diff --git a/tkrzw_langc.cc b/tkrzw_langc.cc
index 01d8b32..fc8e56a 100644
--- a/tkrzw_langc.cc
+++ b/tkrzw_langc.cc
@@ -129,7 +129,7 @@ void tkrzw_set_last_status(int32_t code, const char* message) {
   }
 }
 
-TkrzwStatus tkrzw_get_last_status() {
+TkrzwStatus tkrzw_get_last_status(void) {
   TkrzwStatus status;
   status.code = last_status.GetCode();
   if (last_status.HasMessage()) {
@@ -141,11 +141,11 @@ TkrzwStatus tkrzw_get_last_status() {
   return status;
 }
 
-int32_t tkrzw_get_last_status_code() {
+int32_t tkrzw_get_last_status_code(void) {
   return last_status.GetCode();
 }
 
-const char* tkrzw_get_last_status_message() {
+const char* tkrzw_get_last_status_message(void) {
   if (last_status.HasMessage()) {
     last_message = last_status.GetMessage();
     return last_message.c_str();
@@ -157,7 +157,7 @@ const char* tkrzw_status_code_name(int32_t code) {
   return Status::CodeName(static_cast<Status::Code>(code));
 }
 
-double tkrzw_get_wall_time() {
+double tkrzw_get_wall_time(void) {
   try {
     return GetWallTime();
   } catch (const std::exception& e) {
@@ -166,7 +166,7 @@ double tkrzw_get_wall_time() {
   }
 }
 
-int64_t tkrzw_get_memory_capacity() {
+int64_t tkrzw_get_memory_capacity(void) {
   try {
     return GetMemoryCapacity();
   } catch (const std::exception& e) {
@@ -175,7 +175,7 @@ int64_t tkrzw_get_memory_capacity() {
   }
 }
 
-int64_t tkrzw_get_memory_usage() {
+int64_t tkrzw_get_memory_usage(void) {
   try {
     return GetMemoryUsage();
   } catch (const std::exception& e) {
diff --git a/tkrzw_langc.h b/tkrzw_langc.h
index 0b582ed..7f25a30 100644
--- a/tkrzw_langc.h
+++ b/tkrzw_langc.h
@@ -216,13 +216,13 @@ void tkrzw_set_last_status(int32_t code, const char* message);
  * @details The region of the message string is available until the this function or
  * tkrzw_get_last_status_message function is called next time.
  */
-TkrzwStatus tkrzw_get_last_status();
+TkrzwStatus tkrzw_get_last_status(void);
 
 /**
  * Gets the status code of the last system operation.
  * @return The status code of the last system operation.
  */
-int32_t tkrzw_get_last_status_code();
+int32_t tkrzw_get_last_status_code(void);
 
 /**
  * Gets the status message of the last system operation.
@@ -230,7 +230,7 @@ int32_t tkrzw_get_last_status_code();
  * @details The region of the message string is available until the this function or
  * tkrzw_get_last_status function is called next time.
  */
-const char* tkrzw_get_last_status_message();
+const char* tkrzw_get_last_status_message(void);
 
 /**
  * Gets the string name of a status code.
@@ -243,19 +243,19 @@ const char* tkrzw_status_code_name(int32_t code);
  * Gets the number of seconds since the UNIX epoch.
  * @return The number of seconds since the UNIX epoch with microsecond precision.
  */
-double tkrzw_get_wall_time();
+double tkrzw_get_wall_time(void);
 
 /**
  * Gets the memory capacity of the platform.
  * @return The memory capacity of the platform in bytes, or -1 on failure.
  */
-int64_t tkrzw_get_memory_capacity();
+int64_t tkrzw_get_memory_capacity(void);
 
 /**
  * Gets the current memory usage of the process.
  * @return The current memory usage of the process in bytes, or -1 on failure.
  */
-int64_t tkrzw_get_memory_usage();
+int64_t tkrzw_get_memory_usage(void);
 
 /**
  * Primary hash function for the hash database.
diff --git a/tkrzw_langc_check.c b/tkrzw_langc_check.c
index 53e5319..628b9b1 100644
--- a/tkrzw_langc_check.c
+++ b/tkrzw_langc_check.c
@@ -19,7 +19,7 @@
 
 #include "tkrzw_langc.h"
 
-void print_usage() {
+void print_usage(void) {
   fprintf(stderr, "usage: tkrzw_lang_check [path [params [iters]]]\n");
 }