diff --git a/include/msvcrt/string.h b/include/msvcrt/string.h
index a821aa27619..cc0b25e2bba 100644
--- a/include/msvcrt/string.h
+++ b/include/msvcrt/string.h
@@ -18,8 +18,18 @@ typedef unsigned short wchar_t;
 #endif
 #endif
 
+#ifndef _MSC_VER
+# ifndef __int64
+#  define __int64 long long
+# endif
+#endif
+
 #ifndef _SIZE_T_DEFINED
+#ifdef _WIN64
+typedef unsigned __int64 size_t;
+#else
 typedef unsigned int size_t;
+#endif
 #define _SIZE_T_DEFINED
 #endif