diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index d1996b18..fec08aa9 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -22,6 +22,7 @@
 #define _POSIX_SYNCHRONIZED_IO 200112L
 #define _POSIX_SAVED_IDS 200112L
 #define _POSIX_VDISABLE (-2)
+#define _POSIX_VERSION 200112L
 
 #ifdef __cplusplus
 extern "C"