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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 {
	   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[]; */

       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 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
       expressions  (assuming  their argument is constant); this could be used
       to declare the size of global variables.	 This  may  not	 be  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 domain
       socket using SCM_RIGHTS:

	   struct msghdr msg = { 0 };
	   struct cmsghdr *cmsg;
	   int myfds[NUM_FD];  /* Contains the file descriptors to pass */
	   int *fdptr;
	   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_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(int) * NUM_FD);
	   fdptr = (int *) CMSG_DATA(cmsg);    /* Initialize the payload */
	   memcpy(fdptr, myfds, NUM_FD * sizeof(int));

       recvmsg(2), sendmsg(2)

       RFC 2292

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Linux				  2016-03-15			       CMSG(3)