diff --git a/libc/include/sys/cdefs.h b/libc/include/sys/cdefs.h
index a919a79..ca9374e 100644
--- a/libc/include/sys/cdefs.h
+++ b/libc/include/sys/cdefs.h
@@ -86,6 +86,7 @@
 #define __noreturn __attribute__((__noreturn__))
 #define __mallocfunc  __attribute__((__malloc__))
 #define __packed __attribute__((__packed__))
+#define __returns_twice __attribute__((__returns_twice__))
 #define __unused __attribute__((__unused__))
 #define __used __attribute__((__used__))