diff --git a/i3lock.c b/i3lock.c
index 895d50a..9c55db2 100644
--- a/i3lock.c
+++ b/i3lock.c
@@ -75,7 +75,7 @@ bool tile = false;
  * cold-boot attacks.
  *
  */
-static void clear_password_memory() {
+static void clear_password_memory(void) {
     /* A volatile pointer to the password buffer to prevent the compiler from
      * optimizing this out. */
     volatile char *vpassword = password;
@@ -105,7 +105,7 @@ static void clear_pam_wrong(EV_P_ ev_timer *w, int revents) {
     clear_pam_wrong_timeout = NULL;
 }
 
-static void input_done() {
+static void input_done(void) {
     if (input_position == 0)
         return;
 
@@ -406,7 +406,7 @@ static void handle_mapping_notify(xcb_mapping_notify_event_t *event) {
  * and also redraw the image, if any.
  *
  */
-void handle_screen_resize() {
+void handle_screen_resize(void) {
     xcb_get_geometry_cookie_t geomc;
     xcb_get_geometry_reply_t *geom;
     geomc = xcb_get_geometry(conn, screen->root);
diff --git a/unlock_indicator.c b/unlock_indicator.c
index d603531..3a33ca9 100644
--- a/unlock_indicator.c
+++ b/unlock_indicator.c
@@ -272,7 +272,7 @@ xcb_pixmap_t draw_image(uint32_t *resolution) {
  * Calls draw_image on a new pixmap and swaps that with the current pixmap
  *
  */
-void redraw_screen() {
+void redraw_screen(void) {
     xcb_pixmap_t bg_pixmap = draw_image(last_resolution);
     xcb_change_window_attributes(conn, win, XCB_CW_BACK_PIXMAP, (uint32_t[1]){ bg_pixmap });
     /* XXX: Possible optimization: Only update the area in the middle of the
@@ -303,7 +303,7 @@ static void clear_indicator(EV_P_ ev_timer *w, int revents) {
  * after an unsuccessful authentication attempt.
  *
  */
-void start_clear_indicator_timeout() {
+void start_clear_indicator_timeout(void) {
     if (clear_indicator_timeout) {
         ev_timer_stop(main_loop, clear_indicator_timeout);
         ev_timer_set(clear_indicator_timeout, 1.0, 0.);
@@ -322,7 +322,7 @@ void start_clear_indicator_timeout() {
  * Stops the clear_indicator timeout.
  *
  */
-void stop_clear_indicator_timeout() {
+void stop_clear_indicator_timeout(void) {
     if (clear_indicator_timeout) {
         ev_timer_stop(main_loop, clear_indicator_timeout);
         free(clear_indicator_timeout);
diff --git a/unlock_indicator.h b/unlock_indicator.h
index 8f039d2..e07c2a0 100644
--- a/unlock_indicator.h
+++ b/unlock_indicator.h
@@ -17,8 +17,8 @@ typedef enum {
 } pam_state_t;
 
 xcb_pixmap_t draw_image(uint32_t* resolution);
-void redraw_screen();
-void start_clear_indicator_timeout();
-void stop_clear_indicator_timeout();
+void redraw_screen(void);
+void start_clear_indicator_timeout(void);
+void stop_clear_indicator_timeout(void);
 
 #endif
diff --git a/xinerama.c b/xinerama.c
index 6a3557c..e4b87a4 100644
--- a/xinerama.c
+++ b/xinerama.c
@@ -26,7 +26,7 @@ Rect *xr_resolutions;
 static bool xinerama_active;
 extern bool debug_mode;
 
-void xinerama_init() {
+void xinerama_init(void) {
     if (!xcb_get_extension_data(conn, &xcb_xinerama_id)->present) {
         DEBUG("Xinerama extension not found, disabling.\n");
         return;
@@ -48,7 +48,7 @@ void xinerama_init() {
     xinerama_active = true;
 }
 
-void xinerama_query_screens() {
+void xinerama_query_screens(void) {
     if (!xinerama_active)
         return;
 
diff --git a/xinerama.h b/xinerama.h
index 207a9a8..a0de3c1 100644
--- a/xinerama.h
+++ b/xinerama.h
@@ -11,7 +11,7 @@ typedef struct Rect {
 extern int xr_screens;
 extern Rect *xr_resolutions;
 
-void xinerama_init();
-void xinerama_query_screens();
+void xinerama_init(void);
+void xinerama_query_screens(void);
 
 #endif