d7ef92ffae
Escape sequences like "\n" have to be escaped twice in examples in our
mdoc(7)-based manual pages in order to be displayed properly. The problem
is that otherwise they are interpreted by mdoc(7), which results in:
printf("parent: received '%s'0, buf);
being shown to the user instead of:
printf("parent: received '%s'\n", buf);
Approved by: bcr (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24056
215 lines
5.3 KiB
Groff
215 lines
5.3 KiB
Groff
.\" Written by Jared Yanovich <jaredy@openbsd.org>
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.\" Public domain, July 3, 2005
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.\"
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.\" $FreeBSD$
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.Dd March 13, 2020
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.Dt CMSG_DATA 3
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.Os
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.Sh NAME
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.Nm CMSG_DATA ,
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.Nm CMSG_FIRSTHDR ,
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.Nm CMSG_LEN ,
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.Nm CMSG_NXTHDR ,
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.Nm CMSG_SPACE
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.Nd socket control message routines for ancillary data access
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.Sh SYNOPSIS
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.In sys/socket.h
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.Ft unsigned char *
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.Fn CMSG_DATA "struct cmsghdr *"
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.Ft struct cmsghdr *
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.Fn CMSG_FIRSTHDR "struct msghdr *"
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.Ft size_t
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.Fn CMSG_LEN "size_t"
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.Ft struct cmsghdr *
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.Fn CMSG_NXTHDR "struct msghdr *" "struct cmsghdr *"
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.Ft size_t
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.Fn CMSG_SPACE "size_t"
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.Sh DESCRIPTION
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The control message API is used to construct ancillary data objects for
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use in control messages sent and received across sockets.
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.Pp
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Control messages are passed around by the
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.Xr recvmsg 2
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and
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.Xr sendmsg 2
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system calls.
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The
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.Vt cmsghdr
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structure, described in
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.Xr recvmsg 2 ,
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is used to specify a chain of control messages.
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.Pp
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These routines should be used instead of directly accessing the control
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message header members and data buffers as they ensure that necessary
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alignment constraints are met.
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.Pp
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The following routines are provided:
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.Bl -tag -width Ds
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.It Fn CMSG_DATA cmsg
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This routine accesses the data portion of the control message header
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.Fa cmsg .
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It ensures proper alignment constraints on the beginning of ancillary
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data are met.
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.It Fn CMSG_FIRSTHDR msghdr
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This routine accesses the first control message attached to the
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message
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.Fa msghdr .
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If no control messages are attached to the message, this routine
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returns
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.Dv NULL .
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.It Fn CMSG_LEN len
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This routine determines the size in bytes of a control message,
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which includes the control message header.
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.Fa len
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specifies the length of the data held by the control message.
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This value is what is normally stored in the
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.Fa cmsg_len
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of each control message.
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This routine accounts for any alignment constraints on the beginning of
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ancillary data.
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.It Fn CMSG_NXTHDR msghdr cmsg
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This routine returns the location of the control message following
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.Fa cmsg
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in the message
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.Fa msghdr .
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If
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.Fa cmsg
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is the last control message in the chain, this routine returns
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.Dv NULL .
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.It Fn CMSG_SPACE len
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This routine determines the size in bytes needed to hold a control
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message and its contents of length
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.Fa len ,
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which includes the control message header.
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This value is what is normally stored in
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.Fa msg_msgcontrollen .
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This routine accounts for any alignment constraints on the beginning of
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ancillary data as well as any needed to pad the next control message.
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.El
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.Sh EXAMPLES
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The following example constructs a control message containing a file descriptor
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in the parent process and passes it over a pre-shared socket over the child
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process.
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Then the child process sends a "hello" string to the parent process using the
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received file descriptor.
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.Bd -literal
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#include <sys/socket.h>
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#include <err.h>
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#include <stdio.h>
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#include <string.h>
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#include <sysexits.h>
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#include <unistd.h>
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#define HELLOLEN sizeof("hello")
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int
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main()
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{
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struct msghdr msg;
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union {
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struct cmsghdr hdr;
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unsigned char buf[CMSG_SPACE(sizeof(int))];
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} cmsgbuf;
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char buf[HELLOLEN];
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int hellofd[2];
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int presharedfd[2];
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struct cmsghdr *cmsg;
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if (socketpair(PF_LOCAL, SOCK_STREAM, 0, presharedfd) == -1)
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err(EX_OSERR, "failed to create a pre-shared socket pair");
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memset(&msg, 0, sizeof(msg));
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msg.msg_control = &cmsgbuf.buf;
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msg.msg_controllen = sizeof(cmsgbuf.buf);
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msg.msg_iov = NULL;
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msg.msg_iovlen = 0;
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switch (fork()) {
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case -1:
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err(EX_OSERR, "fork");
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case 0:
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close(presharedfd[0]);
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strlcpy(buf, "hello", HELLOLEN);
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if (recvmsg(presharedfd[1], &msg, 0) == -1)
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err(EX_IOERR, "failed to receive a message");
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if (msg.msg_flags & (MSG_CTRUNC | MSG_TRUNC))
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errx(EX_IOERR, "control message truncated");
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for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL;
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cmsg = CMSG_NXTHDR(&msg, cmsg)) {
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if (cmsg->cmsg_len == CMSG_LEN(sizeof(int)) &&
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cmsg->cmsg_level == SOL_SOCKET &&
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cmsg->cmsg_type == SCM_RIGHTS) {
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hellofd[1] = *(int *)CMSG_DATA(cmsg);
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printf("child: sending '%s'\\n", buf);
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if (write(hellofd[1], buf, HELLOLEN) == -1)
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err(EX_IOERR, "failed to send 'hello'");
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}
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}
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break;
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default:
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close(presharedfd[1]);
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if (socketpair(PF_LOCAL, SOCK_STREAM, 0, hellofd) == -1)
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err(EX_OSERR, "failed to create a 'hello' socket pair");
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cmsg = CMSG_FIRSTHDR(&msg);
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cmsg->cmsg_len = CMSG_LEN(sizeof(int));
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cmsg->cmsg_level = SOL_SOCKET;
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cmsg->cmsg_type = SCM_RIGHTS;
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*(int *)CMSG_DATA(cmsg) = hellofd[1];
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if (sendmsg(presharedfd[0], &msg, 0) == -1)
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err(EX_IOERR, "sendmsg");
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close(hellofd[1]);
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if (read(hellofd[0], buf, HELLOLEN) == -1)
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err(EX_IOERR, "faild to receive 'hello'");
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printf("parent: received '%s'\\n", buf);
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break;
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}
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return (0);
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}
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.Ed
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.Sh SEE ALSO
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.Xr recvmsg 2 ,
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.Xr sendmsg 2 ,
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.Xr socket 2 ,
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.Xr ip 4 ,
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.Xr ip6 4 ,
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.Xr unix 4
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.Sh STANDARDS
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.Bl -item
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.It
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.Rs
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.%A W. Stevens
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.%A M. Thomas
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.%T "Advanced Sockets API for IPv6"
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.%R RFC 2292
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.%D February 1998
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.Re
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.It
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.Rs
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.%A W. Stevens
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.%A M. Thomas
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.%A E. Nordmark
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.%A T. Jinmei
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.%T "Advanced Sockets Application Program Interface (API) for IPv6"
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.%R RFC 3542
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.%D May 2003
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.Re
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.El
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.Sh HISTORY
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The control message API first appeared in
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.Bx 4.2 .
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This manual page was originally written by
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.An Jared Yanovich Aq Mt jaredy@OpenBSD.org
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for
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.Ox 3.8
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and eventually brought to
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.Fx 12.0
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by
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.An Mateusz Piotrowski Aq Mt 0mp@FreeBSD.org .
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