cmsg

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 {
           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[]; */
       };

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 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 be accessed using only 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 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_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).

       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 using only 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 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 domain
       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 3.53 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)