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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);

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 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.

       *  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).

CONFORMING TO

       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.

NOTES

       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.

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 == IPPROTO_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 domain socket using
       SCM_RIGHTS:

           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));

SEE ALSO

       recvmsg(2), sendmsg(2)

       RFC 2292

COLOPHON

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