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       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);


       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 cmsghdr structures with appended data.  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/optmem_max; see socket(7).

       The cmsghdr structure is defined as follows:

           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[]; */

       The  sequence  of cmsghdr structures should never be accessed directly.  Instead, use only
       the following macros:

       *  CMSG_FIRSTHDR() returns a pointer to the first cmsghdr in  the  ancillary  data  buffer
          associated  with  the passed msghdr.  It returns NULL if there isn't enough space for a
          cmsghdr in the buffer.

       *  CMSG_NXTHDR() returns the next valid cmsghdr after the passed cmsghdr.  It returns NULL
          when there isn't enough space left in the buffer.

          When  initializing  a buffer that will contain a series of cmsghdr structures (e.g., to
          be sent with sendmsg(2)), that buffer should first be zero-initialized  to  ensure  the
          correct operation of CMSG_NXTHDR().

       *  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.  The pointer returned
          cannot be assumed to be suitably aligned for accessing arbitrary  payload  data  types.
          Applications  should  not  cast  it  to a pointer type matching the payload, but should
          instead use memcpy(3) to copy data to or from a suitably declared object.

       *  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).


       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), meaning that these values can be used to declare the size of
       global variables.  This may not be portable, however.


       This code looks for the IP_TTL option in a received ancillary buffer:

           struct msghdr msgh;
           struct cmsghdr *cmsg;
           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) {
                   memcpy(&receive_ttl, CMSG_DATA(cmsg), sizeof(int));

           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

           struct msghdr msg = { 0 };
           struct cmsghdr *cmsg;
           int myfds[NUM_FD];  /* Contains the file descriptors to pass */
           char iobuf[1];
           struct iovec io = {
               .iov_base = iobuf,
               .iov_len = sizeof(iobuf)
           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_iov = &io;
           msg.msg_iovlen = 1;
           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);
           memcpy(CMSG_DATA(cmsg), myfds, NUM_FD * sizeof(int));


       recvmsg(2), sendmsg(2)

       RFC 2292


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