diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -502,6 +502,25 @@
  * Our macros begin with two underscores to avoid namespace screwage.
  */
 
+/*
+ * Cope with a now-common error: users defined __BSD_VISIBLE bogusly in the
+ * absence of clear advice during the years FreeBSD didn't have people watching
+ * the namespace and practices well. This will eventually be a #error
+ */
+#ifdef __BSD_VISIBLE
+#undef __BSD_VISIBLE
+#undef _BSD_SOURCE
+#define _BSD_SOURCE
+#endif
+
+#ifdef _BSD_SOURCE
+#define	__POSIX_VISIBLE		202405
+#define	__XSI_VISIBLE		800
+#define	__BSD_VISIBLE		1
+#define	__ISO_C_VISIBLE		2023
+#define	__EXT1_VISIBLE		1
+#else /* for review I've not indented the #if mandness below */
+
 /* Deal with IEEE Std. 1003.1-1990, in which _POSIX_C_SOURCE == 1. */
 #if defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE == 1
 #undef _POSIX_C_SOURCE		/* Probably illegal, but beyond caring now. */
@@ -653,6 +672,7 @@
 #endif
 #endif /* __STDC_WANT_LIB_EXT1__ */
 
+#endif /* ! _BSD_SOURCE */
 /*
  * Nullability qualifiers: currently only supported by Clang.
  */