diff --git a/libc/arch-x86_64/bionic/__set_tls.c b/libc/arch-x86_64/bionic/__set_tls.c
index 68454444e..10fd36f13 100644
--- a/libc/arch-x86_64/bionic/__set_tls.c
+++ b/libc/arch-x86_64/bionic/__set_tls.c
@@ -26,6 +26,7 @@
  * SUCH DAMAGE.
  */
 
+#include <sys/cdefs.h>
 #include <asm/prctl.h>
 #include <stdint.h>