diff --git a/inc/errno.h b/inc/errno.h
index d0c52aa..e9fae4b 100644
--- a/inc/errno.h
+++ b/inc/errno.h
@@ -7,3 +7,7 @@
 
 // TODO: figure out why _Thread_local doesn't work
 extern _Thread_local int errno;
+
+#if __STDC_WANT_LIB_EXT1__ == 1
+    typedef int errno_t;
+#endif
diff --git a/inc/stddef.h b/inc/stddef.h
index 26922a0..f562749 100644
--- a/inc/stddef.h
+++ b/inc/stddef.h
@@ -19,3 +19,6 @@
     typedef int wchar_t;
 #endif
 
+#if __STDC_WANT_LIB_EXT1__ == 1
+    typedef size_t rsize_t;
+#endif
diff --git a/inc/stdint.h b/inc/stdint.h
index 8b6e28a..a1af872 100644
--- a/inc/stdint.h
+++ b/inc/stdint.h
@@ -143,3 +143,11 @@ typedef uint64_t uintmax_t;
 #if defined(_WIN32)
     #define UINT64_C(lit)  lit ## ull
 #endif
+
+#if __STDC_WANT_LIB_EXT1__ == 1
+    #if defined(_WIN32)
+        #define RSIZE_MAX  0xffffffffffffull;
+    #else
+        #define RSIZE_MAX  0xfffffffffffful;
+    #endif
+#endif