diff --git a/lib/libc/alpha/arith.h b/lib/libc/alpha/arith.h
index de6c33dd9f3f..c8af6de23075 100644
--- a/lib/libc/alpha/arith.h
+++ b/lib/libc/alpha/arith.h
@@ -4,6 +4,13 @@
  * $FreeBSD$
  */
 
+/*
+ * NOTE: The definitions in this file must be correct or strtod(3) and
+ * floating point formats in printf(3) will break!  The file can be
+ * generated by running contrib/gdtoa/arithchk.c on the target
+ * architecture.  See contrib/gdtoa/gdtoaimp.h for details.
+ */
+
 #define IEEE_8087
 #define Arith_Kind_ASL 1
 #define Long int
diff --git a/lib/libc/amd64/arith.h b/lib/libc/amd64/arith.h
index 202264da1092..ecb1a33fccb0 100644
--- a/lib/libc/amd64/arith.h
+++ b/lib/libc/amd64/arith.h
@@ -4,6 +4,13 @@
  * $FreeBSD$
  */
 
+/*
+ * NOTE: The definitions in this file must be correct or strtod(3) and
+ * floating point formats in printf(3) will break!  The file can be
+ * generated by running contrib/gdtoa/arithchk.c on the target
+ * architecture.  See contrib/gdtoa/gdtoaimp.h for details.
+ */
+
 #define IEEE_8087
 #define Arith_Kind_ASL 1
 #define Long int
diff --git a/lib/libc/i386/arith.h b/lib/libc/i386/arith.h
index d1bcfb6ed453..2d65061e82a7 100644
--- a/lib/libc/i386/arith.h
+++ b/lib/libc/i386/arith.h
@@ -4,5 +4,12 @@
  * $FreeBSD$
  */
 
+/*
+ * NOTE: The definitions in this file must be correct or strtod(3) and
+ * floating point formats in printf(3) will break!  The file can be
+ * generated by running contrib/gdtoa/arithchk.c on the target
+ * architecture.  See contrib/gdtoa/gdtoaimp.h for details.
+ */
+
 #define IEEE_8087
 #define Arith_Kind_ASL 1
diff --git a/lib/libc/ia64/arith.h b/lib/libc/ia64/arith.h
index 9102826d8925..67265284dbf0 100644
--- a/lib/libc/ia64/arith.h
+++ b/lib/libc/ia64/arith.h
@@ -4,6 +4,13 @@
  * $FreeBSD$
  */
 
+/*
+ * NOTE: The definitions in this file must be correct or strtod(3) and
+ * floating point formats in printf(3) will break!  The file can be
+ * generated by running contrib/gdtoa/arithchk.c on the target
+ * architecture.  See contrib/gdtoa/gdtoaimp.h for details.
+ */
+
 #include <machine/endian.h>
 
 #if _BYTE_ORDER == _LITTLE_ENDIAN
diff --git a/lib/libc/powerpc/arith.h b/lib/libc/powerpc/arith.h
index 9f4cfe4304d0..8e2c9ec597b3 100644
--- a/lib/libc/powerpc/arith.h
+++ b/lib/libc/powerpc/arith.h
@@ -4,6 +4,13 @@
  * $FreeBSD$
  */
 
+/*
+ * NOTE: The definitions in this file must be correct or strtod(3) and
+ * floating point formats in printf(3) will break!  The file can be
+ * generated by running contrib/gdtoa/arithchk.c on the target
+ * architecture.  See contrib/gdtoa/gdtoaimp.h for details.
+ */
+
 #define IEEE_MC68k
 #define Arith_Kind_ASL 2
 #define Double_Align
diff --git a/lib/libc/sparc64/arith.h b/lib/libc/sparc64/arith.h
index c954a9c1ad76..3c35b80f35c3 100644
--- a/lib/libc/sparc64/arith.h
+++ b/lib/libc/sparc64/arith.h
@@ -4,6 +4,13 @@
  * $FreeBSD$
  */
 
+/*
+ * NOTE: The definitions in this file must be correct or strtod(3) and
+ * floating point formats in printf(3) will break!  The file can be
+ * generated by running contrib/gdtoa/arithchk.c on the target
+ * architecture.  See contrib/gdtoa/gdtoaimp.h for details.
+ */
+
 #define IEEE_MC68k
 #define Arith_Kind_ASL 2
 #define Long int