Comment cmsg_align.

src/sys/socket/mod.rs

@@ -392,6 +392,10 @@ pub enum ControlMessage<'a> {
 #[doc(hidden)]
 pub struct UnknownCmsg<'a>(&'a cmsghdr, &'a [u8]);
 
+// Round `len` up to meet the platform's required alignment for
+// `cmsghdr`s and trailing `cmsghdr` data.  This should match the
+// behaviour of CMSG_ALIGN from the Linux headers and do the correct
+// thing on other platforms that don't usually provide CMSG_ALIGN.
 #[inline]
 fn cmsg_align(len: usize) -> usize {
     let align_bytes = mem::size_of::<align_of_cmsg_data>() - 1;