xref: /freebsd/share/man/man9/sysctl.9 (revision 0572ccaa4543b0abef8ef81e384c1d04de9f3da1)
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26.\" $FreeBSD$
27.\"
28.Dd September 12, 2013
29.Dt SYSCTL 9
30.Os
31.Sh NAME
32.Nm SYSCTL_DECL ,
33.Nm SYSCTL_INT ,
34.Nm SYSCTL_LONG ,
35.Nm SYSCTL_NODE ,
36.Nm SYSCTL_OPAQUE ,
37.Nm SYSCTL_PROC ,
38.Nm SYSCTL_QUAD ,
39.Nm SYSCTL_STRING ,
40.Nm SYSCTL_STRUCT ,
41.Nm SYSCTL_UINT ,
42.Nm SYSCTL_ULONG ,
43.Nm SYSCTL_UQUAD
44.Nd Static sysctl declaration functions
45.Sh SYNOPSIS
46.In sys/types.h
47.In sys/sysctl.h
48.Fn SYSCTL_DECL name
49.Fn SYSCTL_INT parent nbr name access ptr val descr
50.Fn SYSCTL_LONG parent nbr name access ptr val descr
51.Fn SYSCTL_NODE parent nbr name access handler descr
52.Fn SYSCTL_OPAQUE parent nbr name access ptr len fmt descr
53.Fn SYSCTL_PROC parent nbr name access ptr arg handler fmt descr
54.Fn SYSCTL_QUAD parent nbr name access ptr val descr
55.Fn SYSCTL_STRING parent nbr name access arg len descr
56.Fn SYSCTL_STRUCT parent nbr name access ptr type descr
57.Fn SYSCTL_UINT parent nbr name access ptr val descr
58.Fn SYSCTL_ULONG parent nbr name access ptr val descr
59.Fn SYSCTL_UQUAD parent nbr name access ptr val descr
60.Sh DESCRIPTION
61The
62.Nm SYSCTL
63kernel interfaces allow code to statically declare
64.Xr sysctl 8
65MIB entries, which will be initialized when the kernel module containing the
66declaration is initialized.
67When the module is unloaded, the sysctl will be automatically destroyed.
68.Pp
69Sysctl nodes are created in a hierarchical tree, with all static nodes being
70represented by named C data structures; in order to create a new node under
71an existing node in the tree, the structure representing the desired parent
72node must be declared in the current context using
73.Fn SYSCTL_DECL .
74.Pp
75New nodes are declared using one of
76.Fn SYSCTL_INT ,
77.Fn SYSCTL_LONG ,
78.Fn SYSCTL_NODE ,
79.Fn SYSCTL_OPAQUE ,
80.Fn SYSCTL_PROC ,
81.Fn SYSCTL_QUAD ,
82.Fn SYSCTL_STRING ,
83.Fn SYSCTL_STRUCT ,
84.Fn SYSCTL_UINT ,
85.Fn SYSCTL_ULONG ,
86and
87.Fn SYSCTL_UQUAD .
88Each macro accepts a parent name, as declared using
89.Fn SYSCTL_DECL ,
90an OID number, typically
91.Dv OID_AUTO ,
92a node name, a set of control and access flags, and a description.
93Depending on the macro, a pointer to a variable supporting the MIB entry, a
94size, a value, and a function pointer implementing the MIB entry may also be
95present.
96.Pp
97For most of the above macros, declaring a type as part of the access flags is
98not necessary \[em] however, when declaring a sysctl implemented by a function,
99including a type in the access mask is required:
100.Bl -tag -width ".Dv CTLTYPE_STRING"
101.It Dv CTLTYPE_NODE
102This is a node intended to be a parent for other nodes.
103.It Dv CTLTYPE_INT
104This is a signed integer.
105.It Dv CTLTYPE_STRING
106This is a nul-terminated string stored in a character array.
107.It Dv CTLTYPE_S64
108This is a 64-bit signed integer.
109.It Dv CTLTYPE_OPAQUE
110This is an opaque data structure.
111.It Dv CTLTYPE_STRUCT
112Alias for
113.Dv CTLTYPE_OPAQUE .
114.It Dv CTLTYPE_UINT
115This is an unsigned integer.
116.It Dv CTLTYPE_LONG
117This is a signed long.
118.It Dv CTLTYPE_ULONG
119This is an unsigned long.
120.It Dv CTLTYPE_U64
121This is a 64-bit unsigned integer.
122.El
123.Pp
124All sysctl types except for new node declarations require one of the following
125flags to be set indicating the read and write disposition of the sysctl:
126.Bl -tag -width ".Dv CTLFLAG_ANYBODY"
127.It Dv CTLFLAG_RD
128This is a read-only sysctl.
129.It Dv CTLFLAG_RDTUN
130This is a read-only sysctl which can be set by a system tunable.
131.It Dv CTLFLAG_WR
132This is a writable sysctl.
133.It Dv CTLFLAG_RW
134This sysctl is readable and writable.
135.It Dv CTLFLAG_RWTUN
136This sysctl is readable and writable and can also be set by a system tunable.
137.El
138.Pp
139Additionally, any of the following optional flags may also be specified:
140.Bl -tag -width ".Dv CTLFLAG_ANYBODY"
141.It Dv CTLFLAG_ANYBODY
142Any user or process can write to this sysctl.
143.It Dv CTLFLAG_SECURE
144This sysctl can be written to only if the effective securelevel of the
145process is \[<=] 0.
146.It Dv CTLFLAG_PRISON
147This sysctl can be written to by processes in
148.Xr jail 2 .
149.It Dv CTLFLAG_SKIP
150When iterating the sysctl name space, do not list this sysctl.
151.It Dv CTLFLAG_TUN
152Advisory flag that a system tunable also exists for this variable.
153.El
154.Pp
155When creating new sysctls, careful attention should be paid to the security
156implications of the monitoring or management interface being created.
157Most sysctls present in the kernel are read-only or writable only by the
158superuser.
159Sysctls exporting extensive information on system data structures and
160operation, especially those implemented using procedures, will wish to
161implement access control to limit the undesired exposure of information about
162other processes, network connections, etc.
163.Pp
164The following top level sysctl name spaces are commonly used:
165.Bl -tag -width ".Va regression"
166.It Va compat
167Compatibility layer information.
168.It Va debug
169Debugging information.
170Various name spaces exist under
171.Va debug .
172.It Va hw
173Hardware and device driver information.
174.It Va kern
175Kernel behavior tuning; generally deprecated in favor of more specific
176name spaces.
177.It Va machdep
178Machine-dependent configuration parameters.
179.It Va net
180Network subsystem.
181Various protocols have name spaces under
182.Va net .
183.It Va regression
184Regression test configuration and information.
185.It Va security
186Security and security-policy configuration and information.
187.It Va sysctl
188Reserved name space for the implementation of sysctl.
189.It Va user
190Configuration settings relating to user application behavior.
191Generally, configuring applications using kernel sysctls is discouraged.
192.It Va vfs
193Virtual file system configuration and information.
194.It Va vm
195Virtual memory subsystem configuration and information.
196.El
197.Sh EXAMPLES
198Sample use of
199.Fn SYSCTL_DECL
200to declare the
201.Va security
202sysctl tree for use by new nodes:
203.Bd -literal -offset indent
204SYSCTL_DECL(_security);
205.Ed
206.Pp
207Examples of integer, opaque, string, and procedure sysctls follow:
208.Bd -literal -offset indent
209/*
210 * Example of a constant integer value.  Notice that the control
211 * flags are CTLFLAG_RD, the variable pointer is NULL, and the
212 * value is declared.
213 */
214SYSCTL_INT(_debug_sizeof, OID_AUTO, bio, CTLFLAG_RD, NULL,
215    sizeof(struct bio), "sizeof(struct bio)");
216
217/*
218 * Example of a variable integer value.  Notice that the control
219 * flags are CTLFLAG_RW, the variable pointer is set, and the
220 * value is 0.
221 */
222static int	doingcache = 1;		/* 1 => enable the cache */
223SYSCTL_INT(_debug, OID_AUTO, vfscache, CTLFLAG_RW, &doingcache, 0,
224    "Enable name cache");
225
226/*
227 * Example of a variable string value.  Notice that the control
228 * flags are CTLFLAG_RW, that the variable pointer and string
229 * size are set.  Unlike newer sysctls, this older sysctl uses a
230 * static oid number.
231 */
232char kernelname[MAXPATHLEN] = "/kernel";	/* XXX bloat */
233SYSCTL_STRING(_kern, KERN_BOOTFILE, bootfile, CTLFLAG_RW,
234    kernelname, sizeof(kernelname), "Name of kernel file booted");
235
236/*
237 * Example of an opaque data type exported by sysctl.  Notice that
238 * the variable pointer and size are provided, as well as a format
239 * string for sysctl(8).
240 */
241static l_fp pps_freq;	/* scaled frequence offset (ns/s) */
242SYSCTL_OPAQUE(_kern_ntp_pll, OID_AUTO, pps_freq, CTLFLAG_RD,
243    &pps_freq, sizeof(pps_freq), "I", "");
244
245/*
246 * Example of a procedure based sysctl exporting string
247 * information.  Notice that the data type is declared, the NULL
248 * variable pointer and 0 size, the function pointer, and the
249 * format string for sysctl(8).
250 */
251SYSCTL_PROC(_kern_timecounter, OID_AUTO, hardware, CTLTYPE_STRING |
252    CTLFLAG_RW, NULL, 0, sysctl_kern_timecounter_hardware, "A",
253    "");
254.Ed
255.Sh SYSCTL NAMING
256When adding, modifying, or removing sysctl names, it is important to be
257aware that these interfaces may be used by users, libraries, applications,
258or documentation (such as published books), and are implicitly published application interfaces.
259As with other application interfaces, caution must be taken not to break
260existing applications, and to think about future use of new name spaces so as
261to avoid the need to rename or remove interfaces that might be depended on in
262the future.
263.Pp
264The semantics chosen for a new sysctl should be as clear as possible,
265and the name of the sysctl must closely reflect its semantics.
266Therefore the sysctl name deserves a fair amount of consideration.
267It should be short but yet representative of the sysctl meaning.
268If the name consists of several words, they should be separated by
269underscore characters, as in
270.Va compute_summary_at_mount .
271Underscore characters may be omitted only if the name consists of not more
272than two words, each being not longer than four characters, as in
273.Va bootfile .
274For boolean sysctls, negative logic should be totally avoided.
275That is, do not use names like
276.Va no_foobar
277or
278.Va foobar_disable .
279They are confusing and lead to configuration errors.
280Use positive logic instead:
281.Va foobar ,
282.Va foobar_enable .
283.Pp
284A temporary sysctl node that should not be relied upon must be designated
285as such by a leading underscore character in its name.  For example:
286.Va _dirty_hack .
287.Sh SEE ALSO
288.Xr sysctl 3 ,
289.Xr sysctl 8 ,
290.Xr sysctl_add_oid 9 ,
291.Xr sysctl_ctx_free 9 ,
292.Xr sysctl_ctx_init 9 ,
293.Xr sysctl_remove_oid 9
294.Sh HISTORY
295The
296.Xr sysctl 8
297utility first appeared in
298.Bx 4.4 .
299.Sh AUTHORS
300.An -nosplit
301The
302.Nm sysctl
303implementation originally found in
304.Bx
305has been extensively rewritten by
306.An Poul-Henning Kamp
307in order to add support for name lookups, name space iteration, and dynamic
308addition of MIB nodes.
309.Pp
310This man page was written by
311.An Robert N. M. Watson .
312