+/* The code that uses CMSG_FIRSTHDR is enabled on
+ Linux, MacOS X, FreeBSD, OpenBSD, NetBSD, AIX, OSF/1, Cygwin.
+ The code that uses HAVE_STRUCT_MSGHDR_MSG_ACCRIGHTS is enabled on
+ HP-UX, IRIX, Solaris. */
+
+/* MSG_CMSG_CLOEXEC is defined only on Linux, as of 2011. */