diff --git a/include/config.h b/include/config.h
index c1705d4f..e959a2dc 100644
--- a/include/config.h
+++ b/include/config.h
@@ -277,7 +277,7 @@ void load_configuration(xcb_connection_t *conn, const char *override_configfile,
  * Translates keysymbols to keycodes for all bindings which use keysyms.
  *
  */
-void translate_keysyms();
+void translate_keysyms(void);
 
 /**
  * Ungrabs all keys, to be called before re-grabbing the keys because of a
diff --git a/include/ewmh.h b/include/ewmh.h
index aacd1510..a786069a 100644
--- a/include/ewmh.h
+++ b/include/ewmh.h
@@ -17,7 +17,7 @@
  * and _NET_NUMBER_OF_DESKTOPS - 1.
  *
  */
-void ewmh_update_current_desktop();
+void ewmh_update_current_desktop(void);
 
 /**
  * Updates _NET_ACTIVE_WINDOW with the currently focused window.
@@ -44,6 +44,6 @@ void ewmh_update_client_list_stacking(xcb_window_t *stack, int num_windows);
  * Set up the EWMH hints on the root window.
  *
  */
-void ewmh_setup_hints();
+void ewmh_setup_hints(void);
 
 #endif
diff --git a/include/handlers.h b/include/handlers.h
index ebec34cf..bcebf9ff 100644
--- a/include/handlers.h
+++ b/include/handlers.h
@@ -43,7 +43,7 @@ void handle_event(int type, xcb_generic_event_t *event);
  * received from X11
  *
  */
-void property_handlers_init();
+void property_handlers_init(void);
 
 #if 0
 /**
diff --git a/include/ipc.h b/include/ipc.h
index 9b59fb01..af80fa4b 100644
--- a/include/ipc.h
+++ b/include/ipc.h
@@ -78,7 +78,7 @@ void ipc_send_event(const char *event, uint32_t message_type, const char *payloa
  * when exiting or restarting only!
  *
  */
-void ipc_shutdown();
+void ipc_shutdown(void);
 
 void dump_node(yajl_gen gen, Con *con, bool inplace_restart);
 
diff --git a/include/libi3.h b/include/libi3.h
index 7b7cf576..b88bcb67 100644
--- a/include/libi3.h
+++ b/include/libi3.h
@@ -197,7 +197,7 @@ void set_font(i3Font *font);
  * Frees the resources taken by the current font.
  *
  */
-void free_font();
+void free_font(void);
 
 /**
  * Converts the given string to UTF-8 from UCS-2 big endian. The return value
diff --git a/include/log.h b/include/log.h
index 0eb55744..e5e20dc1 100644
--- a/include/log.h
+++ b/include/log.h
@@ -29,7 +29,7 @@ extern int shmlog_size;
  * XDG_RUNTIME_DIR, see get_process_filename()).
  *
  */
-void init_logging();
+void init_logging(void);
 
 /**
  * Enables the given loglevel.
diff --git a/include/manage.h b/include/manage.h
index 833d614f..c16d295f 100644
--- a/include/manage.h
+++ b/include/manage.h
@@ -27,7 +27,7 @@ void manage_existing_windows(xcb_window_t root);
  * side-effects which are to be expected when continuing to run i3.
  *
  */
-void restore_geometry();
+void restore_geometry(void);
 
 /**
  * Do some sanity checks and then reparent the window.
diff --git a/include/randr.h b/include/randr.h
index 4cefba80..ac527bcc 100644
--- a/include/randr.h
+++ b/include/randr.h
@@ -61,13 +61,13 @@ void init_ws_for_output(Output *output, Con *content);
  * (Re-)queries the outputs via RandR and stores them in the list of outputs.
  *
  */
-void randr_query_outputs();
+void randr_query_outputs(void);
 
 /**
  * Returns the first output which is active.
  *
  */
-Output *get_first_output();
+Output *get_first_output(void);
 
 /**
  * Returns the output with the given name if it is active (!) or NULL.
diff --git a/include/sighandler.h b/include/sighandler.h
index 5ffef2a3..571070b0 100644
--- a/include/sighandler.h
+++ b/include/sighandler.h
@@ -16,6 +16,6 @@
  * Setup signal handlers to safely handle SIGSEGV and SIGFPE
  *
  */
-void setup_signal_handler();
+void setup_signal_handler(void);
 
 #endif
diff --git a/include/tree.h b/include/tree.h
index 81fdbe6a..b9159e3b 100644
--- a/include/tree.h
+++ b/include/tree.h
@@ -42,20 +42,20 @@ void tree_split(Con *con, orientation_t orientation);
  * Moves focus one level up.
  *
  */
-void level_up();
+void level_up(void);
 
 /**
  * Moves focus one level down.
  *
  */
-void level_down();
+void level_down(void);
 
 /**
  * Renders the tree, that is rendering all outputs using render_con() and
  * pushing the changes to X11 using x_push_changes().
  *
  */
-void tree_render();
+void tree_render(void);
 
 /**
  * Closes the current container using tree_close().
diff --git a/include/workspace.h b/include/workspace.h
index 8e682bb0..1b25b425 100644
--- a/include/workspace.h
+++ b/include/workspace.h
@@ -69,31 +69,31 @@ void workspace_show_by_name(const char *num);
  * Returns the next workspace.
  *
  */
-Con* workspace_next();
+Con* workspace_next(void);
 
 /**
  * Returns the previous workspace.
  *
  */
-Con* workspace_prev();
+Con* workspace_prev(void);
 
 /**
  * Returns the next workspace on the same output
  *
  */
-Con* workspace_next_on_output();
+Con* workspace_next_on_output(void);
 
 /**
  * Returns the previous workspace on the same output
  *
  */
-Con* workspace_prev_on_output();
+Con* workspace_prev_on_output(void);
 
 /**
  * Focuses the previously focused workspace.
  *
  */
-void workspace_back_and_forth();
+void workspace_back_and_forth(void);
 
 
 #if 0
diff --git a/include/x.h b/include/x.h
index 36c77431..cb4a8a96 100644
--- a/include/x.h
+++ b/include/x.h
@@ -108,7 +108,7 @@ void x_set_name(Con *con, const char *name);
  * Sets up i3 specific atoms (I3_SOCKET_PATH and I3_CONFIG_PATH)
  *
  */
-void x_set_i3_atoms();
+void x_set_i3_atoms(void);
 
 /**
  * Set warp_to coordinates.  This will trigger on the next call to
diff --git a/include/xcursor.h b/include/xcursor.h
index 2a67bcb7..c341f90b 100644
--- a/include/xcursor.h
+++ b/include/xcursor.h
@@ -20,7 +20,7 @@ enum xcursor_cursor_t {
     XCURSOR_CURSOR_MAX
 };
 
-void xcursor_load_cursors();
+void xcursor_load_cursors(void);
 Cursor xcursor_get_cursor(enum xcursor_cursor_t c);
 int xcursor_get_xcb_cursor(enum xcursor_cursor_t c);
 
diff --git a/include/xinerama.h b/include/xinerama.h
index e6304b56..8c879c07 100644
--- a/include/xinerama.h
+++ b/include/xinerama.h
@@ -19,6 +19,6 @@
  * Xinerama information to setup workspaces for each screen.
  *
  */
-void xinerama_init();
+void xinerama_init(void);
 
 #endif
diff --git a/libi3/font.c b/libi3/font.c
index 4b308efb..0b276b0b 100644
--- a/libi3/font.c
+++ b/libi3/font.c
@@ -87,7 +87,7 @@ void set_font(i3Font *font) {
  * Frees the resources taken by the current font.
  *
  */
-void free_font() {
+void free_font(void) {
     /* Close the font and free the info */
     xcb_close_font(conn, savedFont->id);
     if (savedFont->info)
diff --git a/src/cfgparse.y b/src/cfgparse.y
index 676e0d11..7aeb0a7e 100644
--- a/src/cfgparse.y
+++ b/src/cfgparse.y
@@ -51,7 +51,7 @@ void yyerror(const char *error_message) {
     ELOG("\n");
 }
 
-int yywrap() {
+int yywrap(void) {
     return 1;
 }
 
diff --git a/src/commands.c b/src/commands.c
index 8111b54c..2df26f6c 100644
--- a/src/commands.c
+++ b/src/commands.c
@@ -108,7 +108,7 @@ static void nagbar_cleanup(EV_P_ ev_cleanup *watcher, int revent) {
 }
 #endif
 
-void cmd_MIGRATION_start_nagbar() {
+void cmd_MIGRATION_start_nagbar(void) {
     if (migration_pid != -1) {
         fprintf(stderr, "i3-nagbar already running.\n");
         return;
diff --git a/src/commands_parser.c b/src/commands_parser.c
index 27a782a2..d15fea35 100644
--- a/src/commands_parser.c
+++ b/src/commands_parser.c
@@ -110,7 +110,7 @@ static char *get_string(const char *identifier) {
     return NULL;
 }
 
-static void clear_stack() {
+static void clear_stack(void) {
     DLOG("clearing stack.\n");
     for (int c = 0; c < 10; c++) {
         if (stack[c].str != NULL)
diff --git a/src/config.c b/src/config.c
index de59420a..50ec2823 100644
--- a/src/config.c
+++ b/src/config.c
@@ -81,7 +81,7 @@ Binding *get_binding(uint16_t modifiers, xcb_keycode_t keycode) {
  * Translates keysymbols to keycodes for all bindings which use keysyms.
  *
  */
-void translate_keysyms() {
+void translate_keysyms(void) {
     Binding *bind;
     xcb_keysym_t keysym;
     int col;
diff --git a/src/ewmh.c b/src/ewmh.c
index a11d4658..1c6d918b 100644
--- a/src/ewmh.c
+++ b/src/ewmh.c
@@ -16,7 +16,7 @@
  * and _NET_NUMBER_OF_DESKTOPS - 1.
  *
  */
-void ewmh_update_current_desktop() {
+void ewmh_update_current_desktop(void) {
     Con *focused_ws = con_get_workspace(focused);
     Con *output;
     uint32_t idx = 0;
@@ -59,7 +59,7 @@ void ewmh_update_active_window(xcb_window_t window) {
  * desktop applications to place desktop icons appropriately.
  *
  */
-void ewmh_update_workarea() {
+void ewmh_update_workarea(void) {
     int num_workspaces = 0, count = 0;
     Rect last_rect = {0, 0, 0, 0};
     Con *output;
@@ -131,7 +131,7 @@ void ewmh_update_client_list_stacking(xcb_window_t *stack, int num_windows) {
  * Set up the EWMH hints on the root window.
  *
  */
-void ewmh_setup_hints() {
+void ewmh_setup_hints(void) {
     xcb_atom_t supported_atoms[] = {
 #define xmacro(atom) A_ ## atom,
 #include "atoms.xmacro"
diff --git a/src/floating.c b/src/floating.c
index 0b246847..3b691169 100644
--- a/src/floating.c
+++ b/src/floating.c
@@ -15,7 +15,7 @@ extern xcb_connection_t *conn;
  * Calculates sum of heights and sum of widths of all currently active outputs
  *
  */
-Rect total_outputs_dimensions() {
+static Rect total_outputs_dimensions(void) {
     Output *output;
     /* Use Rect to encapsulate dimensions, ignoring x/y */
     Rect outputs_dimensions = {0, 0, 0, 0};
diff --git a/src/handlers.c b/src/handlers.c
index 688cf2f6..490bf6eb 100644
--- a/src/handlers.c
+++ b/src/handlers.c
@@ -1007,7 +1007,7 @@ static struct property_handler_t property_handlers[] = {
  * received from X11
  *
  */
-void property_handlers_init() {
+void property_handlers_init(void) {
 
     sn_monitor_context_new(sndisplay, conn_screen, startup_monitor_event, NULL, NULL);
 
diff --git a/src/ipc.c b/src/ipc.c
index 3733034d..40ad8684 100644
--- a/src/ipc.c
+++ b/src/ipc.c
@@ -97,7 +97,7 @@ void ipc_send_event(const char *event, uint32_t message_type, const char *payloa
  * when exiting or restarting only!
  *
  */
-void ipc_shutdown() {
+void ipc_shutdown(void) {
     ipc_client *current;
     while (!TAILQ_EMPTY(&all_clients)) {
         current = TAILQ_FIRST(&all_clients);
diff --git a/src/log.c b/src/log.c
index 7b7eca50..fd98081b 100644
--- a/src/log.c
+++ b/src/log.c
@@ -59,7 +59,7 @@ static int logbuffer_shm;
  * Necessary to print the i3 SHM log in the correct order.
  *
  */
-static void store_log_markers() {
+static void store_log_markers(void) {
     i3_shmlog_header *header = (i3_shmlog_header*)logbuffer;
 
     header->offset_next_write = (logwalk - logbuffer);
@@ -74,7 +74,7 @@ static void store_log_markers() {
  * Will be called twice if --shmlog-size is specified.
  *
  */
-void init_logging() {
+void init_logging(void) {
     if (!errorfilename) {
         if (!(errorfilename = get_process_filename("errorlog")))
             ELOG("Could not initialize errorlog\n");
diff --git a/src/main.c b/src/main.c
index c4cffa5c..0d724607 100644
--- a/src/main.c
+++ b/src/main.c
@@ -212,7 +212,7 @@ static void xkb_got_event(EV_P_ struct ev_io *w, int revents) {
  * Exit handler which destroys the main_loop. Will trigger cleanup handlers.
  *
  */
-static void i3_exit() {
+static void i3_exit(void) {
 /* We need ev >= 4 for the following code. Since it is not *that* important (it
  * only makes sure that there are no i3-nagbar instances left behind) we still
  * support old systems with libev 3. */
diff --git a/src/manage.c b/src/manage.c
index b0a6aafa..ea060d97 100644
--- a/src/manage.c
+++ b/src/manage.c
@@ -47,7 +47,7 @@ void manage_existing_windows(xcb_window_t root) {
  * side-effects which are to be expected when continuing to run i3.
  *
  */
-void restore_geometry() {
+void restore_geometry(void) {
     DLOG("Restoring geometry\n");
 
     Con *con;
diff --git a/src/randr.c b/src/randr.c
index 85f0eab3..73adbf0e 100644
--- a/src/randr.c
+++ b/src/randr.c
@@ -60,7 +60,7 @@ Output *get_output_by_name(const char *name) {
  * Returns the first output which is active.
  *
  */
-Output *get_first_output() {
+Output *get_first_output(void) {
     Output *output;
 
     TAILQ_FOREACH(output, &outputs, outputs)
@@ -539,7 +539,7 @@ static void handle_output(xcb_connection_t *conn, xcb_randr_output_t id,
  * (Re-)queries the outputs via RandR and stores them in the list of outputs.
  *
  */
-void randr_query_outputs() {
+void randr_query_outputs(void) {
     Output *output, *other, *first;
     xcb_randr_get_output_primary_cookie_t pcookie;
     xcb_randr_get_screen_resources_current_cookie_t rcookie;
diff --git a/src/sighandler.c b/src/sighandler.c
index 4a0c13b5..fa608ed8 100644
--- a/src/sighandler.c
+++ b/src/sighandler.c
@@ -192,7 +192,7 @@ void handle_signal(int sig, siginfo_t *info, void *data) {
  * Setup signal handlers to safely handle SIGSEGV and SIGFPE
  *
  */
-void setup_signal_handler() {
+void setup_signal_handler(void) {
     struct sigaction action;
 
     action.sa_sigaction = handle_signal;
diff --git a/src/tree.c b/src/tree.c
index 5559908f..f29369c6 100644
--- a/src/tree.c
+++ b/src/tree.c
@@ -19,7 +19,7 @@ struct all_cons_head all_cons = TAILQ_HEAD_INITIALIZER(all_cons);
  * __i3_scratch will live there.
  *
  */
-static Con *_create___i3() {
+static Con *_create___i3(void) {
     Con *__i3 = con_new(croot, NULL);
     FREE(__i3->name);
     __i3->name = sstrdup("__i3");
@@ -378,7 +378,7 @@ void tree_split(Con *con, orientation_t orientation) {
  * Moves focus one level up.
  *
  */
-void level_up() {
+void level_up(void) {
     /* We cannot go up when we are in fullscreen mode at the moment, that would
      * be totally not intuitive */
     if (focused->fullscreen_mode != CF_NONE) {
@@ -399,7 +399,7 @@ void level_up() {
  * Moves focus one level down.
  *
  */
-void level_down() {
+void level_down(void) {
     /* Go down the focus stack of the current node */
     Con *next = TAILQ_FIRST(&(focused->focus_head));
     if (next == TAILQ_END(&(focused->focus_head))) {
@@ -428,7 +428,7 @@ static void mark_unmapped(Con *con) {
  * pushing the changes to X11 using x_push_changes().
  *
  */
-void tree_render() {
+void tree_render(void) {
     if (croot == NULL)
         return;
 
diff --git a/src/util.c b/src/util.c
index 70984f2b..d337963e 100644
--- a/src/util.c
+++ b/src/util.c
@@ -222,7 +222,7 @@ char *get_process_filename(const char *prefix) {
 #define y(x, ...) yajl_gen_ ## x (gen, ##__VA_ARGS__)
 #define ystr(str) yajl_gen_string(gen, (unsigned char*)str, strlen(str))
 
-char *store_restart_layout() {
+char *store_restart_layout(void) {
     setlocale(LC_NUMERIC, "C");
 #if YAJL_MAJOR >= 2
     yajl_gen gen = yajl_gen_alloc(NULL);
diff --git a/src/workspace.c b/src/workspace.c
index 4fc8ba1b..75b8a7b3 100644
--- a/src/workspace.c
+++ b/src/workspace.c
@@ -393,7 +393,7 @@ void workspace_show_by_name(const char *num) {
  * Focuses the next workspace.
  *
  */
-Con* workspace_next() {
+Con* workspace_next(void) {
     Con *current = con_get_workspace(focused);
     Con *next = NULL;
     Con *output;
@@ -463,7 +463,7 @@ workspace_next_end:
  * Focuses the previous workspace.
  *
  */
-Con* workspace_prev() {
+Con* workspace_prev(void) {
     Con *current = con_get_workspace(focused);
     Con *prev = NULL;
     Con *output;
@@ -535,7 +535,7 @@ workspace_prev_end:
  * Focuses the next workspace on the same output.
  *
  */
-Con* workspace_next_on_output() {
+Con* workspace_next_on_output(void) {
     Con *current = con_get_workspace(focused);
     Con *next = NULL;
     Con *output  = con_get_output(focused);
@@ -590,7 +590,7 @@ workspace_next_on_output_end:
  * Focuses the previous workspace on same output.
  *
  */
-Con* workspace_prev_on_output() {
+Con* workspace_prev_on_output(void) {
     Con *current = con_get_workspace(focused);
     Con *prev = NULL;
     Con *output  = con_get_output(focused);
@@ -646,7 +646,7 @@ workspace_prev_on_output_end:
  * Focuses the previously focused workspace.
  *
  */
-void workspace_back_and_forth() {
+void workspace_back_and_forth(void) {
     if (!previous_workspace_name) {
         DLOG("No previous workspace name set. Not switching.");
         return;
diff --git a/src/x.c b/src/x.c
index f2b53251..08eb8fee 100644
--- a/src/x.c
+++ b/src/x.c
@@ -1013,7 +1013,7 @@ void x_set_name(Con *con, const char *name) {
  * Sets up i3 specific atoms (I3_SOCKET_PATH and I3_CONFIG_PATH)
  *
  */
-void x_set_i3_atoms() {
+void x_set_i3_atoms(void) {
     xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root, A_I3_SOCKET_PATH, A_UTF8_STRING, 8,
                         (current_socketpath == NULL ? 0 : strlen(current_socketpath)),
                         current_socketpath);
diff --git a/src/xcursor.c b/src/xcursor.c
index f04a3959..058b7ae0 100644
--- a/src/xcursor.c
+++ b/src/xcursor.c
@@ -31,7 +31,7 @@ static Cursor load_cursor(const char *name) {
     return c;
 }
 
-void xcursor_load_cursors() {
+void xcursor_load_cursors(void) {
     cursors[XCURSOR_CURSOR_POINTER] = load_cursor("left_ptr");
     cursors[XCURSOR_CURSOR_RESIZE_HORIZONTAL] = load_cursor("sb_h_double_arrow");
     cursors[XCURSOR_CURSOR_RESIZE_VERTICAL] = load_cursor("sb_v_double_arrow");
diff --git a/src/xinerama.c b/src/xinerama.c
index c3ad97c3..f377840f 100644
--- a/src/xinerama.c
+++ b/src/xinerama.c
@@ -90,7 +90,7 @@ static void query_screens(xcb_connection_t *conn) {
  * information to setup workspaces for each screen.
  *
  */
-void xinerama_init() {
+void xinerama_init(void) {
     if (!xcb_get_extension_data(conn, &xcb_xinerama_id)->present) {
         DLOG("Xinerama extension not found, disabling.\n");
         disable_randr(conn);