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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);

       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[]; */
       };

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 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.

CONFORMING TO

       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.

NOTES

       For portability, ancillary data should be accessed using only 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.

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 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;

SEE ALSO

       recvmsg(2), sendmsg(2)

       RFC 2292

COLOPHON

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