SYSCTL_ADD_NODE (9)

Leading comments

Copyright (c) 2006 Robert N. M. Watson
All rights reserved.

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(The comments found at the beginning of the groff file "man9/SYSCTL_ADD_NODE.9freebsd".)

NAME

sysctl_add_oid sysctl_move_oid sysctl_remove_oid - runtime sysctl tree manipulation

SYNOPSIS

In sys/types.h In sys/sysctl.h Ft struct sysctl_oid * Fo sysctl_add_oid Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int kind Fa void *arg1 Fa int arg2 Fa int (*handler) (SYSCTL_HANDLER_ARGS) Fa const char *format Fa const char *descr Fc Ft int Fo sysctl_move_oid Fa struct sysctl_oid *oidp Fa struct sysctl_oid_list *parent Fc Ft int Fo sysctl_remove_oid Fa struct sysctl_oid *oidp Fa int del Fa int recurse Fc Ft struct sysctl_oid_list * Fo SYSCTL_CHILDREN Fa struct sysctl_oid *oidp Fc Ft struct sysctl_oid_list * Fo SYSCTL_STATIC_CHILDREN Fa struct sysctl_oid_list OID_NAME Fc Ft struct sysctl_oid * Fo SYSCTL_ADD_OID Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int kind Fa void *arg1 Fa int arg2 Fa int (*handler) (SYSCTL_HANDLER_ARGS) Fa const char *format Fa const char *descr Fc Ft struct sysctl_oid * Fo SYSCTL_ADD_NODE Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int access Fa int (*handler) (SYSCTL_HANDLER_ARGS) Fa const char *descr Fc Ft struct sysctl_oid * Fo SYSCTL_ADD_STRING Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int access Fa char *arg Fa int len Fa const char *descr Fc Ft struct sysctl_oid * Fo SYSCTL_ADD_INT Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int access Fa int *arg Fa int len Fa const char *descr Fc Ft struct sysctl_oid * Fo SYSCTL_ADD_UINT Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int access Fa unsigned int *arg Fa int len Fa const char *descr Fc Ft struct sysctl_oid * Fo SYSCTL_ADD_LONG Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int access Fa long *arg Fa const char *descr Fc Ft struct sysctl_oid * Fo SYSCTL_ADD_ULONG Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int access Fa unsigned long *arg Fa const char *descr Fc Ft struct sysctl_oid * Fo SYSCTL_ADD_QUAD Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int access Fa int64_t *arg Fa const char *descr Fc Ft struct sysctl_oid * Fo SYSCTL_ADD_UQUAD Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int access Fa uint64_t *arg Fa const char *descr Fc Ft struct sysctl_oid * Fo SYSCTL_ADD_OPAQUE Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int access Fa void *arg Fa int len Fa const char *format Fa const char *descr Fc Ft struct sysctl_oid * Fo SYSCTL_ADD_STRUCT Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int access Fa void *arg Fa STRUCT_NAME Fa const char *descr Fc Ft struct sysctl_oid * Fo SYSCTL_ADD_PROC Fa struct sysctl_ctx_list *ctx Fa struct sysctl_oid_list *parent Fa int number Fa const char *name Fa int access Fa void *arg1 Fa int arg2 Fa int (*handler) (SYSCTL_HANDLER_ARGS) Fa const char *format Fa const char *descr Fc

DESCRIPTION

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 In 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 Fn sysctl_remove_oid ) , one of the required parameters is Fa 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 Fn SYSCTL_STATIC_CHILDREN macro, where the Fa 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 Fn SYSCTL_CHILDREN macro, where the Fa oidp argument points to the parent oid of type CTLTYPE_NODE

The Fn 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 Fn sysctl_add_oid are common to the macros. The arguments are as follows:

Fa ctx

A pointer to an optional sysctl context, or NULL See sysctl_ctx_init9 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 Fa ctx is not NULL the newly created oid will be added to this context as its first entry.

Fa parent

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

Fa 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.

Fa name

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

Fa kind

The kind of oid, specified as a bit mask of the type and access values defined in the In 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.

Fa arg1

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

Fa arg2

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

Fa 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 also to define new handlers using the Fn SYSCTL_ADD_PROC macro.

Fa format

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.

Fa descr

A pointer to a textual description of the oid.

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

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

Fa oidp

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

Fa del

If non-zero, Fn sysctl_remove_oid will try to free the oid's resources when the reference count of the oid becomes zero. However, if Fa 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.

Fa recurse

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 Er 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_init9, 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:

Fn SYSCTL_ADD_OID

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

Fn SYSCTL_ADD_NODE

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

Fn SYSCTL_ADD_STRING

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

Fn SYSCTL_ADD_INT

creates an oid that handles an int variable.

Fn SYSCTL_ADD_UINT

creates an oid that handles an unsigned int variable.

Fn SYSCTL_ADD_LONG

creates an oid that handles a long variable.

Fn SYSCTL_ADD_ULONG

creates an oid that handles an unsigned long variable.

Fn SYSCTL_ADD_QUAD

creates an oid that handles an int64_t variable.

Fn SYSCTL_ADD_OPAQUE

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

Fn SYSCTL_ADD_STRUCT

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

Fn SYSCTL_ADD_PROC

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.

EXAMPLES

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";
 ...

root1 = SYSCTL_ADD_NODE( NULL, SYSCTL_STATIC_CHILDREN(/* tree top */),
        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!

SEE ALSO

sysctl(8), sysctl(9), sysctl_ctx_free9, sysctl_ctx_init9

HISTORY

These functions first appeared in Fx 4.2 .

AUTHORS

An Andrzej Bialecki Aq abial@FreeBSD.org

BUGS

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.