freebsd-skq/share/man/man9/sysctl_add_oid.9

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.Dd December 13, 2016
.Dt SYSCTL_ADD_OID 9
.Os
.Sh NAME
.Nm sysctl_add_oid ,
.Nm sysctl_move_oid ,
.Nm sysctl_remove_oid ,
.Nm sysctl_remove_name
.Nd runtime sysctl tree manipulation
.Sh 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 "intmax_t arg2"
.Fa "int (*handler) (SYSCTL_HANDLER_ARGS)"
.Fa "const char *format"
.Fa "const char *descr"
.Fa "const char *label"
.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 int
.Fo sysctl_remove_name
.Fa "struct sysctl_oid *oidp"
.Fa "const char *name"
.Fa "int del"
.Fa "int recurse"
.Fc
.Sh DESCRIPTION
These functions provide the interface for creating and deleting sysctl
OIDs at runtime for example during the lifetime of a module.
The wrapper macros defined by
.Xr sysctl 9
are recommended when creating new OIDs.
.Fn sysctl_add_oid
should not be called directly from the code.
.Pp
Dynamic OIDs of type
.Dv 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.
.Pp
The
.Fn sysctl_add_oid
function creates a raw OID of any type and connects it to its parent node, if any.
If the OID is successfully created,
the function returns a pointer to it else
it returns
.Dv NULL .
Many of the arguments for
.Fn sysctl_add_oid
are common to the wrapper macros defined by
.Xr sysctl 9 .
.Pp
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
.Dv OID_AUTO .
.Pp
The
.Fn sysctl_remove_oid
function removes a dynamically created OID from the tree and
optionally freeing its resources.
It takes the following arguments:
.Bl -tag -width recurse
.It Fa oidp
A pointer to the dynamic OID to be removed.
If the OID is not dynamic, or the pointer is
.Dv NULL ,
the function returns
.Er EINVAL .
.It 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.
.It Fa recurse
If non-zero, attempt to remove the node and all its children.
If
.Pa recurse
is set to 0,
any attempt to remove a node that contains any children
will result in a
.Er ENOTEMPTY
error.
.Em 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
.Xr panic 9
if other code sections continue to use removed subtrees.
.El
.Pp
The
.Fn sysctl_remove_name
function looks up the child node matching the
.Fa name
argument and then invokes the
.Fn sysctl_remove_oid
function on that node, passing along the
.Fa del
and
.Fa recurse
arguments.
If a node having the specified name does not exist an error code of
.Er ENOENT
is returned.
Else the error code from
.Fn sysctl_remove_oid
is returned.
.Pp
In most cases the programmer should use contexts,
as described in
.Xr sysctl_ctx_init 9 ,
to keep track of created OIDs,
and to delete them later in orderly fashion.
.Sh SEE ALSO
.Xr sysctl 8 ,
.Xr sysctl 9 ,
.Xr sysctl_ctx_free 9 ,
.Xr sysctl_ctx_init 9
.Sh HISTORY
These functions first appeared in
.Fx 4.2 .
.Sh AUTHORS
.An Andrzej Bialecki Aq Mt abial@FreeBSD.org
.Sh 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.
.Pp
Many operations on the tree involve traversing linked lists.
For this reason, OID creation and removal is relatively costly.