Add a man page for filemon(4) [r236592].

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+.\" Copyright (c) 2012
+.\"	David E. O'Brien <obrien@FreeBSD.org>.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by David E. O'Brien and
+.\"	contributors.
+.\" 4. Neither the name of the author nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd May 30, 2012
+.Dt FILEMON 4
+.Os
+.Sh NAME
+.Nm filemon
+.Nd the filemon device
+.Sh SYNOPSIS
+.In dev/filemon/filemon.h
+.Sh DESCRIPTION
+The
+.Nm
+device allows a process to collect file operations data of its children.
+The device
+.Pa /dev/filemon
+responds to two
+.Xr ioctl 2
+calls.
+.Pp
+System calls are denoted using the following single letters:
+.Bl -tag -width indent -compact
+.It Dq Li C
+.Xr chdir 2
+.It Dq Li D
+.Xr unlink 2
+.It Dq Li E
+.Xr exec 2
+.It Dq Li F
+.Xr fork 2 ,
+.Xr vfork 2
+.It Dq Li L
+.Xr link 2 ,
+.Xr linkat 2 ,
+.Xr symlink 2 ,
+.Xr symlinkat 2
+.It Dq Li M
+.Xr rename 2
+.It Dq Li R
+.Xr open 2
+for read
+.It Dq Li S
+.Xr stat 2
+.It Dq Li W
+.Xr open 2
+for write
+.It Dq Li X
+.Xr _exit 2
+.El
+.Pp
+Note that
+.Dq R
+following
+.Dq W
+records can represent a single
+.Xr open 2
+for R/W,
+or two seperate
+.Xr open 2
+calls, one for
+R
+and one for
+W.
+.Sh IOCTLS
+User mode programs communicate with the filemon driver through a
+number of ioctls which are described below.
+Each takes a single argument.
+.Bl -tag -width FILEMON_SET_PID
+.It Dv FILEMON_SET_FD
+Write the internal tracing buffer to the supplied open file descriptor.
+.It Dv FILEMON_SET_PID .
+Child process ID to trace.
+.El
+.Pp
+.Sh RETURN VALUES
+The ioctl returns zero on success and non-zero on failure.
+.Sh EXAMPLES
+.Bd -literal -offset indent
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <sys/ioctl.h>
+#include <dev/filemon/filemon.h>
+#include <fcntl.h>
+#include <err.h>
+
+static void
+open_filemon(void)
+{
+	pid_t child;
+	int fm_fd, fm_log;
+
+	if ((fm_fd = open("/dev/filemon", O_RDWR)) == -1)
+		err(1, "open(\"/dev/filemon\", O_RDWR)");
+	if ((fm_log = open("filemon.out",
+	    O_CREAT | O_WRONLY | O_TRUNC, DEFFILEMODE)) == -1)
+		err(1, "open(filemon.out)");
+
+	if (ioctl(fm_fd, FILEMON_SET_FD, &fm_log) < 0)
+		err(1, "Cannot set filemon log file descriptor");
+	/* Set up these two fd's to close on exec. */
+	(void)fcntl(fm_fd, F_SETFD, FD_CLOEXEC);
+	(void)fcntl(fm_log, F_SETFD, FD_CLOEXEC);
+
+	if ((child = fork()) == 0) {
+		/* Do something here. */
+		return 0;
+	} else {
+		if (ioctl(fm_fd, FILEMON_SET_PID, &child) < 0)
+			err(1, "Cannot set filemon PID");
+		wait(&child);
+		close(fm_fd);
+	}
+	return 0;
+}
+.Ed
+.Pp
+Creates a file named
+.Pa filemon.out
+and configures the
+.Nm
+device to write the filemon buffer contents to it.
+.Sh FILES
+.Bl -tag -width /dev/zero
+.It Pa /dev/filemon
+.El
+.Sh SEE ALSO
+.Xr dtrace 1 ,
+.Xr ktrace 1 ,
+.Xr truss 1
+.Sh HISTORY
+A
+.Nm
+device appeared in
+.Fx 9.1 .