---
name: "sysctl"
section: "9"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/SYSCTL_ADD_BOOL"
---

# sysctl(9)

Dynamic and static sysctl MIB creation functions

Part of [SYSCTL\_ADD\_BOOL(9)](/SYSCTL_ADD_BOOL/9.md).

## NAME

**SYSCTL\_DECL**, **SYSCTL\_ADD\_BOOL**, **SYSCTL\_ADD\_COUNTER\_U64**, **SYSCTL\_ADD\_COUNTER\_U64\_ARRAY**, **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\_SBINTIME\_MSEC**, **SYSCTL\_ADD\_SBINTIME\_USEC**, **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\_UMA\_CUR**, **SYSCTL\_ADD\_UMA\_MAX**, **SYSCTL\_ADD\_UQUAD**, **SYSCTL\_CHILDREN**, **SYSCTL\_STATIC\_CHILDREN**, **SYSCTL\_NODE\_CHILDREN**, **SYSCTL\_PARENT**, **SYSCTL\_BOOL**, **SYSCTL\_COUNTER\_U64**, **SYSCTL\_COUNTER\_U64\_ARRAY**, **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\_SBINTIME\_MSEC**, **SYSCTL\_SBINTIME\_USEC**, **SYSCTL\_STRING**, **SYSCTL\_CONST\_STRING**, **SYSCTL\_STRUCT**, **SYSCTL\_TIMEVAL\_SEC**, **SYSCTL\_U8**, **SYSCTL\_U16**, **SYSCTL\_U32**, **SYSCTL\_U64**, **SYSCTL\_UINT**, **SYSCTL\_ULONG**, **SYSCTL\_UMA\_CUR**, **SYSCTL\_UMA\_MAX**, **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\_BOOL**(*struct sysctl\_ctx\_list \*ctx*, *struct sysctl\_oid\_list \*parent*, *int number*, *const char \*name*, *int ctlflags*, *bool \*ptr*, *uint8\_t val*, *const char \*descr*);

*struct* *sysctl\_oid* *\**\

**SYSCTL\_ADD\_COUNTER\_U64**(*struct sysctl\_ctx\_list \*ctx*, *struct sysctl\_oid\_list \*parent*, *int number*, *const char \*name*, *int ctlflags*, *counter\_u64\_t \*ptr*, *const char \*descr*);

*struct* *sysctl\_oid* *\**\

**SYSCTL\_ADD\_COUNTER\_U64\_ARRAY**(*struct sysctl\_ctx\_list \*ctx*, *struct sysctl\_oid\_list \*parent*, *int number*, *const char \*name*, *int ctlflags*, *counter\_u64\_t \*ptr*, *intmax\_t len*, *const char \*descr*);

*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\_SBINTIME\_MSEC**(*struct sysctl\_ctx\_list \*ctx*, *struct sysctl\_oid\_list \*parent*, *int number*, *const char \*name*, *int ctlflags*, *sbintime\_t \*ptr*, *const char \*descr*);

*struct* *sysctl\_oid* *\**\

**SYSCTL\_ADD\_SBINTIME\_USEC**(*struct sysctl\_ctx\_list \*ctx*, *struct sysctl\_oid\_list \*parent*, *int number*, *const char \*name*, *int ctlflags*, *sbintime\_t \*ptr*, *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\_UMA\_CUR**(*struct sysctl\_ctx\_list \*ctx*, *struct sysctl\_oid\_list \*parent*, *int number*, *const char \*name*, *int ctlflags*, *uma\_zone\_t ptr*, *const char \*descr*);

*struct* *sysctl\_oid* *\**\

**SYSCTL\_ADD\_UMA\_MAX**(*struct sysctl\_ctx\_list \*ctx*, *struct sysctl\_oid\_list \*parent*, *int number*, *const char \*name*, *int ctlflags*, *uma\_zone\_t ptr*, *const char \*descr*);

*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\_BOOL**(*parent*, *number*, *name*, *ctlflags*, *ptr*, *val*, *descr*);

**SYSCTL\_COUNTER\_U64**(*parent*, *number*, *name*, *ctlflags*, *ptr*, *descr*);

**SYSCTL\_COUNTER\_U64\_ARRAY**(*parent*, *number*, *name*, *ctlflags*, *ptr*, *len*, *descr*);

**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\_SBINTIME\_MSEC**(*parent*, *number*, *name*, *ctlflags*, *ptr*, *descr*);

**SYSCTL\_SBINTIME\_USEC**(*parent*, *number*, *name*, *ctlflags*, *ptr*, *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*);

**SYSCTL\_UMA\_MAX**(*parent*, *number*, *name*, *ctlflags*, *ptr*, *descr*);

**SYSCTL\_UMA\_CUR**(*parent*, *number*, *name*, *ctlflags*, *ptr*, *descr*);

## DESCRIPTION

The **SYSCTL** kernel interface allows dynamic or static creation of [sysctl(8)](/sysctl/8.md) 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)](/sysctl_ctx_init/9.md) 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)](/strlen/3.md) will be used to get the length of the string at each access to the OID. For array type OIDs the length must be greater than zero.

- *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)](/sysctl/8.md) to apply proper data formatting for display purposes.

  Current formats:

  - **N**

    node

  - **A**

    char \*

  - **C**

    int8\_t

  - **CU**

    uint8\_t

  - **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**

    int16\_t

  - **SU**

    uint16\_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 translation 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)](/tsleep/9.md). 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*. Note that new code should use kernel variables of type *sbintime\_t* instead of tick counts.

The **SYSCTL\_ADD\_SBINTIME\_MSEC**() and **SYSCTL\_ADD\_SBINTIME\_USEC**() functions and **SYSCTL\_SBINTIME\_MSEC**() and **SYSCTL\_SBINTIME\_USEC**() macros all create nodes which export an in-kernel variable of type *sbintime\_t*. These nodes do not export the raw value of the associated variable. Instead, they export a 64-bit integer containing a count of either milliseconds (the MSEC variants) or microseconds (the USEC variants).

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 is zero, the string length is computed at every access to the OID using [strlen(3)](/strlen/3.md). 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\_BOOL**(), **SYSCTL\_COUNTER\_U64**(), **SYSCTL\_COUNTER\_U64\_ARRAY**(), **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\_SBINTIME\_MSEC**(), **SYSCTL\_SBINTIME\_USEC**(), **SYSCTL\_STRING**(), **SYSCTL\_CONST\_STRING**(), **SYSCTL\_STRUCT**(), **SYSCTL\_TIMEVAL\_SEC**(), **SYSCTL\_U8**(), **SYSCTL\_U16**(), **SYSCTL\_U32**(), **SYSCTL\_U64**(), **SYSCTL\_UINT**(), **SYSCTL\_ULONG**(), **SYSCTL\_UQUAD**(), **SYSCTL\_UMA\_CUR**() or **SYSCTL\_UMA\_MAX**() macros.

## CREATING A DYNAMIC SYSCTL

Dynamic nodes are created using one of the **SYSCTL\_ADD\_BOOL**(), **SYSCTL\_ADD\_COUNTER\_U64**(), **SYSCTL\_ADD\_COUNTER\_U64\_ARRAY**(), **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\_SBINTIME\_MSEC**(), **SYSCTL\_ADD\_SBINTIME\_USEC**(), **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\_ADD\_UMA\_CUR**() or **SYSCTL\_ADD\_UMA\_MAX**() functions. See [sysctl\_remove\_oid(9)](/sysctl_remove_oid/9.md) or [sysctl\_ctx\_free(9)](/sysctl_ctx_free/9.md) 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 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 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)](/sysctl/9.md) 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)](/jail/2.md).

- 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 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(_kern, OID_AUTO, hz_max, CTLFLAG_RD, SYSCTL_NULL_INT_PTR, HZ_MAXIMUM,
    "Maximum hz value supported");
/*
 * 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/3.md), [sysctl(8)](/sysctl/8.md), [device\_get\_sysctl(9)](/device_get_sysctl/9.md), [sysctl\_add\_oid(9)](/sysctl_add_oid/9.md), [sysctl\_ctx\_free(9)](/sysctl_ctx_free/9.md), [sysctl\_ctx\_init(9)](/sysctl_ctx_init/9.md), [sysctl\_remove\_oid(9)](/sysctl_remove_oid/9.md)

## HISTORY

The [sysctl(8)](/sysctl/8.md) utility first appeared in 4.4BSD. **SYSCTL\_ADD\_CONST\_STRING** first appeared in FreeBSD 12.1.

## 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.