This control information may include the interface the packet was received on, various rarely used header fields, an extended error description, a set of file descriptors or unix credentials. For instance, control messages can be used to send additional header fields such as IP options. See their manual pages for more information.
Ancillary data is a sequence of struct cmsghdr structures with appended data.
This sequence should only be accessed using the macros described in this manual page and never directly. See the specific protocol man pages for the available control message types. It returns NULL when there isn't enough space left in the buffer. This is a constant expression. This is a constant expression.