diff --git a/.travis.yml b/.travis.yml
index b7897a5d..8098035c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -35,7 +35,7 @@ install:
 script:
   - docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 ${BASENAME} ./travis/check-safe-wrappers.sh
   - docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 ${BASENAME} ./travis/check-formatting.sh
-  - docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 -e CC ${BASENAME} /bin/sh -c 'autoreconf -fi && mkdir -p build && cd build && (../configure || (cat config.log; false)) && make -j CFLAGS="-Wformat -Wformat-security -Wextra -Wno-unused-parameter -Wstrict-prototypes -Werror"'
+  - docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 -e CC ${BASENAME} /bin/sh -c 'autoreconf -fi && mkdir -p build && cd build && (../configure || (cat config.log; false)) && make -j CFLAGS="-Wformat -Wformat-security -Wextra -Wno-unused-parameter -Wstrict-prototypes -Wmissing-prototypes -Werror"'
   - docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 ${BASENAME} ./travis/check-spelling.pl
   - docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 -e CC ${BASENAME} ./travis/run-tests.sh
   - ./travis/skip-pkg.sh || docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 ${BASENAME} ./travis/debian-build.sh deb/debian-amd64/DIST
diff --git a/i3bar/src/child.c b/i3bar/src/child.c
index 1cd7d512..bb5ceaff 100644
--- a/i3bar/src/child.c
+++ b/i3bar/src/child.c
@@ -133,7 +133,7 @@ finish:
  * Stop and free() the stdin- and SIGCHLD-watchers
  *
  */
-void cleanup(void) {
+static void cleanup(void) {
     if (stdin_io != NULL) {
         ev_io_stop(main_loop, stdin_io);
         FREE(stdin_io);
@@ -400,7 +400,7 @@ static bool read_json_input(unsigned char *input, int length) {
  * in statusline
  *
  */
-void stdin_io_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
+static void stdin_io_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
     int rec;
     unsigned char *buffer = get_buffer(watcher, &rec);
     if (buffer == NULL)
@@ -420,7 +420,7 @@ void stdin_io_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
  * whether this is JSON or plain text
  *
  */
-void stdin_io_first_line_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
+static void stdin_io_first_line_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
     int rec;
     unsigned char *buffer = get_buffer(watcher, &rec);
     if (buffer == NULL)
@@ -457,7 +457,7 @@ void stdin_io_first_line_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
  * anymore
  *
  */
-void child_sig_cb(struct ev_loop *loop, ev_child *watcher, int revents) {
+static void child_sig_cb(struct ev_loop *loop, ev_child *watcher, int revents) {
     int exit_status = WEXITSTATUS(watcher->rstatus);
 
     ELOG("Child (pid: %d) unexpectedly exited with status %d\n",
@@ -477,7 +477,7 @@ void child_sig_cb(struct ev_loop *loop, ev_child *watcher, int revents) {
     draw_bars(false);
 }
 
-void child_write_output(void) {
+static void child_write_output(void) {
     if (child.click_events) {
         const unsigned char *output;
         size_t size;
@@ -580,7 +580,7 @@ void start_child(char *command) {
     atexit(kill_child_at_exit);
 }
 
-void child_click_events_initialize(void) {
+static void child_click_events_initialize(void) {
     if (!child.click_events_init) {
         yajl_gen_array_open(gen);
         child_write_output();
@@ -588,7 +588,7 @@ void child_click_events_initialize(void) {
     }
 }
 
-void child_click_events_key(const char *key) {
+static void child_click_events_key(const char *key) {
     yajl_gen_string(gen, (const unsigned char *)key, strlen(key));
 }
 
diff --git a/i3bar/src/ipc.c b/i3bar/src/ipc.c
index 9a7fb21e..df5a12cf 100644
--- a/i3bar/src/ipc.c
+++ b/i3bar/src/ipc.c
@@ -34,7 +34,7 @@ typedef void (*handler_t)(char *);
  * Since i3 does not give us much feedback on commands, we do not much
  *
  */
-void got_command_reply(char *reply) {
+static void got_command_reply(char *reply) {
     /* TODO: Error handling for command replies */
 }
 
@@ -42,7 +42,7 @@ void got_command_reply(char *reply) {
  * Called, when we get a reply with workspaces data
  *
  */
-void got_workspace_reply(char *reply) {
+static void got_workspace_reply(char *reply) {
     DLOG("Got workspace data!\n");
     parse_workspaces_json(reply);
     draw_bars(false);
@@ -53,7 +53,7 @@ void got_workspace_reply(char *reply) {
  * Since i3 does not give us much feedback on commands, we do not much
  *
  */
-void got_subscribe_reply(char *reply) {
+static void got_subscribe_reply(char *reply) {
     DLOG("Got subscribe reply: %s\n", reply);
     /* TODO: Error handling for subscribe commands */
 }
@@ -62,7 +62,7 @@ void got_subscribe_reply(char *reply) {
  * Called, when we get a reply with outputs data
  *
  */
-void got_output_reply(char *reply) {
+static void got_output_reply(char *reply) {
     DLOG("Clearing old output configuration...\n");
     free_outputs();
 
@@ -87,7 +87,7 @@ void got_output_reply(char *reply) {
  * Called when we get the configuration for our bar instance
  *
  */
-void got_bar_config(char *reply) {
+static void got_bar_config(char *reply) {
     DLOG("Received bar config \"%s\"\n", reply);
     /* We initiate the main function by requesting infos about the outputs and
      * workspaces. Everything else (creating the bars, showing the right workspace-
@@ -132,7 +132,7 @@ handler_t reply_handlers[] = {
  * Called, when a workspace event arrives (i.e. the user changed the workspace)
  *
  */
-void got_workspace_event(char *event) {
+static void got_workspace_event(char *event) {
     DLOG("Got workspace event!\n");
     i3_send_msg(I3_IPC_MESSAGE_TYPE_GET_WORKSPACES, NULL);
 }
@@ -141,7 +141,7 @@ void got_workspace_event(char *event) {
  * Called, when an output event arrives (i.e. the screen configuration changed)
  *
  */
-void got_output_event(char *event) {
+static void got_output_event(char *event) {
     DLOG("Got output event!\n");
     i3_send_msg(I3_IPC_MESSAGE_TYPE_GET_OUTPUTS, NULL);
     if (!config.disable_ws) {
@@ -153,7 +153,7 @@ void got_output_event(char *event) {
  * Called, when a mode event arrives (i3 changed binding mode).
  *
  */
-void got_mode_event(char *event) {
+static void got_mode_event(char *event) {
     DLOG("Got mode event!\n");
     parse_mode_json(event);
     draw_bars(false);
@@ -163,7 +163,7 @@ void got_mode_event(char *event) {
  * Called, when a barconfig_update event arrives (i.e. i3 changed the bar hidden_state or mode)
  *
  */
-void got_bar_config_update(char *event) {
+static void got_bar_config_update(char *event) {
     /* check whether this affect this bar instance by checking the bar_id */
     char *expected_id;
     sasprintf(&expected_id, "\"id\":\"%s\"", config.bar_id);
@@ -213,7 +213,7 @@ handler_t event_handlers[] = {
  * Called, when we get a message from i3
  *
  */
-void got_data(struct ev_loop *loop, ev_io *watcher, int events) {
+static void got_data(struct ev_loop *loop, ev_io *watcher, int events) {
     DLOG("Got data!\n");
     int fd = watcher->fd;
 
diff --git a/i3bar/src/main.c b/i3bar/src/main.c
index f90bb312..a818dd97 100644
--- a/i3bar/src/main.c
+++ b/i3bar/src/main.c
@@ -44,7 +44,7 @@ void debuglog(char *fmt, ...) {
  * Glob path, i.e. expand ~
  *
  */
-char *expand_path(char *path) {
+static char *expand_path(char *path) {
     static glob_t globbuf;
     if (glob(path, GLOB_NOCHECK | GLOB_TILDE, NULL, &globbuf) < 0) {
         ELOG("glob() failed\n");
@@ -55,7 +55,7 @@ char *expand_path(char *path) {
     return result;
 }
 
-void print_usage(char *elf_name) {
+static void print_usage(char *elf_name) {
     printf("Usage: %s -b bar_id [-s sock_path] [-h] [-v]\n", elf_name);
     printf("\n");
     printf("-b, --bar_id  <bar_id>\tBar ID for which to get the configuration\n");
@@ -76,7 +76,7 @@ void print_usage(char *elf_name) {
  * in main() with that
  *
  */
-void sig_cb(struct ev_loop *loop, ev_signal *watcher, int revents) {
+static void sig_cb(struct ev_loop *loop, ev_signal *watcher, int revents) {
     switch (watcher->signum) {
         case SIGTERM:
             DLOG("Got a SIGTERM, stopping\n");
diff --git a/i3bar/src/xcb.c b/i3bar/src/xcb.c
index 66d2fca6..66003425 100644
--- a/i3bar/src/xcb.c
+++ b/i3bar/src/xcb.c
@@ -146,13 +146,13 @@ int _xcb_request_failed(xcb_void_cookie_t cookie, char *err_msg, int line) {
     return 0;
 }
 
-uint32_t get_sep_offset(struct status_block *block) {
+static uint32_t get_sep_offset(struct status_block *block) {
     if (!block->no_separator && block->sep_block_width > 0)
         return block->sep_block_width / 2 + block->sep_block_width % 2;
     return 0;
 }
 
-int get_tray_width(struct tc_head *trayclients) {
+static int get_tray_width(struct tc_head *trayclients) {
     trayclient *trayclient;
     int tray_width = 0;
     TAILQ_FOREACH_REVERSE(trayclient, trayclients, tc_head, tailq) {
@@ -193,7 +193,7 @@ static void draw_separator(i3_output *output, uint32_t x, struct status_block *b
     }
 }
 
-uint32_t predict_statusline_length(bool use_short_text) {
+static uint32_t predict_statusline_length(bool use_short_text) {
     uint32_t width = 0;
     struct status_block *block;
 
@@ -245,7 +245,7 @@ uint32_t predict_statusline_length(bool use_short_text) {
 /*
  * Redraws the statusline to the output's statusline_buffer
  */
-void draw_statusline(i3_output *output, uint32_t clip_left, bool use_focus_colors, bool use_short_text) {
+static void draw_statusline(i3_output *output, uint32_t clip_left, bool use_focus_colors, bool use_short_text) {
     struct status_block *block;
 
     color_t bar_color = (use_focus_colors ? colors.focus_bar_bg : colors.bar_bg);
@@ -330,7 +330,7 @@ void draw_statusline(i3_output *output, uint32_t clip_left, bool use_focus_color
  * Hides all bars (unmaps them)
  *
  */
-void hide_bars(void) {
+static void hide_bars(void) {
     if ((config.hide_on_modifier == M_DOCK) || (config.hidden_state == S_SHOW && config.hide_on_modifier == M_HIDE)) {
         return;
     }
@@ -349,7 +349,7 @@ void hide_bars(void) {
  * Unhides all bars (maps them)
  *
  */
-void unhide_bars(void) {
+static void unhide_bars(void) {
     if (config.hide_on_modifier != M_HIDE) {
         return;
     }
@@ -457,7 +457,7 @@ static bool execute_custom_command(xcb_keycode_t input_code, bool event_is_relea
  * wheel was used and change the workspace appropriately
  *
  */
-void handle_button(xcb_button_press_event_t *event) {
+static void handle_button(xcb_button_press_event_t *event) {
     /* Determine, which bar was clicked */
     i3_output *walk;
     xcb_window_t bar = event->event;
@@ -1080,7 +1080,7 @@ static void handle_resize_request(xcb_resize_request_event_t *event) {
  * events from X11, handle them, then flush our outgoing queue.
  *
  */
-void xcb_prep_cb(struct ev_loop *loop, ev_prepare *watcher, int revents) {
+static void xcb_prep_cb(struct ev_loop *loop, ev_prepare *watcher, int revents) {
     xcb_generic_event_t *event;
 
     if (xcb_connection_has_error(xcb_connection)) {
@@ -1177,7 +1177,7 @@ void xcb_prep_cb(struct ev_loop *loop, ev_prepare *watcher, int revents) {
  * are triggered
  *
  */
-void xcb_io_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
+static void xcb_io_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
 }
 
 /*
@@ -1248,7 +1248,7 @@ char *init_xcb_early(void) {
  * in xcb.
  *
  */
-void register_xkb_keyevents(void) {
+static void register_xkb_keyevents(void) {
     const xcb_query_extension_reply_t *extreply;
     extreply = xcb_get_extension_data(conn, &xcb_xkb_id);
     if (!extreply->present) {
@@ -1272,7 +1272,7 @@ void register_xkb_keyevents(void) {
  * Deregister from xkb keyevents.
  *
  */
-void deregister_xkb_keyevents(void) {
+static void deregister_xkb_keyevents(void) {
     xcb_xkb_select_events(conn,
                           XCB_XKB_ID_USE_CORE_KBD,
                           0,
@@ -1338,7 +1338,7 @@ static void send_tray_clientmessage(void) {
  * atom. Afterwards, tray clients will send ClientMessages to our window.
  *
  */
-void init_tray(void) {
+static void init_tray(void) {
     DLOG("Initializing system tray functionality\n");
     /* request the tray manager atom for the X11 display we are running on */
     char atomname[strlen("_NET_SYSTEM_TRAY_S") + 11];
@@ -1567,7 +1567,7 @@ void destroy_window(i3_output *output) {
 
 /* Strut partial tells i3 where to reserve space for i3bar. This is determined
  * by the `position` bar config directive. */
-xcb_void_cookie_t config_strut_partial(i3_output *output) {
+static xcb_void_cookie_t config_strut_partial(i3_output *output) {
     /* A local struct to save the strut_partial property */
     struct {
         uint32_t left;
diff --git a/src/commands.c b/src/commands.c
index 70f3bf18..45ce1a3c 100644
--- a/src/commands.c
+++ b/src/commands.c
@@ -2034,7 +2034,7 @@ void cmd_rename_workspace(I3_CMD, const char *old_name, const char *new_name) {
  * Implementation of 'bar mode dock|hide|invisible|toggle [<bar_id>]'
  *
  */
-bool cmd_bar_mode(const char *bar_mode, const char *bar_id) {
+static bool cmd_bar_mode(const char *bar_mode, const char *bar_id) {
     int mode = M_DOCK;
     bool toggle = false;
     if (strcmp(bar_mode, "dock") == 0)
@@ -2079,7 +2079,7 @@ bool cmd_bar_mode(const char *bar_mode, const char *bar_id) {
  * Implementation of 'bar hidden_state hide|show|toggle [<bar_id>]'
  *
  */
-bool cmd_bar_hidden_state(const char *bar_hidden_state, const char *bar_id) {
+static bool cmd_bar_hidden_state(const char *bar_hidden_state, const char *bar_id) {
     int hidden_state = S_SHOW;
     bool toggle = false;
     if (strcmp(bar_hidden_state, "hide") == 0)
diff --git a/src/handlers.c b/src/handlers.c
index c76f1935..238cf26f 100644
--- a/src/handlers.c
+++ b/src/handlers.c
@@ -949,8 +949,8 @@ static void handle_client_message(xcb_client_message_event_t *event) {
     }
 }
 
-bool handle_window_type(void *data, xcb_connection_t *conn, uint8_t state, xcb_window_t window,
-                        xcb_atom_t atom, xcb_get_property_reply_t *reply) {
+static bool handle_window_type(void *data, xcb_connection_t *conn, uint8_t state, xcb_window_t window,
+                               xcb_atom_t atom, xcb_get_property_reply_t *reply) {
     Con *con;
     if ((con = con_by_window_id(window)) == NULL || con->window == NULL)
         return false;
diff --git a/src/sighandler.c b/src/sighandler.c
index a5115aa4..e49c30ba 100644
--- a/src/sighandler.c
+++ b/src/sighandler.c
@@ -299,7 +299,7 @@ static void sighandler_handle_key_press(xcb_key_press_event_t *event) {
     }
 }
 
-void handle_signal(int sig, siginfo_t *info, void *data) {
+static void handle_signal(int sig, siginfo_t *info, void *data) {
     DLOG("i3 crashed. SIG: %d\n", sig);
 
     struct sigaction action;
diff --git a/src/util.c b/src/util.c
index 594d2bb6..85f359c0 100644
--- a/src/util.c
+++ b/src/util.c
@@ -217,7 +217,7 @@ static char **add_argument(char **original, char *opt_char, char *opt_arg, char
 #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(void) {
+static char *store_restart_layout(void) {
     setlocale(LC_NUMERIC, "C");
     yajl_gen gen = yajl_gen_alloc(NULL);
 
diff --git a/src/workspace.c b/src/workspace.c
index 5f5c8d4f..0cd72cf8 100644
--- a/src/workspace.c
+++ b/src/workspace.c
@@ -308,7 +308,7 @@ bool workspace_is_visible(Con *ws) {
  * XXX: we need to clean up all this recursive walking code.
  *
  */
-Con *_get_sticky(Con *con, const char *sticky_group, Con *exclude) {
+static Con *_get_sticky(Con *con, const char *sticky_group, Con *exclude) {
     Con *current;
 
     TAILQ_FOREACH(current, &(con->nodes_head), nodes) {
diff --git a/testcases/inject_randr1.5.c b/testcases/inject_randr1.5.c
index 5506d67e..520b0213 100644
--- a/testcases/inject_randr1.5.c
+++ b/testcases/inject_randr1.5.c
@@ -34,7 +34,7 @@ static void read_server_x11_packet_cb(EV_P_ ev_io *w, int revents);
 
 static char *sun_path = NULL;
 
-void cleanup_socket(void) {
+static void cleanup_socket(void) {
     if (sun_path != NULL) {
         unlink(sun_path);
         free(sun_path);