diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -515,6 +515,10 @@
 
 /* Deal with various X/Open Portability Guides and Single UNIX Spec. */
 #ifdef _XOPEN_SOURCE
+#if _XOPEN_SOURCE - 0 >= 800
+#define	__XSI_VISIBLE		800
+#undef _POSIX_C_SOURCE
+#define	_POSIX_C_SOURCE		202405
 #if _XOPEN_SOURCE - 0 >= 700
 #define	__XSI_VISIBLE		700
 #undef _POSIX_C_SOURCE