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CMSG(3)			   Linux Programmer's Manual		       CMSG(3)


       #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);

       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 op-
       tions.  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	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 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  cms-
       ghdr  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, ini-
       tialize 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 ms-
       ghdr, 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 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 == SOL_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;

       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, however.

       This ancillary data model conforms to the POSIX.1003.1g draft,  4.4BSD-
       Lite,  the  IPv6	 advanced API described	in RFC2292 and the Single Unix
       specification v2.  CMSG_ALIGN is	a Linux	extension.

       sendmsg(2), recvmsg(2)

       RFC 2292

Linux Man Page			  1998-10-02			       CMSG(3)


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