Provided by: freebsd-manpages_12.2-2_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.
Debian June 11, 2019 SYSCTL(9)