CMSG_DATA(3)						   BSD Library Functions Manual 					      CMSG_DATA(3)

NAME

     CMSG_DATA, CMSG_FIRSTHDR, CMSG_LEN, CMSG_NXTHDR, CMSG_SPACE -- socket control 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) 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 rou-
	     tine accounts for any alignment constraints on the beginning of ancillary data.  This macro might not evaluate to a compile-time con-
	     stant.

     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.

BSD
								   June 20, 2008							       BSD