diff --git a/compat/mingw.c b/compat/mingw.c
index a8218e6f0f..2b5467dead 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -2162,7 +2162,7 @@ int xwcstoutf(char *utf, const wchar_t *wcs, size_t utflen)
 	return -1;
 }
 
-static void setup_windows_environment()
+static void setup_windows_environment(void)
 {
 	char *tmp = getenv("TMPDIR");
 
@@ -2204,7 +2204,7 @@ typedef struct {
 extern int __wgetmainargs(int *argc, wchar_t ***argv, wchar_t ***env, int glob,
 		_startupinfo *si);
 
-static NORETURN void die_startup()
+static NORETURN void die_startup(void)
 {
 	fputs("fatal: not enough memory for initialization", stderr);
 	exit(128);
@@ -2224,7 +2224,7 @@ static char *wcstoutfdup_startup(char *buffer, const wchar_t *wcs, size_t len)
 	return memcpy(malloc_startup(len), buffer, len);
 }
 
-void mingw_startup()
+void mingw_startup(void)
 {
 	int i, maxlen, argc;
 	char *buffer;
diff --git a/compat/mingw.h b/compat/mingw.h
index 69bb43dc35..9a8803b876 100644
--- a/compat/mingw.h
+++ b/compat/mingw.h
@@ -532,8 +532,8 @@ extern CRITICAL_SECTION pinfo_cs;
  * A replacement of main() that adds win32 specific initialization.
  */
 
-void mingw_startup();
-#define main(c,v) dummy_decl_mingw_main(); \
+void mingw_startup(void);
+#define main(c,v) dummy_decl_mingw_main(void); \
 static int mingw_main(c,v); \
 int main(int argc, char **argv) \
 { \
diff --git a/compat/winansi.c b/compat/winansi.c
index 3be60ce1c6..db4a5b0a37 100644
--- a/compat/winansi.c
+++ b/compat/winansi.c
@@ -492,7 +492,7 @@ static inline ioinfo* _pioinfo(int fd)
 			(fd & (IOINFO_ARRAY_ELTS - 1)) * sizeof_ioinfo);
 }
 
-static int init_sizeof_ioinfo()
+static int init_sizeof_ioinfo(void)
 {
 	int istty, wastty;
 	/* don't init twice */