diff --git a/src/openvpn/win32.c b/src/openvpn/win32.c
index 86556a8..edac71e 100644
--- a/src/openvpn/win32.c
+++ b/src/openvpn/win32.c
@@ -103,7 +103,7 @@
  * Set OpenSSL environment variables to a safe directory
  */
 static void
-set_openssl_env_vars();
+set_openssl_env_vars(void);
 
 void
 init_win32(void)
@@ -1518,7 +1518,7 @@
 }
 
 static void
-set_openssl_env_vars()
+set_openssl_env_vars(void)
 {
     const WCHAR *ssl_fallback_dir = L"C:\\Windows\\System32";
 
diff --git a/src/openvpnserv/common.c b/src/openvpnserv/common.c
index a88a724..74bec6e 100644
--- a/src/openvpnserv/common.c
+++ b/src/openvpnserv/common.c
@@ -181,7 +181,7 @@
 
 
 LPCTSTR
-GetLastErrorText()
+GetLastErrorText(void)
 {
     DWORD error;
     static TCHAR buf[256];
diff --git a/src/openvpnserv/service.c b/src/openvpnserv/service.c
index 054fc5f..84fd11a 100644
--- a/src/openvpnserv/service.c
+++ b/src/openvpnserv/service.c
@@ -54,7 +54,7 @@
 }
 
 static int
-CmdInstallServices()
+CmdInstallServices(void)
 {
     SC_HANDLE service;
     SC_HANDLE svc_ctl_mgr;
@@ -146,7 +146,7 @@
 
 
 static int
-CmdRemoveServices()
+CmdRemoveServices(void)
 {
     SC_HANDLE service;
     SC_HANDLE svc_ctl_mgr;
diff --git a/src/openvpnserv/service.h b/src/openvpnserv/service.h
index da20433..c5f587b 100644
--- a/src/openvpnserv/service.h
+++ b/src/openvpnserv/service.h
@@ -86,7 +86,7 @@
 
 BOOL ReportStatusToSCMgr(SERVICE_STATUS_HANDLE service, SERVICE_STATUS *status);
 
-LPCTSTR GetLastErrorText();
+LPCTSTR GetLastErrorText(void);
 
 DWORD MsgToEventLog(DWORD flags, LPCTSTR lpszMsg, ...);