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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  creden-
       tials.  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 mes-
       sage 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 struc-
       ture, 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_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 information on the msghdr, see

       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  exten-

       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  pro-

       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.

       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

           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 information about reporting bugs, can be found at http://www.ker-

Linux                             2008-11-20                           CMSG(3)

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