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+## 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.adoc[`nn_sendmsg`] or received by xref:nn_recvmsg.adoc[`nn_recvmsg`].
+
+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
+
+xref:nn_recvmsg.adoc[nn_recvmsg],
+xref:nn_sendmsg.adoc[nn_sendmsg]