cmsg(3)



CMSG(3)                    Linux Programmer's Manual                   CMSG(3)

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 ancil-
          lary data buffer associated with the passed msghdr.

       *  CMSG_NXTHDR() returns the next valid cmsghdr after the  passed  cms-
          ghdr.   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 expres-
          sion.

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

SEE ALSO
       recvmsg(2), sendmsg(2)

       RFC 2292

COLOPHON
       This  page  is  part of release 4.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                           CMSG(3)

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