xref: /freebsd/share/man/man9/sysctl.9 (revision 1e413cf93298b5b97441a21d9a50fdcd0ee9945e)
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28.Dd November 23, 2006
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_STRING ,
39.Nm SYSCTL_STRUCT ,
40.Nm SYSCTL_UINT ,
41.Nm SYSCTL_ULONG ,
42.Nm SYSCTL_XINT ,
43.Nm SYSCTL_XLONG
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_STRING parent nbr name access arg len descr
55.Fn SYSCTL_STRUCT parent nbr name access ptr type descr
56.Fn SYSCTL_UINT parent nbr name access ptr val descr
57.Fn SYSCTL_ULONG parent nbr name access ptr val descr
58.Fn SYSCTL_XINT parent nbr name access ptr val descr
59.Fn SYSCTL_XLONG 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_STRING ,
82.Fn SYSCTL_STRUCT ,
83.Fn SYSCTL_UINT ,
84.Fn SYSCTL_ULONG ,
85.Fn SYSCTL_XINT ,
86and
87.Fn SYSCTL_XLONG .
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_QUAD
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.El
121.Pp
122All sysctl types except for new node declarations require one or more flags
123to be set indicating the read and write disposition of the sysctl:
124.Bl -tag -width ".Dv CTLFLAG_ANYBODY"
125.It Dv CTLFLAG_RD
126This is a read-only sysctl.
127.It Dv CTLFLAG_WR
128This is a writable sysctl.
129.It Dv CTLFLAG_RW
130This sysctl is readable and writable.
131.It Dv CTLFLAG_ANYBODY
132Any user or process can write to this sysctl.
133.It Dv CTLFLAG_SECURE
134This sysctl can be written to only if the effective securelevel of the
135process is \[<=] 0.
136.It Dv CTLFLAG_PRISON
137This sysctl can be written to by processes in
138.Xr jail 2 .
139.It Dv CTLFLAG_SKIP
140When iterating the sysctl name space, do not list this sysctl.
141.It Dv CTLFLAG_TUN
142Also declare a system tunable with the same name to initialize this variable.
143.It Dv CTLFLAG_RDTUN
144Also declare a system tunable with the same name to initialize this variable;
145however, the run-time variable is read-only.
146.El
147.Pp
148When creating new sysctls, careful attention should be paid to the security
149implications of the monitoring or management interface being created.
150Most sysctls present in the kernel are read-only or writable only by the
151superuser.
152Sysctls exporting extensive information on system data structures and
153operation, especially those implemented using procedures, will wish to
154implement access control to limit the undesired exposure of information about
155other processes, network connections, etc.
156.Pp
157The following top level sysctl name spaces are commonly used:
158.Bl -tag -width ".Va regression"
159.It Va compat
160Compatibility layer information.
161.It Va debug
162Debugging information.
163Various name spaces exist under
164.Va debug .
165.It Va hw
166Hardware and device driver information.
167.It Va kern
168Kernel behavior tuning; generally deprecated in favor of more specific
169name spaces.
170.It Va machdep
171Machine-dependent configuration parameters.
172.It Va net
173Network subsystem.
174Various protocols have name spaces under
175.Va net .
176.It Va regression
177Regression test configuration and information.
178.It Va security
179Security and security-policy configuration and information.
180.It Va sysctl
181Reserved name space for the implementation of sysctl.
182.It Va user
183Configuration settings relating to user application behavior.
184Generally, configuring applications using kernel sysctls is discouraged.
185.It Va vfs
186Virtual file system configuration and information.
187.It Va vm
188Virtual memory subsystem configuration and information.
189.El
190.Sh EXAMPLES
191Sample use of
192.Fn SYSCTL_DECL
193to declare the
194.Va security
195sysctl tree for use by new nodes:
196.Bd -literal -offset indent
197SYSCTL_DECL(_security);
198.Ed
199.Pp
200Examples of integer, opaque, string, and procedure sysctls follow:
201.Bd -literal -offset indent
202/*
203 * Example of a constant integer value.  Notice that the control
204 * flags are CTLFLAG_RD, the variable pointer is NULL, and the
205 * value is declared.
206 * If sysctl(8) should print this value in hex, use 'SYSCTL_XINT'.
207 */
208SYSCTL_INT(_debug_sizeof, OID_AUTO, bio, CTLFLAG_RD, NULL,
209    sizeof(struct bio), "sizeof(struct bio)");
210
211/*
212 * Example of a variable integer value.  Notice that the control
213 * flags are CTLFLAG_RW, the variable pointer is set, and the
214 * value is 0.
215 */
216static int	doingcache = 1;		/* 1 => enable the cache */
217SYSCTL_INT(_debug, OID_AUTO, vfscache, CTLFLAG_RW, &doingcache, 0,
218    "Enable name cache");
219
220/*
221 * Example of a variable string value.  Notice that the control
222 * flags are CTLFLAG_RW, that the variable pointer and string
223 * size are set.  Unlike newer sysctls, this older sysctl uses a
224 * static oid number.
225 */
226char kernelname[MAXPATHLEN] = "/kernel";	/* XXX bloat */
227SYSCTL_STRING(_kern, KERN_BOOTFILE, bootfile, CTLFLAG_RW,
228    kernelname, sizeof(kernelname), "Name of kernel file booted");
229
230/*
231 * Example of an opaque data type exported by sysctl.  Notice that
232 * the variable pointer and size are provided, as well as a format
233 * string for sysctl(8).
234 */
235static l_fp pps_freq;	/* scaled frequence offset (ns/s) */
236SYSCTL_OPAQUE(_kern_ntp_pll, OID_AUTO, pps_freq, CTLFLAG_RD,
237    &pps_freq, sizeof(pps_freq), "I", "");
238
239/*
240 * Example of a procedure based sysctl exporting string
241 * information.  Notice that the data type is declared, the NULL
242 * variable pointer and 0 size, the function pointer, and the
243 * format string for sysctl(8).
244 */
245SYSCTL_PROC(_kern_timecounter, OID_AUTO, hardware, CTLTYPE_STRING |
246    CTLFLAG_RW, NULL, 0, sysctl_kern_timecounter_hardware, "A",
247    "");
248.Ed
249.Sh SYSCTL NAMING
250When adding, modifying, or removing sysctl names, it is important to be
251aware that these interfaces may be used by users, libraries, applications,
252or documentation (such as published books), and are implicitly published application interfaces.
253As with other application interfaces, caution must be taken not to break
254existing applications, and to think about future use of new name spaces so as
255to avoid the need to rename or remove interfaces that might be depended on in
256the future.
257.Pp
258The semantics chosen for a new sysctl should be as clear as possible,
259and the name of the sysctl must closely reflect its semantics.
260Therefore the sysctl name deserves a fair amount of consideration.
261It should be short but yet representative of the sysctl meaning.
262If the name consists of several words, they should be separated by
263underscore characters, as in
264.Va compute_summary_at_mount .
265Underscore characters may be omitted only if the name consists of not more
266than two words, each being not longer than four characters, as in
267.Va bootfile .
268For boolean sysctls, negative logic should be totally avoided.
269That is, do not use names like
270.Va no_foobar
271or
272.Va foobar_disable .
273They are confusing and lead to configuration errors.
274Use positive logic instead:
275.Va foobar ,
276.Va foobar_enable .
277.Pp
278A temporary sysctl node that should not be relied upon must be designated
279as such by a leading underscore character in its name.  For example:
280.Va _dirty_hack .
281.Sh SEE ALSO
282.Xr sysctl 8 ,
283.Xr sysctl_add_oid 9 ,
284.Xr sysctl_ctx_free 9 ,
285.Xr sysctl_ctx_init 9 ,
286.Xr sysctl_remove_oid 9
287.Sh HISTORY
288The
289.Xr sysctl 8
290utility first appeared in
291.Bx 4.4 .
292.Sh AUTHORS
293.An -nosplit
294The
295.Nm sysctl
296implementation originally found in
297.Bx
298has been extensively rewritten by
299.An Poul-Henning Kamp
300in order to add support for name lookups, name space iteration, and dynamic
301addition of MIB nodes.
302.Pp
303This man page was written by
304.An Robert N. M. Watson .
305