diff --git a/sdk/include/crt/io.h b/sdk/include/crt/io.h
index 404407d7dba..e6a452cc0ac 100644
--- a/sdk/include/crt/io.h
+++ b/sdk/include/crt/io.h
@@ -474,6 +474,7 @@ _getcwd(
 #include <string.h>
 #endif
 
+  _Check_return_
   __CRT_INLINE
   intptr_t
   __cdecl
@@ -492,6 +493,7 @@ _getcwd(
     return ret;
   }
 
+  _Check_return_
   __CRT_INLINE
   int
   __cdecl
diff --git a/sdk/include/crt/stdlib.h b/sdk/include/crt/stdlib.h
index 3985a547536..e3548a84a71 100644
--- a/sdk/include/crt/stdlib.h
+++ b/sdk/include/crt/stdlib.h
@@ -1432,7 +1432,7 @@ extern "C" {
   _Check_return_ long long __cdecl llabs(_In_ long long _j);
   #pragma function(llabs)
 #endif
-  __MINGW_EXTENSION __CRT_INLINE long long __cdecl llabs(_In_ long long _j) { return (_j >= 0 ? _j : -_j); }
+  __MINGW_EXTENSION _Check_return_ __CRT_INLINE long long __cdecl llabs(_In_ long long _j) { return (_j >= 0 ? _j : -_j); }
 
   __MINGW_EXTENSION long long  __cdecl strtoll(const char* __restrict__, char** __restrict, int);
   __MINGW_EXTENSION unsigned long long  __cdecl strtoull(const char* __restrict__, char** __restrict__, int);
diff --git a/sdk/include/crt/string.h b/sdk/include/crt/string.h
index 23cf26a9ce1..1dd1fe0aba1 100644
--- a/sdk/include/crt/string.h
+++ b/sdk/include/crt/string.h
@@ -508,7 +508,7 @@ extern "C" {
   _strupr_s_l(
     _Inout_updates_z_(_Size) char *_Str,
     _In_ size_t _Size,
-    _locale_t _Locale);
+    _In_opt_ _locale_t _Locale);
 
   _Check_return_wat_
   _CRTIMP