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
       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

          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

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

       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
       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.

       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)