NAME¶
SYSCTL_DECL,
SYSCTL_INT,
SYSCTL_LONG,
SYSCTL_NODE,
SYSCTL_OPAQUE,
SYSCTL_PROC,
SYSCTL_STRING,
SYSCTL_STRUCT,
SYSCTL_UINT,
SYSCTL_ULONG,
SYSCTL_XINT,
SYSCTL_XLONG,
SYSCTL_QUAD —
Static sysctl
declaration functions
SYNOPSIS¶
#include <sys/types.h>
#include <sys/sysctl.h>
SYSCTL_DECL(
name);
SYSCTL_INT(
parent,
nbr,
name,
access,
ptr,
val,
descr);
SYSCTL_LONG(
parent,
nbr,
name,
access,
ptr,
val,
descr);
SYSCTL_NODE(
parent,
nbr,
name,
access,
handler,
descr);
SYSCTL_OPAQUE(
parent,
nbr,
name,
access,
ptr,
len,
fmt,
descr);
SYSCTL_PROC(
parent,
nbr,
name,
access,
ptr,
arg,
handler,
fmt,
descr);
SYSCTL_STRING(
parent,
nbr,
name,
access,
arg,
len,
descr);
SYSCTL_STRUCT(
parent,
nbr,
name,
access,
ptr,
type,
descr);
SYSCTL_UINT(
parent,
nbr,
name,
access,
ptr,
val,
descr);
SYSCTL_ULONG(
parent,
nbr,
name,
access,
ptr,
val,
descr);
SYSCTL_XINT(
parent,
nbr,
name,
access,
ptr,
val,
descr);
SYSCTL_XLONG(
parent,
nbr,
name,
access,
ptr,
val,
descr);
SYSCTL_QUAD(
parent,
nbr,
name,
access,
ptr,
val,
descr);
DESCRIPTION¶
The
SYSCTL kernel interfaces allow code to statically declare
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.
Sysctl nodes are created in a hierarchical 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
SYSCTL_DECL().
New nodes are declared using one of
SYSCTL_INT(),
SYSCTL_LONG(),
SYSCTL_NODE(),
SYSCTL_OPAQUE(),
SYSCTL_PROC(),
SYSCTL_STRING(),
SYSCTL_STRUCT(),
SYSCTL_UINT(),
SYSCTL_ULONG(),
SYSCTL_XINT(),
SYSCTL_XLONG(), and
SYSCTL_QUAD(). Each macro accepts a parent name, as declared
using
SYSCTL_DECL(), an OID number, typically
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.
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:
CTLTYPE_NODE
- This is a node intended to be a parent for other
nodes.
CTLTYPE_INT
- This is a signed integer.
CTLTYPE_STRING
- This is a nul-terminated string stored in a character
array.
CTLTYPE_QUAD
- This is a 64-bit signed integer.
CTLTYPE_OPAQUE
- This is an opaque data structure.
CTLTYPE_STRUCT
- Alias for
CTLTYPE_OPAQUE.
CTLTYPE_UINT
- This is an unsigned integer.
CTLTYPE_LONG
- This is a signed long.
CTLTYPE_ULONG
- This is an unsigned long.
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:
CTLFLAG_RD
- This is a read-only sysctl.
CTLFLAG_WR
- This is a writable sysctl.
CTLFLAG_RW
- This sysctl is readable and writable.
CTLFLAG_ANYBODY
- Any user or process can write to this sysctl.
CTLFLAG_SECURE
- This sysctl can be written to only if the effective
securelevel of the process is ≤ 0.
CTLFLAG_PRISON
- This sysctl can be written to by processes in
jail(2).
CTLFLAG_SKIP
- When iterating the sysctl name space, do not list this
sysctl.
CTLFLAG_TUN
- Also declare a system tunable with the same name to
initialize this variable.
CTLFLAG_RDTUN
- Also declare a system tunable with the same name to
initialize this variable; however, the run-time variable is
read-only.
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.
The following top level sysctl name spaces are commonly used:
- compat
- Compatibility layer information.
- debug
- Debugging information. Various name spaces exist under
debug.
- hw
- Hardware and device driver information.
- kern
- Kernel behavior tuning; generally deprecated in favor of
more specific name spaces.
- machdep
- Machine-dependent configuration parameters.
- net
- Network subsystem. Various protocols have name spaces under
net.
- regression
- Regression test configuration and information.
- security
- Security and security-policy configuration and
information.
- sysctl
- Reserved name space for the implementation of sysctl.
- user
- Configuration settings relating to user application
behavior. Generally, configuring applications using kernel sysctls is
discouraged.
- vfs
- Virtual file system configuration and information.
- vm
- Virtual memory subsystem configuration and
information.
EXAMPLES¶
Sample use of
SYSCTL_DECL() to declare the
security sysctl tree for use by new nodes:
Examples of integer, opaque, string, and procedure sysctls follow:
/*
* Example of a constant integer value. Notice that the control
* flags are CTLFLAG_RD, the variable pointer is NULL, and the
* value is declared.
* If sysctl(8) should print this value in hex, use 'SYSCTL_XINT'.
*/
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",
"");
SYSCTL NAMING¶
When adding, modifying, or removing sysctl names, it is important to be aware
that these interfaces may be used by users, libraries, applications, or
documentation (such as published 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.
The semantics chosen for a new sysctl should be as clear as possible, and the
name of the sysctl must closely reflect its semantics. Therefore the sysctl
name deserves a fair amount of consideration. It should be short but yet
representative of the sysctl meaning. If the name consists of several words,
they should be separated by underscore characters, as in
compute_summary_at_mount. Underscore characters may be
omitted only if the name consists of not more than two words, each being not
longer than four characters, as in
bootfile. For boolean
sysctls, negative logic should be totally avoided. That is, do not use names
like
no_foobar or
foobar_disable.
They are confusing and lead to configuration errors. Use positive logic
instead:
foobar,
foobar_enable.
A temporary sysctl node that should not be relied upon must be designated as
such by a leading underscore character in its name. For example:
_dirty_hack.
SEE ALSO¶
sysctl(8),
sysctl_add_oid(9),
sysctl_ctx_free(9),
sysctl_ctx_init(9),
sysctl_remove_oid(9)
HISTORY¶
The
sysctl(8) utility first appeared in
4.4BSD.
AUTHORS¶
The
sysctl implementation originally found in
BSD has been extensively rewritten by
Poul-Henning Kamp in order to add support for name
lookups, name space iteration, and dynamic addition of MIB nodes.
This man page was written by
Robert N. M. Watson.