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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);

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 cmsghdr structures with appended  data.
     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).

     The cmsghdr structure is defined as follows:

         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[]; */ };

     The sequence of cmsghdr structures should never be  accessed  directly.
     Instead, use only the following macros:

CMSG_FIRSTHDR() returns a pointer to the first cmsghdr in the ancil-

lary data buffer associated with the passed msghdr.

CMSG_NXTHDR() returns the next valid cmsghdr after the passed cms-

ghdr. 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 expres-
        sion.

     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 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
     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

     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; char
         iobuf[1]; struct iovec io = {
             .iov_base = iobuf,
             .iov_len = sizeof(iobuf) }; 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_iov  =  &io;  msg.msg_iovlen  = 1; 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));