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+= nn_cmsg(3compat)
+//
+// Copyright 2018 Staysail Systems, Inc. <info@staysail.tech>
+// Copyright 2018 Capitar IT Group BV <info@capitar.com>
+//
+// This document is supplied under the terms of the MIT License, a
+// copy of which should be located in the distribution where this
+// file was obtained (LICENSE.txt).  A copy of the license may also be
+// found online at https://opensource.org/licenses/MIT.
+//
+
+== NAME
+
+nn_cmsg - message control data (compatible API)
+
+== SYNOPSIS
+
+[source,c]
+----
+#include <nanomsg/nn.h>
+
+struct nn_cmsghdr {
+    size_t cmsg_len;
+    int    cmsg_level;
+    int    cmsg_type;
+};
+----
+
+== DESCRIPTION
+
+The `nn_cmsghdr` structure describes a block of control data that is
+associated with a message either sent by xref:nn_sendmsg.3compat.adoc[`nn_sendmsg()`]
+or received by xref:nn_recvmsg.3compat.adoc[`nn_recvmsg()`].
+
+NOTE: This structure and supporting macros are provided for API
+xref:nng_compat.3compat.adoc[compatibility] with legacy _libnanomsg_.
+Consider using the relevant xref:libnng.3.adoc[modern API] instead.
+
+Each header is followed by `cmsg_len` bytes of data, plus any padding required
+to align the structure.
+
+The only defined ancillary data at this time is the protocol headers used by
+the protocols.
+This uses `cmsg_level` set to `PROTO_SP` and the `cmsg_type` set to
+`SP_HDR`.
+The actual data for this will vary from depending on the protocol used.
+
+Convenience macros are provided to make working with these fields easier.
+
+`struct nn_cmsghdr *NN_CMSG_FIRSTHDR(struct nn_msghdr *__hdr__)`::
+This macro returns the first `struct nn_cmsghdr` header in _hdr_.
+
+`struct nn_cmsghdr *NN_CMSG_NXTHDR(struct nn_msghdr *__hdr__, struct nn_cmsghdr *__ch__)`::
+This macro returns a pointer to the next `struct nn_cmsghdr` in _hdr_ after _ch_.
+
+`void *NN_CMSG_DATA(struct nn_cmsghdr *__ch__)`::
+This macro returns a pointer to the header-specific data for _ch_.
+
+`size_t NN_CMSG_ALIGN(size_t __len__)`::
+This macro returns the length specified by _len_, plus any padding required to
+provide the necessary alignment for another structure.
+
+`size_t NN_CMSG_SPACE(size_t __len__)`::
+This macro returns the amount of space required for a header, with _len_
+bytes of following data, and any necessary padding.
+
+`size_t NN_CMSG_LEN(size_t __len__)`::
+This macro evaluates to the length of the header (including alignment),
+and the associated data of length _len_, but without any trailing padding
+to align for another header.
+
+== SEE ALSO
+
+[.text-left]
+xref:nn_recvmsg.3compat.adoc[nn_recvmsg(3compat)],
+xref:nn_sendmsg.3compat.adoc[nn_sendmsg(3compat)],
+xref:nng_compat.3compat.adoc[nng_compat(3compat)],
+xref:nng.7.adoc[nng(7)]