diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -551,19 +551,31 @@
 #ifdef _XOPEN_SOURCE
 #if _XOPEN_SOURCE - 0 >= 800
 #define	__XSI_VISIBLE		800
+#if _POSIX_C_SOURCE - 0 > 0 && _POSIX_C_SOURCE - 0 != 202405
+#warning "_POSIX_C_SOURCE must be 202405 for _XOPEN_SOURCE == 800"
 #undef _POSIX_C_SOURCE
+#endif
 #define	_POSIX_C_SOURCE		202405
 #elif _XOPEN_SOURCE - 0 >= 700
 #define	__XSI_VISIBLE		700
+#if _POSIX_C_SOURCE - 0 > 0 && _POSIX_C_SOURCE - 0 != 200809
+#warning "_POSIX_C_SOURCE must be 200809 for _XOPEN_SOURCE == 700"
 #undef _POSIX_C_SOURCE
+#endif
 #define	_POSIX_C_SOURCE		200809
 #elif _XOPEN_SOURCE - 0 >= 600
 #define	__XSI_VISIBLE		600
+#if _POSIX_C_SOURCE - 0 > 0 && _POSIX_C_SOURCE - 0 != 200112
+#warning "_POSIX_C_SOURCE must be 200112 for _XOPEN_SOURCE == 600"
 #undef _POSIX_C_SOURCE
+#endif
 #define	_POSIX_C_SOURCE		200112
 #elif _XOPEN_SOURCE - 0 >= 500
 #define	__XSI_VISIBLE		500
+#if _POSIX_C_SOURCE - 0 > 0 && _POSIX_C_SOURCE - 0 != 199506
+#warning "_POSIX_C_SOURCE must be 199506 for _XOPEN_SOURCE == 500"
 #undef _POSIX_C_SOURCE
+#endif
 #define	_POSIX_C_SOURCE		199506
 #else
 /* #define	_POSIX_C_SOURCE		199209 */