-.IR msg_accrightslen .
-Access rights are currently limited to file descriptors,
-which each occupy the size of an
-.BR int .
-.SH "RETURN VALUE
-These calls return the number of bytes received, or \-1
+.Fa msg_controllen ,
+points to a buffer for other protocol control related messages
+or other miscellaneous ancillary data.
+The messages are of the form:
+.Bd -literal
+struct cmsghdr {
+ u_int cmsg_len; /* data byte count, including hdr */
+ int cmsg_level; /* originating protocol */
+ int cmsg_type; /* protocol-specific type */
+/* followed by
+ u_char cmsg_data[]; */
+};
+.Ed
+As an example, one could use this to learn of changes in the data-stream
+in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
+a recvmsg with no data buffer provided immediately after an
+.Fn accept
+call.
+.Pp
+Open file descriptors are now passed as ancillary data for
+.Dv AF_UNIX
+domain sockets, with
+.Fa cmsg_level
+set to
+.Dv SOL_SOCKET
+and
+.Fa cmsg_type
+set to
+.Dv SCM_RIGHTS .
+.Pp
+The
+.Fa msg_flags
+field is set on return according to the message received.
+.Dv MSG_EOR
+indicates end-of-record;
+the data returned completed a record (generally used with sockets of type
+.Dv SOCK_SEQPACKET ) .
+.Dv MSG_TRUNC
+indicates that
+the trailing portion of a datagram was discarded because the datagram
+was larger than the buffer supplied.
+.Dv MSG_CTRUNC
+indicates that some
+control data were discarded due to lack of space in the buffer
+for ancillary data.
+.Dv MSG_OOB
+is returned to indicate that expedited or out-of-band data were received.
+.Pp
+.Sh RETURN VALUES
+These calls return the number of bytes received, or -1