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CMSG(3) 		   Linux Programmers Manual		      CMSG(3)



       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);

       struct cmsghdr {
	   socklen_t cmsg_len;	  /* data byte count, including header */
	   int	     cmsg_level;  /* originating protocol */
	   int	     cmsg_type;   /* protocol-specific type */
	   /* followed by unsigned char cmsg_data[]; */

       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 isnt enough space left in the buffer.

       CMSG_ALIGN(),  given a length, returns it including the required align
       ment.  This is a constant expression.

       CMSG_SPACE() returns the number of bytes an ancillary element with pay
       load  of  the  passed data length occupies.  This is a constant expres

       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).

       When the control message buffer is too short to store all messages, the
       MSG_CTRUNC flag is set in the msg_flags member of the msghdr.

       This  ancillary data model conforms to the POSIX.1g draft, 4.4BSD-Lite,
       the  IPv6  advanced  API  described  in	 RFC 2292   and   the	SUSv2.
       CMSG_ALIGN() is a Linux extension.

       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.

       In  Linux,  CMSG_LEN(),	CMSG_DATA(),  and  CMSG_ALIGN()  are  constant
       expressions (assuming their argument is constant); this could  be  used
       to  declare  the  size  of global variables.  This may be not portable,

       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(&msgh); cmsg != NULL;
		   cmsg = CMSG_NXTHDR(&msgh,cmsg)) {
	       if (cmsg->cmsg_level == IPPROTO_IP
		       && cmsg->cmsg_type == IP_TTL) {
		   ttlptr = (int *) CMSG_DATA(cmsg);
		   received_ttl = *ttlptr;
	   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	socket
       using SCM_RIGHTS:

	   struct msghdr msg = {0};
	   struct cmsghdr *cmsg;
	   int myfds[NUM_FD]; /* Contains the file descriptors to pass. */
	   char buf[CMSG_SPACE(sizeof myfds)];	/* ancillary data buffer */
	   int *fdptr;

	   msg.msg_control = buf;
	   msg.msg_controllen = sizeof buf;
	   cmsg = CMSG_FIRSTHDR(&msg);
	   cmsg->cmsg_level = SOL_SOCKET;
	   cmsg->cmsg_type = SCM_RIGHTS;
	   cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);
	   /* Initialize the payload: */
	   fdptr = (int *) CMSG_DATA(cmsg);
	   memcpy(fdptr, myfds, NUM_FD * sizeof(int));
	   /* Sum of the length of all control messages in the buffer: */
	   msg.msg_controllen = cmsg->cmsg_len;

       recvmsg(2), sendmsg(2)

       RFC 2292

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  1998-10-02			       CMSG(3)

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