NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

CMSG(3)                   Linux Programmer's Manual                  CMSG(3)

NAME         top

       CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR - access ancillary
       data

SYNOPSIS         top

       #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         top

       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         top

       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.

NOTES         top

       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); this could be used
       to declare the size of global variables.  This may not be portable,
       however.

EXAMPLE         top

       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 */
           int *fdptr;
           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_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);
           fdptr = (int *) CMSG_DATA(cmsg);    /* Initialize the payload */
           memcpy(fdptr, myfds, NUM_FD * sizeof(int));

SEE ALSO         top

       recvmsg(2), sendmsg(2)

       RFC 2292

COLOPHON         top

       This page is part of release 4.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
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Linux                            2016-03-15                          CMSG(3)