Provided by: freebsd-manpages_12.2-1_all 
NAME
SYSCTL_DECL, SYSCTL_ADD_INT, SYSCTL_ADD_LONG, SYSCTL_ADD_NODE, SYSCTL_ADD_NODE_WITH_LABEL,
SYSCTL_ADD_OPAQUE, SYSCTL_ADD_PROC, SYSCTL_ADD_QUAD, SYSCTL_ADD_ROOT_NODE, SYSCTL_ADD_S8,
SYSCTL_ADD_S16, SYSCTL_ADD_S32, SYSCTL_ADD_S64, SYSCTL_ADD_STRING, SYSCTL_ADD_CONST_STRING,
SYSCTL_ADD_STRUCT, SYSCTL_ADD_TIMEVAL_SEC, SYSCTL_ADD_U8, SYSCTL_ADD_U16, SYSCTL_ADD_U32,
SYSCTL_ADD_U64, SYSCTL_ADD_UAUTO, SYSCTL_ADD_UINT, SYSCTL_ADD_ULONG, SYSCTL_ADD_UQUAD,
SYSCTL_CHILDREN, SYSCTL_STATIC_CHILDREN, SYSCTL_NODE_CHILDREN, SYSCTL_PARENT, SYSCTL_INT,
SYSCTL_INT_WITH_LABEL, SYSCTL_LONG, sysctl_msec_to_ticks, SYSCTL_NODE,
SYSCTL_NODE_WITH_LABEL, SYSCTL_OPAQUE, SYSCTL_PROC, SYSCTL_QUAD, SYSCTL_ROOT_NODE,
SYSCTL_S8, SYSCTL_S16, SYSCTL_S32, SYSCTL_S64, SYSCTL_STRING, SYSCTL_CONST_STRING,
SYSCTL_STRUCT, SYSCTL_TIMEVAL_SEC, SYSCTL_U8, SYSCTL_U16, SYSCTL_U32, SYSCTL_U64,
SYSCTL_UINT, SYSCTL_ULONG, SYSCTL_UQUAD — Dynamic and static sysctl MIB creation functions
SYNOPSIS
#include <sys/param.h>
#include <sys/sysctl.h>
SYSCTL_DECL(name);
struct sysctl_oid *
SYSCTL_ADD_INT(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, int *ptr, int val, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_LONG(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, long *ptr, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_NODE(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, int (*handler)(SYSCTL_HANDLER_ARGS), const char *descr);
struct sysctl_oid *
SYSCTL_ADD_NODE_WITH_LABEL(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent,
int number, const char *name, int ctlflags, int (*handler)(SYSCTL_HANDLER_ARGS),
const char *descr, const char *label);
struct sysctl_oid *
SYSCTL_ADD_OPAQUE(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, void *ptr, intptr_t len, const char *format,
const char *descr);
struct sysctl_oid *
SYSCTL_ADD_PROC(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, void *arg1, intptr_t arg2,
int (*handler) (SYSCTL_HANDLER_ARGS), const char *format, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_QUAD(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, int64_t *ptr, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_ROOT_NODE(struct sysctl_ctx_list *ctx, int number, const char *name,
int ctlflags, int (*handler)(SYSCTL_HANDLER_ARGS), const char *descr);
struct sysctl_oid *
SYSCTL_ADD_S8(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, int8_t *ptr, int8_t val, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_S16(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, int16_t *ptr, int16_t val, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_S32(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, int32_t *ptr, int32_t val, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_S64(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, int64_t *ptr, int64_t val, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_STRING(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, char *ptr, intptr_t len, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_CONST_STRING(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent,
int number, const char *name, int ctlflags, const char *ptr, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_STRUCT(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, void *ptr, struct_type, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_TIMEVAL_SEC(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent,
int number, const char *name, int ctlflags, struct timeval *ptr, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_U8(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, uint8_t *ptr, uint8_t val, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_U16(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, uint16_t *ptr, uint16_t val, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_U32(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, uint32_t *ptr, uint32_t val, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_U64(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, uint64_t *ptr, uint64_t val, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_UINT(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, unsigned int *ptr, unsigned int val, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_ULONG(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, unsigned long *ptr, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_UQUAD(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, uint64_t *ptr, const char *descr);
struct sysctl_oid *
SYSCTL_ADD_UAUTO(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number,
const char *name, int ctlflags, void *ptr, const char *descr);
struct sysctl_oid_list *
SYSCTL_CHILDREN(struct sysctl_oid *oidp);
struct sysctl_oid_list *
SYSCTL_STATIC_CHILDREN(struct sysctl_oid_list OID_NAME);
struct sysctl_oid_list *
SYSCTL_NODE_CHILDREN(parent, name);
struct sysctl_oid *
SYSCTL_PARENT(struct sysctl_oid *oid);
SYSCTL_INT(parent, number, name, ctlflags, ptr, val, descr);
SYSCTL_INT_WITH_LABEL(parent, number, name, ctlflags, ptr, val, descr, label);
SYSCTL_LONG(parent, number, name, ctlflags, ptr, val, descr);
int
sysctl_msec_to_ticks(SYSCTL_HANDLER_ARGS);
SYSCTL_NODE(parent, number, name, ctlflags, handler, descr);
SYSCTL_NODE_WITH_LABEL(parent, number, name, ctlflags, handler, descr, label);
SYSCTL_OPAQUE(parent, number, name, ctlflags, ptr, len, format, descr);
SYSCTL_PROC(parent, number, name, ctlflags, arg1, arg2, handler, format, descr);
SYSCTL_QUAD(parent, number, name, ctlflags, ptr, val, descr);
SYSCTL_ROOT_NODE(number, name, ctlflags, handler, descr);
SYSCTL_S8(parent, number, name, ctlflags, ptr, val, descr);
SYSCTL_S16(parent, number, name, ctlflags, ptr, val, descr);
SYSCTL_S32(parent, number, name, ctlflags, ptr, val, descr);
SYSCTL_S64(parent, number, name, ctlflags, ptr, val, descr);
SYSCTL_STRING(parent, number, name, ctlflags, arg, len, descr);
SYSCTL_CONST_STRING(parent, number, name, ctlflags, arg, descr);
SYSCTL_STRUCT(parent, number, name, ctlflags, ptr, struct_type, descr);
SYSCTL_TIMEVAL_SEC(parent, number, name, ctlflags, ptr, descr);
SYSCTL_U8(parent, number, name, ctlflags, ptr, val, descr);
SYSCTL_U16(parent, number, name, ctlflags, ptr, val, descr);
SYSCTL_U32(parent, number, name, ctlflags, ptr, val, descr);
SYSCTL_U64(parent, number, name, ctlflags, ptr, val, descr);
SYSCTL_UINT(parent, number, name, ctlflags, ptr, val, descr);
SYSCTL_ULONG(parent, number, name, ctlflags, ptr, val, descr);
SYSCTL_UQUAD(parent, number, name, ctlflags, ptr, val, descr);
DESCRIPTION
The SYSCTL kernel interface allows dynamic or static creation of sysctl(8) MIB entries. All
static sysctls are automatically destroyed when the module which they are part of is
unloaded. Most top level categories are created statically and are available to all kernel
code and its modules.
DESCRIPTION OF ARGUMENTS
ctx Pointer to sysctl context or NULL, if no context. See sysctl_ctx_init(9) for how
to create a new sysctl context. Programmers are strongly advised to use contexts
to organize the dynamic OIDs which they create because when a context is destroyed
all belonging sysctls are destroyed as well. This makes the sysctl cleanup code
much simpler. Else deletion of all created OIDs is required at module unload.
parent A pointer to a struct sysctl_oid_list, which is the head of the parent's list of
children. This pointer is retrieved using the SYSCTL_STATIC_CHILDREN() macro for
static sysctls and the SYSCTL_CHILDREN() macro for dynamic sysctls. The
SYSCTL_PARENT() macro can be used to get the parent of an OID. The macro returns
NULL if there is no parent.
number The OID number that will be assigned to this OID. In almost all cases this should
be set to OID_AUTO, which will result in the assignment of the next available OID
number.
name The name of the OID. The newly created OID will contain a copy of the name.
ctlflags A bit mask of sysctl control flags. See the section below describing all the
control flags.
arg1 First callback argument for procedure sysctls.
arg2 Second callback argument for procedure sysctls.
len The length of the data pointed to by the ptr argument. For string type OIDs a
length of zero means that strlen(3) will be used to get the length of the string
at each access to the OID.
ptr Pointer to sysctl variable or string data. For sysctl values the pointer can be
SYSCTL_NULL_XXX_PTR which means the OID is read-only and the returned value should
be taken from the val argument.
val If the ptr argument is SYSCTL_NULL_XXX_PTR, gives the constant value returned by
this OID. Else this argument is not used.
struct_type
Name of structure type.
handler A pointer to the function that is responsible for handling read and write requests
to this OID. There are several standard handlers that support operations on
nodes, integers, strings and opaque objects. It is possible to define custom
handlers using the SYSCTL_PROC() macro or the SYSCTL_ADD_PROC() function.
format A pointer to a string which specifies the format of the OID in a symbolic way.
This format is used as a hint by sysctl(8) to apply proper data formatting for
display purposes.
Current formats:
N node
A char *
I int
IK[n] temperature in Kelvin, multiplied by an optional single digit power
of ten scaling factor: 1 (default) gives deciKelvin, 0 gives Kelvin,
3 gives milliKelvin
IU unsigned int
L long
LU unsigned long
Q quad_t
QU u_quad_t
S,TYPE struct TYPE structures
descr A pointer to a textual description of the OID.
label A pointer to an aggregation label for this component of the OID. To make it
easier to export sysctl data to monitoring systems that support aggregations
through labels (e.g., Prometheus), this argument can be used to attach a label
name to an OID. The label acts as a hint that this component's name should not be
part of the metric's name, but attached to the metric as a label instead.
Labels should only be applied to siblings that are structurally similar and encode
the same type of value, as aggregation is of no use otherwise.
NODE VALUE TYPES
Most of the macros and functions used to create sysctl nodes export a read-only constant or
in-kernel variable whose type matches the type of the node's value. For example,
SYSCTL_INT() reports the raw value of an associated variable of type int. However, nodes
may also export a value that is a translatation of an internal representation.
The sysctl_msec_to_ticks() handler can be used with SYSCTL_PROC() or SYSCTL_ADD_PROC() to
export a millisecond time interval. When using this handler, the arg2 parameter points to
an in-kernel variable of type int which stores a tick count suitable for use with functions
like tsleep(9). The sysctl_msec_to_ticks() function converts this value to milliseconds
when reporting the node's value. Similarly, sysctl_msec_to_ticks() accepts new values in
milliseconds and stores an equivalent value in ticks to *arg2.
The SYSCTL_ADD_TIMEVAL_SEC() function and SYSCTL_TIMEVAL_SEC() macro create nodes which
export an in-kernel variable of type struct timeval. These nodes do not export full value
of the associated structure. Instead, they export a count in seconds as a simple integer
which is stored in the tv_sec field of the associated variable. This function and macro are
intended to be used with variables which store a non-negative interval rather than an
absolute time. As a result, they reject attempts to store negative values.
CREATING ROOT NODES
Sysctl MIBs or OIDs are created in a hierarchical tree. The nodes at the bottom of the tree
are called root nodes, and have no parent OID. To create bottom tree nodes the
SYSCTL_ROOT_NODE() macro or the SYSCTL_ADD_ROOT_NODE() function needs to be used. By
default all static sysctl node OIDs are global and need a SYSCTL_DECL() statement prior to
their SYSCTL_NODE() definition statement, typically in a so-called header file.
CREATING SYSCTL STRINGS
Zero terminated character strings sysctls are created either using the SYSCTL_STRING() macro
or the SYSCTL_ADD_STRING() function. If the len argument in zero, the string length is
computed at every access to the OID using strlen(3). Use the SYSCTL_CONST_STRING() macro or
the SYSCTL_ADD_CONST_STRING() function to add a sysctl for a constant string.
CREATING OPAQUE SYSCTLS
The SYSCTL_OPAQUE() or SYSCTL_STRUCT() macros or the SYSCTL_ADD_OPAQUE() or
SYSCTL_ADD_STRUCT() functions create an OID that handle any chunk of data of the size
specified by the len argument and data pointed to by the ptr argument. When using the
structure version the type is encoded as part of the created sysctl.
CREATING CUSTOM SYSCTLS
The SYSCTL_PROC() macro and the SYSCTL_ADD_PROC() function create OIDs with the specified
handler function. The handler is responsible for handling all read and write requests to
the OID. This OID type is especially useful if the kernel data is not easily accessible, or
needs to be processed before exporting.
CREATING A STATIC SYSCTL
Static sysctls are declared using one of the SYSCTL_INT(), SYSCTL_INT_WITH_LABEL(),
SYSCTL_LONG(), SYSCTL_NODE(), SYSCTL_NODE_WITH_LABEL(), SYSCTL_OPAQUE(), SYSCTL_PROC(),
SYSCTL_QUAD(), SYSCTL_ROOT_NODE(), SYSCTL_S8(), SYSCTL_S16(), SYSCTL_S32(), SYSCTL_S64(),
SYSCTL_STRING(), SYSCTL_CONST_STRING(), SYSCTL_STRUCT(), SYSCTL_TIMEVAL_SEC(), SYSCTL_U8(),
SYSCTL_U16(), SYSCTL_U32(), SYSCTL_U64(), SYSCTL_UINT(), SYSCTL_ULONG() or SYSCTL_UQUAD()
macros.
CREATING A DYNAMIC SYSCTL
Dynamic nodes are created using one of the SYSCTL_ADD_INT(), SYSCTL_ADD_LONG(),
SYSCTL_ADD_NODE(), SYSCTL_ADD_NODE_WITH_LABEL(), SYSCTL_ADD_OPAQUE(), SYSCTL_ADD_PROC(),
SYSCTL_ADD_QUAD(), SYSCTL_ADD_ROOT_NODE(), SYSCTL_ADD_S8(), SYSCTL_ADD_S16(),
SYSCTL_ADD_S32(), SYSCTL_ADD_S64(), SYSCTL_ADD_STRING(), SYSCTL_ADD_CONST_STRING(),
SYSCTL_ADD_STRUCT(), SYSCTL_ADD_TIMEVAL_SEC(), SYSCTL_ADD_U8(), SYSCTL_ADD_U16(),
SYSCTL_ADD_U32(), SYSCTL_ADD_U64(), SYSCTL_ADD_UAUTO(), SYSCTL_ADD_UINT(),
SYSCTL_ADD_ULONG(), or SYSCTL_UQUAD() functions. See sysctl_remove_oid(9) or
sysctl_ctx_free(9) for more information on how to destroy a dynamically created OID.
CONTROL FLAGS
For most of the above functions and 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_S8 This is an 8-bit signed integer.
CTLTYPE_S16 This is a 16-bit signed integer.
CTLTYPE_S32 This is a 32-bit signed integer.
CTLTYPE_S64 This is a 64-bit signed integer.
CTLTYPE_OPAQUE This is an opaque data structure.
CTLTYPE_STRUCT Alias for CTLTYPE_OPAQUE.
CTLTYPE_U8 This is an 8-bit unsigned integer.
CTLTYPE_U16 This is a 16-bit unsigned integer.
CTLTYPE_U32 This is a 32-bit unsigned integer.
CTLTYPE_U64 This is a 64-bit unsigned integer.
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 of the following flags to be
set indicating the read and write disposition of the sysctl:
CTLFLAG_RD This is a read-only sysctl.
CTLFLAG_RDTUN This is a read-only sysctl and tunable which is tried fetched once from the
system environment early during module load or system boot.
CTLFLAG_WR This is a writable sysctl.
CTLFLAG_RW This sysctl is readable and writable.
CTLFLAG_RWTUN This is a readable and writeable sysctl and tunable which is tried fetched
once from the system environment early during module load or system boot.
CTLFLAG_NOFETCH In case the node is marked as a tunable using the CTLFLAG_[XX]TUN, this
flag will prevent fetching the initial value from the system environment.
Typically this flag should only be used for very early low level system
setup code, and not by common drivers and modules.
CTLFLAG_MPSAFE This sysctl(9) handler is MP safe. Do not grab Giant around calls to this
handler. This should only be used for SYSCTL_PROC() entries.
Additionally, any of the following optional flags may also be specified:
CTLFLAG_ANYBODY Any user or process can write to this sysctl.
CTLFLAG_CAPRD A process in capability mode can read from this sysctl.
CTLFLAG_CAPWR A process in capability mode 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 Advisory flag that a system tunable also exists for this variable. The
initial sysctl value is tried fetched once from the system environment
early during module load or system boot.
CTLFLAG_DYN Dynamically created OIDs automatically get this flag set.
CTLFLAG_VNET OID references a VIMAGE-enabled variable.
EXAMPLES
Sample use of SYSCTL_DECL() to declare the security sysctl tree for use by new nodes:
SYSCTL_DECL(_security);
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 SYSCTL_NULL_INT_PTR,
* and the value is declared.
*/
SYSCTL_INT(_debug_sizeof, OID_AUTO, bio, CTLFLAG_RD, SYSCTL_NULL_INT_PTR,
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 frequency 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",
"");
The following is an example of how to create a new top-level category and how to hook up
another subtree to an existing static node. This example does not use contexts, which
results in tedious management of all intermediate oids, as they need to be freed later on:
#include <sys/sysctl.h>
...
/*
* Need to preserve pointers to newly created subtrees,
* to be able to free them later:
*/
static struct sysctl_oid *root1;
static struct sysctl_oid *root2;
static struct sysctl_oid *oidp;
static int a_int;
static char *string = "dynamic sysctl";
...
root1 = SYSCTL_ADD_ROOT_NODE(NULL,
OID_AUTO, "newtree", CTLFLAG_RW, 0, "new top level tree");
oidp = SYSCTL_ADD_INT(NULL, SYSCTL_CHILDREN(root1),
OID_AUTO, "newint", CTLFLAG_RW, &a_int, 0, "new int leaf");
...
root2 = SYSCTL_ADD_NODE(NULL, SYSCTL_STATIC_CHILDREN(_debug),
OID_AUTO, "newtree", CTLFLAG_RW, 0, "new tree under debug");
oidp = SYSCTL_ADD_STRING(NULL, SYSCTL_CHILDREN(root2),
OID_AUTO, "newstring", CTLFLAG_RD, string, 0, "new string leaf");
This example creates the following subtrees:
debug.newtree.newstring
newtree.newint
Care should be taken to free all OIDs once they are no longer needed!
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 OID 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(3), 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.
SECURITY CONSIDERATIONS
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.