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26 .\" $FreeBSD: src/share/man/man9/sysctl.9,v 1.3 2006/04/28 23:21:36 keramida Exp $
45 .Nd Static sysctl declaration functions
165 kernel interfaces allow code to statically declare
167 MIB entries, which will be initialized when the kernel module containing the
168 declaration is initialized.
169 When the module is unloaded, the sysctl will be automatically destroyed.
171 Sysctl nodes are created in a hierarchical tree, with all static nodes being
172 represented by named C data structures; in order to create a new node under
173 an existing node in the tree, the structure representing the desired parent
174 node must be declared in the current context using
177 New nodes are declared using one of
191 Each macro accepts a parent name, as declared using
193 an OID number, typically
195 a node name, a set of control and access flags, and a description.
196 Depending on the macro, a pointer to a variable supporting the MIB entry, a
197 size, a value, and a function pointer implementing the MIB entry may also be
200 For most of the above macros, declaring a type as part of the access flags is
201 not necessary -- however, when declaring a sysctl implemented by a function,
202 including a type in the access mask is required:
203 .Bl -tag -width ".Dv CTLTYPE_STRING"
205 This is a node intended to be a parent for other nodes.
207 This is a signed integer.
208 .It Dv CTLTYPE_STRING
209 This is a nul-terminated string stored in a character array.
211 This is a 64-bit signed integer.
212 .It Dv CTLTYPE_OPAQUE
213 This is an opaque data structure.
214 .It Dv CTLTYPE_STRUCT
218 This is a 32-bit unsigned integer.
220 This is an unsigned integer.
222 This is a signed long.
224 This is an unsigned long.
226 This is a 64-bit unsigned integer.
229 All sysctl types except for new node declarations require one or more flags
230 to be set indicating the read and write disposition of the sysctl:
231 .Bl -tag -width ".Dv CTLFLAG_ANYBODY"
233 This is a read-only sysctl.
235 This is a writable sysctl.
237 This sysctl is readable and writable.
238 .It Dv CTLFLAG_ANYBODY
239 Any user or process can write to this sysctl.
240 .It Dv CTLFLAG_SECURE
241 This sysctl can be written to only if the effective securelevel of the
243 .It Dv CTLFLAG_PRISON
244 This sysctl can be written to by processes in
246 .\".It Dv CTLFLAG_SKIP
247 .\"When iterating the sysctl name space, do not list this sysctl.
250 When creating new sysctls, careful attention should be paid to the security
251 implications of the monitoring or management interface being created.
252 Most sysctls present in the kernel are read-only or writable only by the
254 Sysctls exporting extensive information on system data structures and
255 operation, especially those implemented using procedures, will wish to
256 implement access control to limit the undesired exposure of information about
257 other processes, network connections, etc.
259 The following top level sysctl name spaces are commonly used:
260 .Bl -tag -width ".Va machdep"
262 Compatibility layer information.
264 Debugging information.
265 Various name spaces exist under
268 Hardware and device driver information.
270 Information about the
274 Kernel behavior tuning; generally deprecated in favor of more specific
277 Machine-dependent configuration parameters.
280 Various protocols have name spaces under
283 Reserved name space for the implementation of sysctl.
285 Configuration settings relating to user application behavior.
286 Generally, configuring applications using kernel sysctls is discouraged.
288 Virtual file system configuration and information.
290 Virtual memory subsystem configuration and information.
295 to declare the "machdep" sysctl tree for use by new nodes:
296 .Bd -literal -offset indent
297 SYSCTL_DECL(_machdep);
300 Examples of integer, opaque, string, and procedure sysctls follow:
301 .Bd -literal -offset indent
303 * Example of a constant integer value. Notice that the control
304 * flags are CTLFLAG_RD, the variable pointer is NULL, and the
307 SYSCTL_INT(_debug_sizeof, OID_AUTO, bio, CTLFLAG_RD, NULL,
308 sizeof(struct bio), "sizeof(struct bio)");
311 * Example of a variable integer value. Notice that the control
312 * flags are CTLFLAG_RW, the variable pointer is set, and the
315 static int doingcache = 1; /* 1 => enable the cache */
316 SYSCTL_INT(_debug, OID_AUTO, vfscache, CTLFLAG_RW, &doingcache, 0,
317 "Enable name cache");
320 * Example of a variable string value. Notice that the control
321 * flags are CTLFLAG_RW, that the variable pointer and string
322 * size are set. Unlike newer sysctls, this older sysctl uses a
325 char kernelname[MAXPATHLEN] = "/boot/kernel"; /* XXX bloat */
326 SYSCTL_STRING(_kern, KERN_BOOTFILE, bootfile, CTLFLAG_RW,
327 kernelname, sizeof(kernelname), "Name of kernel file booted");
330 * Example of an opaque data type exported by sysctl. Notice that
331 * the variable pointer and size are provided, as well as a format
332 * string for sysctl(8).
334 static l_fp pps_freq; /* scaled frequency offset (ns/s) */
335 SYSCTL_OPAQUE(_kern_ntp_pll, OID_AUTO, pps_freq, CTLFLAG_RD,
336 &pps_freq, sizeof(pps_freq), "I", "");
339 * Example of a procedure based sysctl exporting string
340 * information. Notice that the data type is declared, the NULL
341 * variable pointer and 0 size, the function pointer, and the
342 * format string for sysctl(8).
344 SYSCTL_PROC(_kern, OID_AUTO, msgbuf, CTLTYPE_STRING | CTLFLAG_RD,
345 0, 0, sysctl_kern_msgbuf, "A", "Contents of kernel message buffer");
348 When adding, modifying, or removing sysctl names, it is important to be
349 aware that these interfaces may be used by users, libraries, applications,
350 or documentation (such as published books), and are implicitly published
351 application interfaces.
352 As with other application interfaces, caution must be taken not to break
353 existing applications, and to think about future use of new name spaces so as
354 to avoid the need to rename or remove interfaces that might be depended on in
358 .Xr sysctl_add_oid 9 ,
359 .Xr sysctl_ctx_free 9 ,
360 .Xr sysctl_ctx_init 9 ,
361 .Xr sysctl_remove_oid 9
368 The sysctl implementation originally found in
370 has been extensively rewritten by
371 .An Poul-Henning Kamp
372 in order to add support for name lookups, name space iteration, and dynamic
373 addition of MIB nodes.
375 This man page was written by
376 .An Robert N. M. Watson .