diff --git a/cont.c b/cont.c
index a3c8a3241f..3014dc19a2 100644
--- a/cont.c
+++ b/cont.c
@@ -818,7 +818,7 @@ root_fiber_alloc(rb_thread_t *th)
 }
 
 VALUE
-rb_fiber_current()
+rb_fiber_current(void)
 {
     rb_thread_t *th = GET_THREAD();
     if (th->fiber == 0) {
diff --git a/dln.c b/dln.c
index 0aeb3e979f..e9d0bb1508 100644
--- a/dln.c
+++ b/dln.c
@@ -445,14 +445,14 @@ undef_print(char *key, char *value)
 }
 
 static void
-dln_print_undef()
+dln_print_undef(void)
 {
     fprintf(stderr, " Undefined symbols:\n");
     st_foreach(undef_tbl, undef_print, NULL);
 }
 
 static void
-dln_undefined()
+dln_undefined(void)
 {
     if (undef_tbl->num_entries > 0) {
 	fprintf(stderr, "dln: Calling undefined function\n");
diff --git a/eval.c b/eval.c
index b1b04985ac..c9aaa1dc84 100644
--- a/eval.c
+++ b/eval.c
@@ -542,7 +542,7 @@ rb_block_given_p(void)
 }
 
 int
-rb_iterator_p()
+rb_iterator_p(void)
 {
     return rb_block_given_p();
 }
@@ -550,7 +550,7 @@ rb_iterator_p()
 VALUE rb_eThreadError;
 
 void
-rb_need_block()
+rb_need_block(void)
 {
     if (!rb_block_given_p()) {
 	rb_vm_localjump_error("no block given", Qnil, 0);
diff --git a/ruby.c b/ruby.c
index e43fc2c9e5..01550929e5 100644
--- a/ruby.c
+++ b/ruby.c
@@ -1112,7 +1112,7 @@ true_value(void)
     rb_define_virtual_variable((name), (val) ? true_value : false_value, 0)
 
 static VALUE
-uscore_get()
+uscore_get(void)
 {
     VALUE line;
 
@@ -1176,7 +1176,7 @@ rb_f_gsub(argc, argv)
  */
 
 static VALUE
-rb_f_chop()
+rb_f_chop(void)
 {
     VALUE str = rb_funcall3(uscore_get(), rb_intern("chop"), 0, 0);
     rb_lastline_set(str);
diff --git a/signal.c b/signal.c
index f3cfbc84d6..0348f0c995 100644
--- a/signal.c
+++ b/signal.c
@@ -515,7 +515,7 @@ sighandler(int sig)
 }
 
 int
-rb_signal_buff_size()
+rb_signal_buff_size(void)
 {
     return signal_buff.size;
 }
@@ -1044,7 +1044,7 @@ init_sigchld(int sig)
 #endif
 
 void
-ruby_sig_finalize()
+ruby_sig_finalize(void)
 {
     sighandler_t oldfunc;
 
diff --git a/st.c b/st.c
index 54e10721b0..4b2be3d8c7 100644
--- a/st.c
+++ b/st.c
@@ -145,7 +145,7 @@ static int collision = 0;
 static int init_st = 0;
 
 static void
-stat_col()
+stat_col(void)
 {
     FILE *f = fopen("/tmp/col", "w");
     fprintf(f, "collision: %d\n", collision);
diff --git a/thread.c b/thread.c
index bc3e46274f..fa14f28cbb 100644
--- a/thread.c
+++ b/thread.c
@@ -823,14 +823,14 @@ sleep_timeval(rb_thread_t *th, struct timeval tv)
 }
 
 void
-rb_thread_sleep_forever()
+rb_thread_sleep_forever(void)
 {
     thread_debug("rb_thread_sleep_forever\n");
     sleep_forever(GET_THREAD(), 0);
 }
 
 static void
-rb_thread_sleep_deadly()
+rb_thread_sleep_deadly(void)
 {
     thread_debug("rb_thread_sleep_deadly\n");
     sleep_forever(GET_THREAD(), 1);
@@ -2010,7 +2010,7 @@ vm_living_thread_num(rb_vm_t *vm)
 }
 
 int
-rb_thread_alone()
+rb_thread_alone(void)
 {
     int num = 1;
     if (GET_THREAD()->vm->living_threads) {