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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);

       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 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/optmem_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 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_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, however.


       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

           struct msghdr msg = {0};
           struct cmsghdr *cmsg;
           int myfds[NUM_FD]; /* Contains the file descriptors to pass. */
           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;
           int *fdptr;

           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);
           /* Initialize the payload: */
           fdptr = (int *) CMSG_DATA(cmsg);
           memcpy(fdptr, myfds, NUM_FD * sizeof(int));


       recvmsg(2), sendmsg(2)

       RFC 2292


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