diff --git a/include/stdlib.h b/include/stdlib.h
index 6bacd952b..59b339dd5 100644
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -62,6 +62,7 @@ void __fastcall__ free (void* block);
 
 /* Non standard memory management functions */
 
+#if __CC65_STD__ == __CC65_STD_CC65__
 int __fastcall__ posix_memalign (void** memptr, size_t alignment, size_t size);
 /* Allocate a block of memory with the given "size", which is aligned to a
  * memory address that is a multiple of "alignment".  "alignment" MUST NOT be
@@ -71,6 +72,7 @@ int __fastcall__ posix_memalign (void** memptr, size_t alignment, size_t size);
  * will return the address of the allocated memory.  Use free() to release that
  * allocated block.
  */
+#endif
 
 void __fastcall__ _heapadd (void* mem, size_t size);
 /* Add a block to the heap */