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

NAME

       CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR - Access ancillary data.

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);

       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[]; */
       };

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 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 maxi-
       mum 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 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.

       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,  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
       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.

EXAMPLE

       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;
			      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 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;

NOTES

       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.

CONFORMS TO

       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.

SEE ALSO

       sendmsg(2), recvmsg(2)

       RFC 2292

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