= 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 `<<nn_sendmsg.3compat#,nn_sendmsg()>>`
or received by `<<nn_recvmsg.3compat#,nn_recvmsg()>>`.

NOTE: This structure and supporting macros are provided for API
<<nng_compat.3compat#,compatibility>> with legacy _libnanomsg_.
Consider using the relevant <<libnng.3#,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 evalutes 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

<<nn_recvmsg.3compat#,nn_recvmsg(3compat)>>,
<<nn_sendmsg.3compat#,nn_sendmsg(3compat)>>,
<<nng_compat.3compat#,nng_compat(3compat)>>,
<<nng.7#,nng(7)>>