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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);

       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[]; */

       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 only be
       accessed using 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

       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  con-
       trol  message buffer.  Use CMSG_FIRSTHDR() on the msghdr to get the first control message and CMSG_NEXTHDR() 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  informa-
       tion 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.

       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.

       For  portability,  ancillary  data  should be accessed only using 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  con-
       stant); this could be used to declare the size of global variables.  This may be not portable, however.

       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;
           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 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;

       recvmsg(2), sendmsg(2)

       RFC 2292

       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at

Linux                             2008-11-20                           CMSG(3)