diff --git a/libc/sock/struct/ifreq.h b/libc/sock/struct/ifreq.h
index 3a3bd7209..9424276e8 100644
--- a/libc/sock/struct/ifreq.h
+++ b/libc/sock/struct/ifreq.h
@@ -5,7 +5,9 @@
 COSMOPOLITAN_C_START_
 
 #define IF_NAMESIZE 16
+#ifndef IFNAMSIZ
 #define IFNAMSIZ    IF_NAMESIZE
+#endif
 
 struct ifreq {
   union {