SYSCTL_ADD_OID(9) MidnightBSD Kernel Developer’s Manual SYSCTL_ADD_OID(9)


sysctl_add_oid, sysctl_move_oid, sysctl_remove_oid — runtime sysctl tree manipulation


#include <sys/types.h>
#include <sys/sysctl.h>

struct sysctl_oid *

sysctl_add_oid(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number, const char *name, int kind, void *arg1, int arg2, int (*handler) (SYSCTL_HANDLER_ARGS), const char *format, const char *descr);


sysctl_move_oid(struct sysctl_oid *oidp, struct sysctl_oid_list *parent);


sysctl_remove_oid(struct sysctl_oid *oidp, int del, int recurse);

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 *

SYSCTL_ADD_OID(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number, const char *name, int kind, void *arg1, int arg2, int (*handler) (SYSCTL_HANDLER_ARGS), const char *format, 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 access, int (*handler) (SYSCTL_HANDLER_ARGS), 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 access, char *arg, int 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 access, int *arg, int len, 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 access, unsigned int *arg, int len, 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 access, long *arg, 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 access, unsigned long *arg, const char *descr);

struct sysctl_oid *

SYSCTL_ADD_OPAQUE(struct sysctl_ctx_list *ctx, struct sysctl_oid_list *parent, int number, const char *name, int access, void *arg, int len, const char *format, 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 access, void *arg, STRUCT_NAME, 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 access, void *arg1, int arg2, int (*handler) (SYSCTL_HANDLER_ARGS), const char *format, const char *descr);


These functions and macros provide an interface for creating and deleting sysctl oids at runtime (e.g. during lifetime of a module). The alternative method, based on linker sets (see <sys/linker_set.h> and src/sys/kern/kern_sysctl.c for details), only allows creation and deletion on module load and unload respectively.

Dynamic oids of type CTLTYPE_NODE are reusable so that several code sections can create and delete them, but in reality they are allocated and freed based on their reference count. As a consequence, it is possible for two or more code sections to create partially overlapping trees that they both can use. It is not possible to create overlapping leaves, nor to create different child types with the same name and parent.

Newly created oids are connected to their parent nodes. In all these functions and macros (with the exception of sysctl_remove_oid()), one of the required parameters is parent, which points to the head of the parent’s list of children.

Most top level categories are created statically. When connecting to existing static oids, this pointer can be obtained with the SYSCTL_STATIC_CHILDREN() macro, where the OID_NAME argument is name of the parent oid of type CTLTYPE_NODE (i.e., the name displayed by sysctl(8), preceded by underscore, and with all dots replaced with underscores).

When connecting to an existing dynamic oid, this pointer can be obtained with the SYSCTL_CHILDREN() macro, where the oidp argument points to the parent oid of type CTLTYPE_NODE.

The sysctl_add_oid() function creates raw oids of any type. If the oid is successfully created, the function returns a pointer to it; otherwise it returns NULL. Many of the arguments for sysctl_add_oid() are common to the macros. The arguments are as follows:


A pointer to an optional sysctl context, or NULL. See sysctl_ctx_init(9) for details. Programmers are strongly advised to use contexts to organize the dynamic oids which they create, unless special creation and deletion sequences are required. If ctx is not NULL, the newly created oid will be added to this context as its first entry.


A pointer to a struct sysctl_oid_list, which is the head of the parent’s list of children.


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.


The name of the oid. The newly created oid will contain a copy of the name.


The kind of oid, specified as a bit mask of the type and access values defined in the <sys/sysctl.h> header file. Oids created dynamically always have the CTLFLAG_DYN flag set. Access flags specify whether this oid is read-only or read-write, and whether it may be modified by all users or by the superuser only.


A pointer to any data that the oid should reference, or NULL.


The size of arg1, or 0 if arg1 is NULL.


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 also to define new handlers using the SYSCTL_ADD_PROC() macro.


A pointer to a string which specifies the format of the oid symbolically. This format is used as a hint by sysctl(8) to apply proper data formatting for display purposes. Currently used format names are: ‘‘N’’ for node, ‘‘A’’ for char *, ‘‘I’’ for int, ‘‘IU’’ for unsigned int, ‘‘L’’ for long, ‘‘LU’’ for unsigned long and ‘‘S,TYPE’’ for struct TYPE structures.


A pointer to a textual description of the oid.

The sysctl_move_oid() function reparents an existing oid. The oid is assigned a new number as if it had been created with number set to OID_AUTO.

The sysctl_remove_oid() function removes a dynamically created oid from the tree, optionally freeing its resources. It takes the following arguments:


A pointer to the dynamic oid to be removed. If the oid is not dynamic, or the pointer is NULL, the function returns EINVAL.


If non-zero, sysctl_remove_oid() will try to free the oid’s resources when the reference count of the oid becomes zero. However, if del is set to 0, the routine will only deregister the oid from the tree, without freeing its resources. This behaviour is useful when the caller expects to rollback (possibly partially failed) deletion of many oids later.


If non-zero, attempt to remove the node and all its children. If recurse is set to 0, any attempt to remove a node that contains any children will result in a ENOTEMPTY error. WARNING: use recursive deletion with extreme caution! Normally it should not be needed if contexts are used. Contexts take care of tracking inter-dependencies between users of the tree. However, in some extreme cases it might be necessary to remove part of the subtree no matter how it was created, in order to free some other resources. Be aware, though, that this may result in a system panic(9) if other code sections continue to use removed subtrees.

Again, in most cases the programmer should use contexts, as described in sysctl_ctx_init(9), to keep track of created oids, and to delete them later in orderly fashion.

There is a set of macros defined that helps to create oids of given type.
They are as follows:


creates a raw oid. This macro is functionally equivalent to the sysctl_add_oid() function.


creates an oid of type CTLTYPE_NODE, to which child oids may be added.


creates an oid that handles a zero-terminated character string.


creates an oid that handles an int variable.


creates an oid that handles an unsigned int variable.


creates an oid that handles a long variable.


creates an oid that handles an unsigned long variable.


creates an oid that handles any chunk of opaque data of the size specified by the len argument, which is a pointer to a size_t *.


creates an oid that handles a struct TYPE structure. The format parameter will be set to ‘‘S,TYPE’’ to provide proper hints to the sysctl(8) utility.


creates an oid with the specified handler function. The handler is responsible for handling 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.


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.
struct sysctl_oid *root1, *root2, *oidp;
int a_int;
char *string = "dynamic sysctl";


OID_AUTO, "newtree", CTLFLAG_RW, 0, "new top level tree");


OID_AUTO, "newint", CTLFLAG_RW, &a_int, 0, "new int leaf");


OID_AUTO, "newtree", CTLFLAG_RW, 0, "new tree under debug");


OID_AUTO, "newstring", CTLFLAG_RD, string, 0, "new string leaf");

This example creates the following subtrees:


Care should be taken to free all oids once they are no longer needed!


sysctl(8), sysctl(9), sysctl_ctx_free(9), sysctl_ctx_init(9)


These functions first appeared in FreeBSD 4.2.


Andrzej Bialecki 〈〉


Sharing nodes between many code sections causes interdependencies that sometimes may lock the resources. For example, if module A hooks up a subtree to an oid created by module B, module B will be unable to delete that oid. These issues are handled properly by sysctl contexts.

Many operations on the tree involve traversing linked lists. For this reason, oid creation and removal is relatively costly.

MidnightBSD 0.3 July 15, 2000 MidnightBSD 0.3