.\" Written by Jared Yanovich <jaredy@openbsd.org>
.\" Public domain, July 3, 2005
.\"
.\" $FreeBSD$
.Dd March 13, 2020
.Dt CMSG_DATA 3
.Os
.Sh NAME
.Nm CMSG_DATA ,
.Nm CMSG_FIRSTHDR ,
.Nm CMSG_LEN ,
.Nm CMSG_NXTHDR ,
.Nm CMSG_SPACE
.Nd socket control message routines for ancillary data access
.Sh SYNOPSIS
.In sys/socket.h
.Ft unsigned char *
.Fn CMSG_DATA "struct cmsghdr *"
.Ft struct cmsghdr *
.Fn CMSG_FIRSTHDR "struct msghdr *"
.Ft size_t
.Fn CMSG_LEN "size_t"
.Ft struct cmsghdr *
.Fn CMSG_NXTHDR "struct msghdr *" "struct cmsghdr *"
.Ft size_t
.Fn CMSG_SPACE "size_t"
.Sh DESCRIPTION
The control message API is used to construct ancillary data objects for
use in control messages sent and received across sockets.
.Pp
Control messages are passed around by the
.Xr recvmsg 2
and
.Xr sendmsg 2
system calls.
The
.Vt cmsghdr
structure, described in
.Xr recvmsg 2 ,
is used to specify a chain of control messages.
.Pp
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.
.Pp
The following routines are provided:
.Bl -tag -width Ds
.It Fn CMSG_DATA cmsg
This routine accesses the data portion of the control message header
.Fa cmsg .
It ensures proper alignment constraints on the beginning of ancillary
data are met.
.It Fn CMSG_FIRSTHDR msghdr
This routine accesses the first control message attached to the
message
.Fa msghdr .
If no control messages are attached to the message, this routine
returns
.Dv NULL .
.It Fn CMSG_LEN len
This routine determines the size in bytes of a control message,
which includes the control message header.
.Fa len
specifies the length of the data held by the control message.
This value is what is normally stored in the
.Fa cmsg_len
of each control message.
This routine accounts for any alignment constraints on the beginning of
ancillary data.
.It Fn CMSG_NXTHDR msghdr cmsg
This routine returns the location of the control message following
.Fa cmsg
in the message
.Fa msghdr .
If
.Fa cmsg
is the last control message in the chain, this routine returns
.Dv NULL .
.It Fn CMSG_SPACE len
This routine determines the size in bytes needed to hold a control
message and its contents of length
.Fa len ,
which includes the control message header.
This value is what is normally stored in
.Fa 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.
.El
.Sh EXAMPLES
The following example constructs a control message containing a file descriptor
in the parent process and passes it over a pre-shared socket over the child
process.
Then the child process sends a "hello" string to the parent process using the
received file descriptor.
.Bd -literal
#include <sys/socket.h>

#include <err.h>
#include <stdio.h>
#include <string.h>
#include <sysexits.h>
#include <unistd.h>

#define	HELLOLEN    sizeof("hello")

int
main()
{
	struct msghdr msg;
	union {
		struct cmsghdr hdr;
		unsigned char	 buf[CMSG_SPACE(sizeof(int))];
	} cmsgbuf;
	char buf[HELLOLEN];
	int hellofd[2];
	int presharedfd[2];
	struct cmsghdr *cmsg;

	if (socketpair(PF_LOCAL, SOCK_STREAM, 0, presharedfd) == -1)
		err(EX_OSERR, "failed to create a pre-shared socket pair");

	memset(&msg, 0, sizeof(msg));
	msg.msg_control = &cmsgbuf.buf;
	msg.msg_controllen = sizeof(cmsgbuf.buf);
	msg.msg_iov = NULL;
	msg.msg_iovlen = 0;

	switch (fork()) {
	case -1:
		err(EX_OSERR, "fork");
	case 0:
		close(presharedfd[0]);
		strlcpy(buf, "hello", HELLOLEN);

		if (recvmsg(presharedfd[1], &msg, 0) == -1)
			err(EX_IOERR, "failed to receive a message");
		if (msg.msg_flags & (MSG_CTRUNC | MSG_TRUNC))
			errx(EX_IOERR, "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) {
				hellofd[1] = *(int *)CMSG_DATA(cmsg);
				printf("child: sending '%s'\\n", buf);
				if (write(hellofd[1], buf, HELLOLEN) == -1)
				    err(EX_IOERR, "failed to send 'hello'");
			}
		}
		break;
	default:
		close(presharedfd[1]);

		if (socketpair(PF_LOCAL, SOCK_STREAM, 0, hellofd) == -1)
			err(EX_OSERR, "failed to create a 'hello' socket pair");

		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) = hellofd[1];

		if (sendmsg(presharedfd[0], &msg, 0) == -1)
			err(EX_IOERR, "sendmsg");
		close(hellofd[1]);

		if (read(hellofd[0], buf, HELLOLEN) == -1)
			err(EX_IOERR, "failed to receive 'hello'");
		printf("parent: received '%s'\\n", buf);
		break;
	}

	return (0);
}
.Ed
.Sh SEE ALSO
.Xr recvmsg 2 ,
.Xr sendmsg 2 ,
.Xr socket 2 ,
.Xr ip 4 ,
.Xr ip6 4 ,
.Xr unix 4
.Sh STANDARDS
.Bl -item
.It
.Rs
.%A W. Stevens
.%A M. Thomas
.%T "Advanced Sockets API for IPv6"
.%R RFC 2292
.%D February 1998
.Re
.It
.Rs
.%A W. Stevens
.%A M. Thomas
.%A E. Nordmark
.%A T. Jinmei
.%T "Advanced Sockets Application Program Interface (API) for IPv6"
.%R RFC 3542
.%D May 2003
.Re
.El
.Sh HISTORY
The control message API first appeared in
.Bx 4.2 .
This manual page was originally written by
.An Jared Yanovich Aq Mt jaredy@OpenBSD.org
for
.Ox 3.8
and eventually brought to
.Fx 12.0
by
.An Mateusz Piotrowski Aq Mt 0mp@FreeBSD.org .