diff --git a/py/gc.c b/py/gc.c
index 4a3d1dabee..7d4f4762ef 100644
--- a/py/gc.c
+++ b/py/gc.c
@@ -115,7 +115,7 @@ void gc_init(void *start, void *end) {
         } \
     } while (0)
 
-static void gc_drain_stack() {
+static void gc_drain_stack(void) {
     while (gc_sp > gc_stack) {
         // pop the next block off the stack
         machine_uint_t block = *--gc_sp;
@@ -135,7 +135,7 @@ static void gc_drain_stack() {
     }
 }
 
-static void gc_deal_with_stack_overflow() {
+static void gc_deal_with_stack_overflow(void) {
     while (gc_stack_overflow) {
         gc_stack_overflow = 0;
         gc_sp = gc_stack;
@@ -151,7 +151,7 @@ static void gc_deal_with_stack_overflow() {
     }
 }
 
-static void gc_sweep() {
+static void gc_sweep(void) {
     // free unmarked heads and their tails
     int free_tail = 0;
     for (machine_uint_t block = 0; block < gc_alloc_table_byte_len * BLOCKS_PER_ATB; block++) {
@@ -174,7 +174,7 @@ static void gc_sweep() {
     }
 }
 
-void gc_collect_start() {
+void gc_collect_start(void) {
     gc_stack_overflow = 0;
     gc_sp = gc_stack;
 }
@@ -187,7 +187,7 @@ void gc_collect_root(void **ptrs, machine_uint_t len) {
     }
 }
 
-void gc_collect_end() {
+void gc_collect_end(void) {
     gc_deal_with_stack_overflow();
     gc_sweep();
 }
@@ -336,7 +336,7 @@ void *gc_realloc(void *ptr, machine_uint_t n_bytes) {
 }
 
 /*
-static void gc_dump_at() {
+static void gc_dump_at(void) {
     for (machine_uint_t bl = 0; bl < gc_alloc_table_byte_len * BLOCKS_PER_ATB; bl++) {
         printf("block % 6u ", bl);
         switch (ATB_GET_KIND(bl)) {
@@ -349,7 +349,7 @@ static void gc_dump_at() {
     }
 }
 
-int main() {
+int main(void) {
     machine_uint_t len = 1000;
     machine_uint_t *heap = malloc(len);
     gc_init(heap, heap + len / sizeof(machine_uint_t));
diff --git a/py/gc.h b/py/gc.h
index d5b5fcc91c..711212ac40 100644
--- a/py/gc.h
+++ b/py/gc.h
@@ -1,8 +1,8 @@
 void gc_init(void *start, void *end);
-void gc_collect_start();
+void gc_collect_start(void);
 void gc_collect_root(void **ptrs, machine_uint_t len);
-void gc_collect_end();
-void gc_collect();
+void gc_collect_end(void);
+void gc_collect(void);
 void *gc_alloc(machine_uint_t n_bytes);
 void gc_free(void *ptr);
 machine_uint_t gc_nbytes(void *ptr);
diff --git a/py/malloc.c b/py/malloc.c
index 8775f68aa3..2f8b5f78b5 100644
--- a/py/malloc.c
+++ b/py/malloc.c
@@ -51,6 +51,6 @@ void *m_realloc(void *ptr, int num_bytes) {
     return ptr;
 }
 
-int m_get_total_bytes_allocated() {
+int m_get_total_bytes_allocated(void) {
     return total_bytes_allocated;
 }
diff --git a/py/misc.h b/py/misc.h
index c67d9df8f9..87cbbd98b6 100644
--- a/py/misc.h
+++ b/py/misc.h
@@ -25,7 +25,7 @@ void *m_malloc(int num_bytes);
 void *m_malloc0(int num_bytes);
 void *m_realloc(void *ptr, int num_bytes);
 
-int m_get_total_bytes_allocated();
+int m_get_total_bytes_allocated(void);
 
 /** unichar / UTF-8 *********************************************/
 
@@ -67,7 +67,7 @@ typedef struct _vstr_t {
 
 void vstr_init(vstr_t *vstr);
 void vstr_clear(vstr_t *vstr);
-vstr_t *vstr_new();
+vstr_t *vstr_new(void);
 void vstr_free(vstr_t *vstr);
 void vstr_reset(vstr_t *vstr);
 bool vstr_had_error(vstr_t *vstr);
@@ -88,7 +88,7 @@ void vstr_cut_tail(vstr_t *vstr, int len);
 
 typedef unsigned int qstr;
 
-void qstr_init();
+void qstr_init(void);
 qstr qstr_from_str_static(const char *str);
 qstr qstr_from_str_take(char *str);
 qstr qstr_from_strn_copy(const char *str, int len);
diff --git a/py/nlr.h b/py/nlr.h
index fa4e2f45f0..8ca8a9dc62 100644
--- a/py/nlr.h
+++ b/py/nlr.h
@@ -24,5 +24,5 @@ struct _nlr_buf_t {
 };
 
 unsigned int nlr_push(nlr_buf_t *);
-void nlr_pop();
+void nlr_pop(void);
 void nlr_jump(void *val) __attribute__((noreturn));
diff --git a/py/qstr.c b/py/qstr.c
index 33d15c7e73..6fa69ddb95 100644
--- a/py/qstr.c
+++ b/py/qstr.c
@@ -7,7 +7,7 @@ static int qstrs_alloc;
 static int qstrs_len;
 static const char **qstrs;
 
-void qstr_init() {
+void qstr_init(void) {
     qstrs_alloc = 400;
     qstrs_len = 1;
     qstrs = m_new(const char*, qstrs_alloc);
diff --git a/py/runtime.c b/py/runtime.c
index 41d696a2e3..9da5688763 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -521,7 +521,7 @@ py_obj_t py_builtin_range(py_obj_t o_arg) {
 FILE *fp_native = NULL;
 #endif
 
-void rt_init() {
+void rt_init(void) {
     q_append = qstr_from_str_static("append");
     q_print = qstr_from_str_static("print");
     q_len = qstr_from_str_static("len");
@@ -560,7 +560,7 @@ void rt_init() {
 #endif
 }
 
-void rt_deinit() {
+void rt_deinit(void) {
 #ifdef WRITE_NATIVE
     if (fp_native != NULL) {
         fclose(fp_native);
@@ -576,7 +576,7 @@ int rt_get_unique_code_id(bool is_main_module) {
     }
 }
 
-static void alloc_unique_codes() {
+static void alloc_unique_codes(void) {
     if (unique_codes == NULL) {
         unique_codes = m_new(py_code_t, next_unique_code_id);
         for (int i = 0; i < next_unique_code_id; i++) {
@@ -901,7 +901,7 @@ py_obj_t rt_load_global(qstr qstr) {
     return elem->value;
 }
 
-py_obj_t rt_load_build_class() {
+py_obj_t rt_load_build_class(void) {
     DEBUG_OP_printf("load_build_class\n");
     py_map_elem_t *elem = py_qstr_map_lookup(&map_builtins, q___build_class__, false);
     if (elem == NULL) {
@@ -1635,7 +1635,7 @@ void rt_f_vector(rt_fun_kind_t fun_kind) {
 // temporary way of making C modules
 // hack: use class to mimic a module
 
-py_obj_t py_module_new() {
+py_obj_t py_module_new(void) {
     py_obj_base_t *o = m_new(py_obj_base_t, 1);
     o->kind = O_CLASS;
     o->u_class.locals = py_map_new(MAP_QSTR, 0);
diff --git a/py/runtime.h b/py/runtime.h
index dedbf1bad4..326ca0833b 100644
--- a/py/runtime.h
+++ b/py/runtime.h
@@ -79,18 +79,18 @@ typedef enum {
 extern void *const rt_fun_table[RT_F_NUMBER_OF];
 
 typedef machine_ptr_t py_obj_t; // must be of pointer size
-typedef py_obj_t (*py_fun_0_t)();
+typedef py_obj_t (*py_fun_0_t)(void);
 typedef py_obj_t (*py_fun_1_t)(py_obj_t);
 typedef py_obj_t (*py_fun_2_t)(py_obj_t, py_obj_t);
-typedef py_obj_t (*py_fun_t)();
+typedef py_obj_t (*py_fun_t)(void);
 
 extern py_obj_t py_const_none;
 extern py_obj_t py_const_false;
 extern py_obj_t py_const_true;
 extern py_obj_t py_const_stop_iteration; // special object indicating end of iteration (not StopIteration exception!)
 
-void rt_init();
-void rt_deinit();
+void rt_init(void);
+void rt_deinit(void);
 int rt_get_unique_code_id(bool is_main_module);
 void rt_assign_byte_code(int unique_code_id, byte *code, uint len, int n_args, int n_locals, int n_stack, bool is_generator);
 void rt_assign_native_code(int unique_code_id, py_fun_t f, uint len, int n_args);
@@ -102,7 +102,7 @@ qstr py_get_qstr(py_obj_t arg);
 py_obj_t rt_load_const_str(qstr qstr);
 py_obj_t rt_load_name(qstr qstr);
 py_obj_t rt_load_global(qstr qstr);
-py_obj_t rt_load_build_class();
+py_obj_t rt_load_build_class(void);
 void rt_store_name(qstr qstr, py_obj_t obj);
 void rt_store_global(qstr qstr, py_obj_t obj);
 py_obj_t rt_unary_op(int op, py_obj_t arg);
@@ -133,4 +133,4 @@ py_obj_t rt_getiter(py_obj_t o);
 py_obj_t rt_iternext(py_obj_t o);
 
 // temporary way of making C modules
-py_obj_t py_module_new();
+py_obj_t py_module_new(void);
diff --git a/py/vstr.c b/py/vstr.c
index fc7a772452..076c3a30de 100644
--- a/py/vstr.c
+++ b/py/vstr.c
@@ -24,7 +24,7 @@ void vstr_clear(vstr_t *vstr) {
     vstr->buf = NULL;
 }
 
-vstr_t *vstr_new() {
+vstr_t *vstr_new(void) {
     vstr_t *vstr = m_new(vstr_t, 1);
     if (vstr == NULL) {
         return NULL;
@@ -193,7 +193,7 @@ void vstr_printf(vstr_t *vstr, const char *fmt, ...) {
 /** testing *****************************************************/
 
 /*
-int main() {
+int main(void) {
     vstr_t *vstr = vstr_new();
     int i;
     for (i = 0; i < 10; i++) {
diff --git a/unix/main.c b/unix/main.c
index 12aca6ddf2..ebab646275 100644
--- a/unix/main.c
+++ b/unix/main.c
@@ -28,7 +28,7 @@ char *str_join(const char *s1, int sep_char, const char *s2) {
     return s;
 }
 
-void do_repl() {
+void do_repl(void) {
     for (;;) {
         char *line = readline(">>> ");
         if (line == NULL) {