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CMSG_DATA(3)               Library Functions Manual               CMSG_DATA(3)

NAME
     CMSG_DATA, CMSG_FIRSTHDR, CMSG_LEN, CMSG_NXTHDR, CMSG_SPACE -- socket
     control message routines

SYNOPSIS
     #include <&lt;sys/socket.h>&gt;

     void *
     CMSG_DATA(struct cmsghdr *);

     struct cmsghdr *
     CMSG_FIRSTHDR(struct msghdr *);

     size_t
     CMSG_LEN(size_t);

     struct cmsghdr *
     CMSG_NXTHDR(struct msghdr *, struct cmsghdr *);

     size_t
     CMSG_SPACE(size_t);

DESCRIPTION
     The control message API is used to construct ancillary data objects for
     use in control messages sent and received across sockets.

     Control messages are passed around by the recvmsg(2) and sendmsg(2)
     system calls.  The cmsghdr structure, described in recvmsg(2), is used to
     specify a chain of control messages.

     These routines should be used instead of directly accessing the control
     message header members and data buffers as they ensure that necessary
     alignment constraints are met.

     The following routines are provided:

     CMSG_DATA(cmsg)
             This routine accesses the data portion of the control message
             header cmsg.  It ensures proper alignment constraints on the
             beginning of ancillary data are met.

     CMSG_FIRSTHDR(mhdr)
             This routine accesses the first control message attached to the
             message msg.  If no control messages are attached to the message,
             this routine returns NULL.

     CMSG_LEN(len)
             This routine determines the size in bytes of a control message,
             which includes the control message header.  len specifies the
             length of the data held by the control message.  This value is
             what is normally stored in the cmsg_len of each control message.
             This routine accounts for any alignment constraints on the
             beginning of ancillary data.  This macro might not evaluate to a
             compile-time constant.

     CMSG_NXTHDR(mhdr, cmsg)
             This routine returns the location of the control message
             following cmsg in the message mhdr.  If cmsg is the last control
             message in the chain, this routine returns NULL.

     CMSG_SPACE(len)
             This routine determines the size in bytes needed to hold a
             control message and its contents of length len, which includes
             the control message header.  This value is what is normally
             stored in msg_msgcontrollen.  This routine accounts for any
             alignment constraints on the beginning of ancillary data as well
             as any needed to pad the next control message.  This macro might
             not evaluate to a compile-time constant.

EXAMPLES
     The following example constructs a control message containing a file
     descriptor and passes it over a socket:

           struct msghdr    msg;
           struct cmsghdr  *cmsg;
           /* We use a union to make sure hdr is aligned */
           union {
                   struct cmsghdr hdr;
                   unsigned char    buf[CMSG_SPACE(sizeof(int))];
           } *cmsgbuf;

           /*
            * We allocate in the heap instead of the stack to avoid C99
            * variable stack allocation, which breaks gcc -fstack-protector.
            */
           if ((cmsgbuf = malloc(sizeof(*cmsgbuf))) == NULL)
                   err(1, "malloc");
           (void)memset(&msg, 0, sizeof(msg));
           msg.msg_control = cmsgbuf->buf;
           msg.msg_controllen = sizeof(cmsgbuf->buf);

           cmsg = CMSG_FIRSTHDR(&msg);
           cmsg->cmsg_len = CMSG_LEN(sizeof(int));
           cmsg->cmsg_level = SOL_SOCKET;
           cmsg->cmsg_type = SCM_RIGHTS;
           *(int *)CMSG_DATA(cmsg) = fd;

           if (sendmsg(s, &msg, 0) == -1)
                   err(1, "sendmsg");
           free(cmsgbuf);

     And an example that receives and decomposes the control message:

           struct msghdr    msg;
           struct cmsghdr  *cmsg;
           union {
                   struct cmsghdr hdr;
                   unsigned char    buf[CMSG_SPACE(sizeof(int))];
           } *cmsgbuf;

           if ((cmsgbuf = malloc(sizeof(*cmsgbuf))) == NULL)
                   err(1, "malloc");
           (void)memset(&msg, 0, sizeof(msg));
           msg.msg_control = cmsgbuf->buf;
           msg.msg_controllen = sizeof(cmsgbuf->buf);

           if (recvmsg(s, &msg, 0) == -1)
                   err(1, "recvmsg");
           if ((msg.msg_flags & (MSG_TRUNC|MSG_CTRUNC))
                   errx(1, "control message truncated");
           for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL;
               cmsg = CMSG_NXTHDR(&msg, cmsg)) {
                   if (cmsg->cmsg_len == CMSG_LEN(sizeof(int)) &&
                       cmsg->cmsg_level == SOL_SOCKET &&
                       cmsg->cmsg_type == SCM_RIGHTS) {
                           fd = *(int *)CMSG_DATA(cmsg);
                           /* Do something with the descriptor. */
                   }
           }
           free(cmsgbuf);

SEE ALSO
     recvmsg(2), sendmsg(2), socket(2)

HISTORY
     The control message API first appeared in 4.2BSD.

NetBSD 6.1.5                     June 20, 2008                    NetBSD 6.1.5