Add a basic man page for the sysctl(9) macro interfaces. Previously man

pages existed only for the dynamic sysctl interfaces.  There's probably
more complete and accurate content, better advice, etc, that could be added
here.

Per scottl's suggest, add a small piece of moralizing text regarding the
fact that sysctl names quickly get embedded in system configuration files,
libraries, third party applications, and even books, so renaming and
removing names after they've been published is a tricky issue.

MFC after:	1 month

1 files changed, 343 insertions, 0 deletions

diff --git a/share/man/man9/sysctl.9 b/share/man/man9/sysctl.9
new file mode 100644
index 0000000..3db0618
--- /dev/null
+++ b/share/man/man9/sysctl.9
@@ -0,0 +1,343 @@
+.\"
+.\" Copyright (c) 2006 Robert N. M. Watson
+.\" 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.
+.\"
+.\" 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 April 28, 2006
+.Dt SYSCTL 9
+.Os
+.Sh NAME
+.Nm SYSCTL_DECL ,
+.Nm SYSCTL_INT ,
+.Nm SYSCTL_LONG ,
+.Nm SYSCTL_NODE ,
+.Nm SYSCTL_OPAQUE ,
+.Nm SYSCTL_PROC ,
+.Nm SYSCTL_STRING ,
+.Nm SYSCTL_STRUCT ,
+.Nm SYSCTL_UINT ,
+.Nm SYSCTL_ULONG
+.Nd Static sysctl declaration functions
+.Sh SYNOPSIS
+.In sys/types.h
+.In sys/sysctl.h
+.Fo SYSCTL_DECL
+.Fa "name"
+.Fc
+.Fo SYSCTL_INT
+.Fa "parent"
+.Fa "nbr"
+.Fa "name"
+.Fa "access"
+.Fa "ptr"
+.Fa "val"
+.Fa "descr"
+.Fc
+.Fo SYSCTL_LONG
+.Fa "parent"
+.Fa "nbr"
+.Fa "name"
+.Fa "access"
+.Fa "ptr"
+.Fa "val"
+.Fa "descr"
+.Fc
+.Fo SYSCTL_NODE
+.Fa "parent"
+.Fa "nbr"
+.Fa "name"
+.Fa "access"
+.Fa "handler"
+.Fa "descr"
+.Fc
+.Fo SYSCTL_OPAQUE
+.Fa "parent"
+.Fa "nbr"
+.Fa "name"
+.Fa "access"
+.Fa "ptr"
+.Fa "len"
+.Fa "fmt"
+.Fa "descr"
+.Fc
+.Fo SYSCTL_PROC
+.Fa "parent"
+.Fa "nbr"
+.Fa "name"
+.Fa "access"
+.Fa "ptr"
+.Fa "arg"
+.Fa "handler"
+.Fa "fmt"
+.Fa "descr"
+.Fc
+.Fo SYSCTL_STRING
+.Fa "parent"
+.Fa "nbr"
+.Fa "name"
+.Fa "access"
+.Fa "arg"
+.Fa "len"
+.Fa "descr"
+.Fc
+.Fo SYSCTL_STRUCT
+.Fa "parent"
+.Fa "nbr"
+.Fa "name"
+.Fa "access"
+.Fa "ptr"
+.Fa "type"
+.Fa "descr"
+.Fc
+.Fo SYSCTL_UINT
+.Fa "parent"
+.Fa "nbr"
+.Fa "name"
+.Fa "access"
+.Fa "ptr"
+.Fa "val"
+.Fa "descr"
+.Fc
+.Fo SYSCTL_ULONG
+.Fa "parent"
+.Fa "nbr"
+.Fa "name"
+.Fa "access"
+.Fa "ptr"
+.Fa "val"
+.Fa "descr"
+.Fc
+.Sh DESCRIPTION
+The
+.Nm
+kernel interfaces allow code to statically declare
+.Xr sysctl 8
+MIB entries, which will be initialized when the kernel module containing the
+declaration is initialized.
+When the module is unloaded, the sysctl will be automatically destroyed.
+.Pp
+Sysctl nodes are created in a hierarchal tree, with all static nodes being
+represented by named C data structures; in order to create a new node under
+an existing node in the tree, the structure representing the desired parent
+node must be declared in the current context using
+.Fn SYSCTL_DECL .
+.Pp
+New nodes are declared using one of
+.Nm SYSCTL_INT ,
+.Nm SYSCTL_LONG ,
+.Nm SYSCTL_NODE ,
+.Nm SYSCTL_OPAQUE ,
+.Nm SYSCTL_PROC ,
+.Nm SYSCTL_STRING ,
+.Nm SYSCTL_STRUCT ,
+.Nm SYSCTL_UINT ,
+and
+.Nm SYSCTL_ULONG .
+Each macro accepts a parent name, as declared using
+.Nm SYSCTL_DECL ,
+an OID number, typically
+.Dv OID_AUTO ,
+a node name, a set of control and access flags, and a description.
+Depending on the macro, a pointer to a variable supporting the MIB entry, a
+size, a value, and a function pointer implementing the MIB entry may also be
+present.
+.Pp
+For most of the above macros, declaring a type as part of the access flags is
+not necessary -- however, when declaring a sysctl implemented by a function,
+including a type in the access mask is required:
+.Bl -tag -width CTLTYPE_STRING
+.It Dv CTLTYPE_NODE
+This is a node intended to be a parent for other nodes.
+.It Dv CTLTYPE_INT
+This is a signed integer.
+.It Dv CTLTYPE_STRING
+This is a nul-terminated string stored in a character array.
+.It Dv CTLTYPE_QUAD
+This is a 64-bit signed integer.
+.It Dv CTLTYPE_OPAQUE
+This is an opaque data structure.
+.It Dv CTLTYPE_STRUCT
+Alias for
+.Dv CTLTYPE_OPAQUE.
+.It Dv CTLTYPE_UINT
+This is an unsigned integer.
+.It Dv CTLTYPE_LONG
+This is a signed long.
+.It Dv CTLTYPE_ULONG
+This is an insigned long.
+.El
+.Pp
+All sysctl types except for new node declarations require one or more flags
+to be set indicating the read and write disposition of the sysctl:
+.Bl -tag -width CTLFLAG_ANYBODY
+.It Dv CTLFLAG_RD
+This is a read-only sysctl.
+It Dv CTLFLAG_WR
+This is a writable sysctl.
+.It Dv CTLFLAG_RW
+This sysctl is readable and writable.
+.It Dv CTLFLAG_ANYBODY
+Any user or process can write to this sysctl.
+.It Dv CTLFLAG_SECURE
+This sysctl can be written to only if the effective securelevel of the
+process is <= 0.
+.It Dv CTLFLAG_PRISON
+This sysctl can be written to by processes in
+.Xr jail 2 .
+.It Dv CTLFLAG_SKIP
+When iterating the sysctl name space, do not list this sysctl.
+.It Dv CTLFLAG_TUN
+Also declare a system tunable with the same name to initialize this variable.
+.It Dv CTLFLAG_RDTUN
+Also declare a system tunable with the same name to initalize this variable;
+however, the run-time variable is read-only.
+.El
+.Pp
+When creating new sysctls, careful attention should be paid to the security
+implications of the monitoring or management interface being created.
+Most sysctls present in the kernel are read-only or writable only by the
+superuser.
+Sysctls exporting extensive information on system data structures and
+operation, especially those implemented using procedures, will wish to
+implement access control to limit the undesired exposure of information about
+other processes, network connections, etc.
+.Pp
+The following top level sysctl name spaces are commonly used:
+.Bl -tag -width regression
+.It Dv compat
+Compatibility layer information.
+.It Dv debug
+Debugging information.
+Various name spaces exist under
+.Dv debug .
+.It Dv hw
+Hardware and device driver information.
+.It Dv kern
+Kernel behavior tuning; generally deprecated in favor of more specific
+name spaces.
+.It Dv machdep
+Machine-dependent configuration parameters.
+.It Dv net
+Network subsystem.
+Various protocols have name spaces under
+.Dv net .
+.It Dv regression
+Regression test configuration and information.
+.It Dv security
+Security and security policy configuration and information.
+.It Dv sysctl
+Reserved name space for the implementation of sysctl.
+.It Dv user
+Configuration settings relating to user application behavior.
+Generally, configuring applications using kernel sysctls is disouraged.
+.It Dv vfs
+Virtual file system configuration and information.
+.It Dv vm
+Virtual memory subsystem configuration and information.
+.El
+.Sh EXAMPLES
+Sample use of
+.Nm SYSCTL_DECL
+to declare the "security" sysctl tree for use by new nodes:
+.Bd -literal -offset indent
+SYSCTL_DECL(_security);
+.Ed
+.Pp
+Examples of integer, opaque, string, and procedure sysctls follow:
+.Bd -literal -offset indent
+/*
+ * Example of a constant integer value.  Notice that the control
+ * flags are CTLFLAG_RD, the variable pointer is NULL, and the
+ * value is declared.
+ */
+SYSCTL_INT(_debug_sizeof, OID_AUTO, bio, CTLFLAG_RD, NULL,
+    sizeof(struct bio), "sizeof(struct bio)");
+
+/*
+ * Example of a variable integer value.  Notice that the control
+ * flags are CTLFLAG_RW, the variable pointer is set, and the
+ * value is 0.
+ */
+static int	doingcache = 1;		/* 1 => enable the cache */
+SYSCTL_INT(_debug, OID_AUTO, vfscache, CTLFLAG_RW, &doingcache, 0,
+    "Enable name cache");
+
+/*
+ * Example of a variable string value.  Notice that the control
+ * flags are CTLFLAG_RW, that the variable pointer and string
+ * size are set.  Unlike newer sysctls, this older sysctl uses a
+ * static oid number.
+ */
+char kernelname[MAXPATHLEN] = "/kernel";	/* XXX bloat */
+SYSCTL_STRING(_kern, KERN_BOOTFILE, bootfile, CTLFLAG_RW,
+    kernelname, sizeof(kernelname), "Name of kernel file booted");
+
+/*
+ * Example of an opaque data type exported by sysctl.  Notice that
+ * the variable pointer and size are provided, as well as a format
+ * string for sysctl(8).
+ */
+static l_fp pps_freq;	/* scaled frequence offset (ns/s) */
+SYSCTL_OPAQUE(_kern_ntp_pll, OID_AUTO, pps_freq, CTLFLAG_RD,
+    &pps_freq, sizeof(pps_freq), "I", "");
+
+/*
+ * Example of a procedure based sysctl exporting string
+ * information.  Notice that the data type is declared, the NULL
+ * variable pointer and 0 size, the function pointer, and the
+ * format string for sysctl(8).
+ */
+SYSCTL_PROC(_kern_timecounter, OID_AUTO, hardware, CTLTYPE_STRING |
+    CTLFLAG_RW, NULL, 0, sysctl_kern_timecounter_hardware, "A",
+    "");
+.Ed
+.Pp
+When adding, modifying, or removing sysctl names, it is important to be
+aware that these interfaces may be used by users, libraries, applications,
+and even books, and are implicitly published application interfaces.
+As with other application interfaces, caution must be taken not to break
+existing applications, and to think about future use of new name spaces so as
+to avoid the need to rename or remove interfaces that might be depended on in
+the future.
+.Sh SEE ALSO
+.Xr sysctl 8 ,
+.Xr sysctl_add_oid 9 ,
+.Xr sysctl_ctx_free 9 ,
+.Xr sysctl_ctx_init 9 ,
+.Xr sysctl_remove_oid 9
+.Sh HISTORY
+.Xr sysctl 8
+first appeared in
+.Bx 4.4 .
+.Sh AUTHORS
+The sysctl implementation originally found in
+.Bx
+has been extensively rewritten by
+.An Poul-Henning Kamp
+in order to add support for name lookups, name space iteration, and dynamic
+addition of MIB nodes.
+.Pp
+This man page was written by
+.An Robert N. M. Watson .