MirBSD manpage: CMSG_DATA(3), CMSG_FIRSTHDR(3), CMSG_LEN(3), CMSG_NXTHDR(3), CMSG_SPACE(3)

CMSG_DATA(3)               BSD Programmer's Manual                CMSG_DATA(3)

NAME

     CMSG_DATA, CMSG_FIRSTHDR, CMSG_LEN, CMSG_NXTHDR, CMSG_SPACE - socket con-
     trol message routines

SYNOPSIS

     #include <sys/socket.h>

     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) sys-
     tem 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 be-
             ginning 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 routine ac-
             counts for any alignment constraints on the beginning of ancil-
             lary data.

     CMSG_NXTHDR(mhdr, cmsg)
             This routine returns the location of the control message follow-
             ing 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 con-
             trol message and its contents of length len, which includes the
             control message header. This routine accounts for any alignment
             constraints on the beginning of ancillary data as well as any
             needed to pad the next control message.

EXAMPLES

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

           struct msghdr    msg;
           struct cmsghdr  *cmsg;
           unsigned char    buf[CMSG_SPACE(sizeof(int))];

           memset(&msg, 0, sizeof(msg));
           msg.msg_control = buf;
           msg.msg_controllen = CMSG_LEN(sizeof(int));

           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");

     And an example that receives and decomposes the control message:

           struct msghdr    msg;
           struct cmsghdr  *cmsg;
           unsigned char    buf[CMSG_SPACE(sizeof(int))];

           memset(&msg, 0, sizeof(msg));
           msg.msg_control = buf;
           msg.msg_controllen = sizeof(buf);

           if (recvmsg(s, &msg, 0) == -1)
                   err(1, "recvmsg");
           if ((msg.msg_flags & MSG_TRUNC) || (msg.msg_flags & 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. */
                   }
           }

SEE ALSO

     recvmsg(2), sendmsg(2), socket(2)

HISTORY

     The control message API first appeared in 4.2BSD.

MirBSD #10-current               July 3, 2005                                1