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


       #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 op-
       tions.  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.  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 cms-
          ghdr.  It returns NULL when there isn't enough  space  left  in  the

          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_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  ex-

       *  CMSG_DATA() returns a pointer to the data portion of a cmsghdr.  The
          pointer returned cannot be assumed to be suitably  aligned  for  ac-
          cessing  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 expres-

       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  ex-
       pressions  (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 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));

       recvmsg(2), sendmsg(2)

       RFC 2292

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Linux                             2020-02-09                           CMSG(3)
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