DESCRIPTION

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.

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.

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

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");

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

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.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. */
	}
}