diff --git a/libc/include/stdio.h b/libc/include/stdio.h
index 8727a9f..ce60fd7 100644
--- a/libc/include/stdio.h
+++ b/libc/include/stdio.h
@@ -325,6 +325,11 @@
 int	 putchar_unlocked(int);
 #endif /* __POSIX_VISIBLE >= 199506 */
 
+#if __POSIX_VISIBLE >= 200809
+FILE* fmemopen(void*, size_t, const char*);
+FILE* open_memstream(char**, size_t*);
+#endif /* __POSIX_VISIBLE >= 200809 */
+
 __END_DECLS
 
 #endif /* __BSD_VISIBLE || __POSIX_VISIBLE || __XPG_VISIBLE */
diff --git a/libc/include/wchar.h b/libc/include/wchar.h
index 1898c7e..ae10d93 100644
--- a/libc/include/wchar.h
+++ b/libc/include/wchar.h
@@ -166,6 +166,7 @@
 extern wctrans_t wctrans(const char*);
 
 #if __POSIX_VISIBLE >= 200809
+FILE* open_wmemstream(wchar_t**, size_t*);
 wchar_t* wcsdup(const wchar_t*);
 size_t wcsnlen(const wchar_t*, size_t);
 #endif