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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 be accessed using only 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 /proc/sys/net/core/opt-
       mem_max;	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  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_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).

       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 SUSv2.  CMSG_ALIGN() is
       a Linux extension.

       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  ex-
       pressions  (assuming their argument is constant); this could be used to
       declare the size	of global variables.  This may not be  portable,  how-

       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	domain
       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 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at

Linux				  2008-11-20			       CMSG(3)


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