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

       This  ancillary data model conforms to the POSIX.1g draft, 4.4BSD-Lite,
       the IPv6 advanced API described in RFC 2292 and the SUSv2.   CMSG_ALIGN
       is a Linux extension.

       recvmsg(2), sendmsg(2)

       RFC 2292

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