Ubuntu Manpage: CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR

NAME

       CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR - access ancillary data

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/socket.h>

       struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgh);
       struct cmsghdr *CMSG_NXTHDR(struct msghdr *msgh,
                                   struct cmsghdr *cmsg);
       size_t CMSG_ALIGN(size_t length);
       size_t CMSG_SPACE(size_t length);
       size_t CMSG_LEN(size_t length);
       unsigned char *CMSG_DATA(struct cmsghdr *cmsg);

DESCRIPTION

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 cmsghdr structures with appended data. See the specific
protocol man pages for the available control message types. The maximum ancillary buffer
size allowed per socket can be set using /proc/sys/net/core/optmem_max; see socket(7).

The cmsghdr structure is defined as follows:

struct cmsghdr {
size_t cmsg_len; /* Data byte count, including header
(type is socklen_t in POSIX) */
int cmsg_level; /* Originating protocol */
int cmsg_type; /* Protocol-specific type */
/* followed by
unsigned char cmsg_data[]; */
};

The sequence of cmsghdr structures should never be accessed directly. Instead, use only
the following macros:

CMSG_FIRSTHDR()
returns a pointer to the first cmsghdr in the ancillary data buffer associated with
the passed msghdr. It returns NULL if there isn't enough space for a cmsghdr in
the buffer.

CMSG_NXTHDR()
returns the next valid cmsghdr after the passed cmsghdr. It returns NULL when
there isn't enough space left in the buffer.

When initializing a buffer that will contain a series of cmsghdr structures (e.g.,
to be sent with sendmsg(2)), that buffer should first be zero-initialized to ensure
the correct operation of CMSG_NXTHDR().

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. The pointer returned cannot be
assumed to be suitably aligned for accessing arbitrary payload data types.
Applications should not cast it to a pointer type matching the payload, but should
instead use memcpy(3) to copy data to or from a suitably declared object.

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_NXTHDR() 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).

STANDARDS

       This ancillary data model conforms to the POSIX.1g draft, 4.4BSD-Lite, the  IPv6  advanced
       API  described in RFC 2292 and SUSv2.  CMSG_FIRSTHDR(), CMSG_NXTHDR(), and CMSG_DATA() are
       specified in POSIX.1-2008.  CMSG_SPACE() and CMSG_LEN() will be included in the next POSIX
       release (Issue 8).

       CMSG_ALIGN() is a Linux extension.

NOTES

       For  portability,  ancillary data should be accessed using only the macros described here.
       CMSG_ALIGN() is a Linux extension and should not be used in portable programs.

       In Linux, CMSG_LEN(), CMSG_DATA(), and CMSG_ALIGN()  are  constant  expressions  (assuming
       their  argument is constant), meaning that these values can be used to declare the size of
       global variables.  This may not be portable, however.

EXAMPLES

       This code looks for the IP_TTL option in a received ancillary buffer:

           struct msghdr msgh;
           struct cmsghdr *cmsg;
           int received_ttl;

           /* Receive auxiliary data in msgh */

           for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL;
                   cmsg = CMSG_NXTHDR(&msgh, cmsg)) {
               if (cmsg->cmsg_level == IPPROTO_IP
                       && cmsg->cmsg_type == IP_TTL) {
                   memcpy(&receive_ttl, CMSG_DATA(cmsg), sizeof(received_ttl));
                   break;
               }
           }

           if (cmsg == NULL) {
               /* Error: IP_TTL not enabled or small buffer or I/O error */
           }

       The code below passes an array of  file  descriptors  over  a  UNIX  domain  socket  using
       SCM_RIGHTS:

           struct msghdr msg = { 0 };
           struct cmsghdr *cmsg;
           int myfds[NUM_FD];  /* Contains the file descriptors to pass */
           char iobuf[1];
           struct iovec io = {
               .iov_base = iobuf,
               .iov_len = sizeof(iobuf)
           };
           union {         /* Ancillary data buffer, wrapped in a union
                              in order to ensure it is suitably aligned */
               char buf[CMSG_SPACE(sizeof(myfds))];
               struct cmsghdr align;
           } u;

           msg.msg_iov = &io;
           msg.msg_iovlen = 1;
           msg.msg_control = u.buf;
           msg.msg_controllen = sizeof(u.buf);
           cmsg = CMSG_FIRSTHDR(&msg);
           cmsg->cmsg_level = SOL_SOCKET;
           cmsg->cmsg_type = SCM_RIGHTS;
           cmsg->cmsg_len = CMSG_LEN(sizeof(myfds));
           memcpy(CMSG_DATA(cmsg), myfds, sizeof(myfds));

       For  a  complete  code  example  that shows passing of file descriptors over a UNIX domain
       socket, see seccomp_unotify(2).