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SYSCTL(9) Kernel Developer's Manual SYSCTL(9)

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_STRUCT, 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_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_STRUCT, 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_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_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);

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_STRUCT(parent, number, name, ctlflags, ptr, struct_type, 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:

node
[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
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.

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).

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_STRUCT(), 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_STRUCT(), 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:
This is a node intended to be a parent for other nodes.
This is a signed integer.
This is a nul-terminated string stored in a character array.
This is an 8-bit signed integer.
This is a 16-bit signed integer.
This is a 32-bit signed integer.
This is a 64-bit signed integer.
This is an opaque data structure.
Alias for CTLTYPE_OPAQUE.
This is an 8-bit unsigned integer.
This is a 16-bit unsigned integer.
This is a 32-bit unsigned integer.
This is a 64-bit unsigned integer.
This is an unsigned integer.
This is a signed long.
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:

This is a read-only sysctl.
This is a read-only sysctl and tunable which is tried fetched once from the system environment early during module load or system boot.
This is a writable sysctl.
This sysctl is readable and writable.
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.
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.
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:

Any user or process can write to this sysctl.
A process in capability mode can read from this sysctl.
A process in capability mode can write to this sysctl.
This sysctl can be written to only if the effective securelevel of the process is ≤ 0.
This sysctl can be written to by processes in jail(2).
When iterating the sysctl name space, do not list this sysctl.
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.
Dynamically created OIDs automatically get this flag set.
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.
June 1, 2018 Linux 4.19.0-6-amd64