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);

       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[]; */
       };

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 mes-
       sages 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/opt-
       mem_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 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 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 por-
       tion 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
       the SUSv2.  CMSG_ALIGN() is a Linux extension.

NOTES
       For  portability, ancillary data should be accessed only using the macros described here.  CMSG_ALIGN() is a Linux exten-
       sion and should be not 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 be not 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 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;

SEE ALSO
       recvmsg(2), sendmsg(2)

       RFC 2292

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2008-11-20                                                    CMSG(3)