var, "_" #var)
 
-void obt_prop_startup()
+void obt_prop_startup(void)
 {
     if (prop_started) return;
     prop_started = TRUE;
diff --git a/obt/xevent.c b/obt/xevent.c
index 9732c8f9..1cc32a94 100644
--- a/obt/xevent.c
+++ b/obt/xevent.c
@@ -44,7 +44,7 @@ static void xevent_handler(const XEvent *e, gpointer data);
 static guint window_hash(Window *w) { return *w; }
 static gboolean window_comp(Window *w1, Window *w2) { return *w1 == *w2; }
 
-ObtXEventHandler* xevent_new()
+ObtXEventHandler* xevent_new(void)
 {
     ObtXEventHandler *h;
 
diff --git a/openbox/mouse.c b/openbox/mouse.c
index ae9a6fc2..0233ac38 100644
--- a/openbox/mouse.c
+++ b/openbox/mouse.c
@@ -202,7 +202,7 @@ static gboolean fire_binding(ObMouseAction a, ObFrameContext context,
     return TRUE;
 }
 
-void mouse_replay_pointer()
+void mouse_replay_pointer(void)
 {
     if (replay_pointer_needed) {
         /* replay the pointer event before any windows move */
diff --git a/openbox/openbox.c b/openbox/openbox.c
index f7d820e0..d6da5ec7 100644
--- a/openbox/openbox.c
+++ b/openbox/openbox.c
@@ -467,7 +467,7 @@ static void signal_handler(gint signal, gpointer data)
     }
 }
 
-static void print_version()
+static void print_version(void)
 {
     g_print("Openbox %s\n", PACKAGE_VERSION);
     g_print(_("Copyright (c)"));
@@ -479,7 +479,7 @@ static void print_version()
     g_print("under certain conditions. See the file COPYING for details.\n\n");
 }
 
-static void print_help()
+static void print_help(void)
 {
     g_print(_("Syntax: openbox [options]\n"));
     g_print(_("\nOptions:\n"));
@@ -512,7 +512,7 @@ static void remove_args(gint *argc, gchar **argv, gint index, gint num)
     *argc -= num;
 }
 
-static void parse_env()
+static void parse_env(void)
 {
     /* unset this so we don't pass it on unknowingly */
     unsetenv("DESKTOP_STARTUP_ID");
@@ -644,13 +644,13 @@ void ob_restart_other(const gchar *path)
     ob_restart();
 }
 
-void ob_restart()
+void ob_restart(void)
 {
     restart = TRUE;
     ob_exit(0);
 }
 
-void ob_reconfigure()
+void ob_reconfigure(void)
 {
     reconfigure = TRUE;
     ob_exit(0);
@@ -662,7 +662,7 @@ void ob_exit(gint code)
     obt_main_loop_exit(ob_main_loop);
 }
 
-void ob_exit_replace()
+void ob_exit_replace(void)
 {
     exitcode = 0;
     being_replaced = TRUE;
@@ -681,7 +681,7 @@ KeyCode ob_keycode(ObKey key)
     return keys[key];
 }
 
-ObState ob_state()
+ObState ob_state(void)
 {
     return state;
 }
diff --git a/openbox/session.c b/openbox/session.c
index 267d17ef..3822d414 100644
--- a/openbox/session.c
+++ b/openbox/session.c
@@ -159,7 +159,7 @@ void session_shutdown(gboolean permanent)
 }
 
 /*! Connect to the session manager and set up our callback functions */
-static gboolean session_connect()
+static gboolean session_connect(void)
 {
     SmcCallbacks cb;
     gchar *oldid;
@@ -193,7 +193,7 @@ static gboolean session_connect()
     return sm_conn != NULL;
 }
 
-static void session_setup_program()
+static void session_setup_program(void)
 {
     SmPropValue vals = {
         .value = sm_argv[0],
@@ -212,7 +212,7 @@ static void session_setup_program()
     g_free(prop.type);
 }
 
-static void session_setup_user()
+static void session_setup_user(void)
 {
     char *user = g_strdup(g_get_user_name());
 
@@ -255,7 +255,7 @@ static void session_setup_restart_style(gboolean restart)
     g_free(prop.type);
 }
 
-static void session_setup_pid()
+static void session_setup_pid(void)
 {
     gchar *pid = g_strdup_printf("%ld", (glong) getpid());
 
@@ -278,7 +278,7 @@ static void session_setup_pid()
 }
 
 /*! This is a gnome-session-manager extension */
-static void session_setup_priority()
+static void session_setup_priority(void)
 {
     gchar priority = 20; /* 20 is a lower prioity to run before other apps */
 
@@ -299,7 +299,7 @@ static void session_setup_priority()
     g_free(prop.type);
 }
 
-static void session_setup_clone_command()
+static void session_setup_clone_command(void)
 {
     gint i;
 
@@ -325,7 +325,7 @@ static void session_setup_clone_command()
     g_free(vals);
 }
 
-static void session_setup_restart_command()
+static void session_setup_restart_command(void)
 {
     gint i;
 
@@ -367,7 +367,7 @@ static void session_setup_restart_command()
     g_free(vals);
 }
 
-static ObSMSaveData *sm_save_get_data()
+static ObSMSaveData *sm_save_get_data(void)
 {
     ObSMSaveData *savedata = g_new0(ObSMSaveData, 1);
     /* save the active desktop and client.