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RedHat 9 (Linux i386) - man page for cmsg_firsthdr (redhat section 3)

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

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

       #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  mes-
       sages  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.opt-
       mem_max sysctl; see socket(7).

       CMSG_FIRSTHDR  returns a pointer to the first cmsghdr in the ancillary data buffer associ-
       ated 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 con-
       stant 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, tak-
       ing 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 mes-
       sage, 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 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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