## 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]