These macros are used to create and access control messages (also called ancillary data) that are not a part of the socket payload. 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. Ancillary data is sent by calling sendmsg(2) and received by calling recvmsg(2). 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. The maximum ancillary buffer size allowed per socket can be set using the net.core.optmem_max sysctl; see socket(7).
CMSG_FIRSTHDR returns a pointer to the first cmsghdr in the ancillary data buffer associated with the passed msghdr.
CMSG_NXTHDR returns the next valid cmsghdr after the passed cmsghdr. It returns NULL when there isn't enough space left in the buffer.
CMSG_ALIGN, given a length, returns it including the required alignment. This is a constant expression.
CMSG_SPACE returns the number of bytes an ancillary element with payload of the passed data length occupies. This is a constant expression.
CMSG_DATA returns a pointer to the data portion of a cmsghdr.
CMSG_LEN returns the value to store in the cmsg_len member of the cmsghdr structure, taking into account any necessary alignment. It takes the data length as an argument. This is a constant expression.
To create ancillary data, first initialize the msg_controllen member of the msghdr with the length of the control message buffer. Use CMSG_FIRSTHDR on the msghdr to get the first control message and CMSG_NEXTHDR to get all subsequent ones. In each control message, initialize cmsg_len (with CMSG_LEN), the other cmsghdr header fields, and the data portion using CMSG_DATA. Finally, the msg_controllen field of the msghdr should be set to the sum of the CMSG_SPACE of the length of all control messages in the buffer. For more information on the msghdr, see recvmsg(2).
This code looks for the IP_TTL option in a received ancillary buffer:
struct msghdr msgh; struct cmsghdr cmsg; int *ttlptr; int received_ttl; / Receive auxiliary data in msgh */ for (cmsg = CMSG_FIRSTHDR(
The code below passes an array of file descriptors over a Unix socket using SCM_RIGHTS:
For portability, ancillary data should be accessed only using the macros described here. CMSG_ALIGN is a Linux extension and should be not used in portable programs.