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.Dd April 4, 2000
.Dt KOBJ 9
.Os FreeBSD
.Sh NAME
.Nm kobj
.Nd a kernel object system for FreeBSD
.Sh SYNOPSIS
.Fd #include <sys/param.h>
.Fd #include <sys/kobj.h>
.Ft void
.Fn kobj_class_compile "kobj_class_t cls"
.Ft void
.Fn kobj_class_free "kobj_class_t cls"
.Ft kobj_t
.Fn kobj_create "kobj_class_t cls" "struct malloc_type *mtype" "int mflags"
.Ft void
.Fn kobj_init "kobj_t obj" "kobj_class_t cls"
.Ft void
.Fn kobj_delete "kobj_t obj" "struct malloc_type *mtype"
.Fn DEFINE_CLASS name methods size
.Sh DESCRIPTION
.Pp
The kernel object system implements an object-oriented programming
system in the FreeBSD kernel.
The system is based around the concepts of interfaces, which are
descriptions of sets of methods; classes, which are lists of functions
implementing certain methods from those interfaces; and objects,
which combine a class with a structure in memory.
.Pp
Methods are called using a dynamic method dispatching algorithm which
is designed to allow new interfaces and classes to be introduced into
the system at runtime.
The method dispatch algorithm is designed to be both fast and robust
and is only slightly more expensive than a direct function call,
making kernel objects suitable for performance-critical algorithms.
.Pp
Suitable uses for kernel objects are any algorithms which need some
kind of polymorphism (i.e. many different objects which can be treated 
in a uniform way).
The common behaviour of the objects is described by a suitable
interface and each different type of object is implemented by a
suitable class.
.Pp
The simplest way to create a kernel object is to call
.Fn kobj_create
with a suitable class, malloc type and flags (see
.Xr malloc 9
for a description of the malloc type and flags).
This will allocate memory for the object based on the object size
specified by the class and initialise it be zeroing the memory and
installing a pointer to the class' method dispatch table.
Objects created in this way should be freed by calling
.Fn kobj_free .
.Pp
Clients which would like to manage the allocation of memory
themselves should call
.Fn kobj_init
with a pointer to the memory for the object and the class which
implements it.
It is also possible to use
.Fn kobj_init
to change the class for an object.
This should be done with care as the classes must agree on the layout
of the object.
The device framework uses this feature to associate drivers with
devices.
.Pp
The functions
.Fn kobj_class_compile
and
.Fn kobj_class_free
are used to process a class description to make method dispatching
efficient.
A client should not normally need to call these since a class
will automatically be compiled the first time it is used.
.Pp
To define a class, first define a simple array of
.Dv kobj_method_t .
Each method which the class implements should be entered into the
table using the macro
.Fn KOBJMETHOD
which takes the name of the method (including its interface) and a
pointer to a function which implements it.
The table should be terminated with two zeros.
The macro
.Fn DEFINE_CLASS
can then be used to initialise a
.Dv kobj_class_t
structure.
The size argument to
.Fn DEFINE_CLASS
specifies how much memory should be allocated for each object.
.Sh HISTORY
Some of the concepts for this interface appeared in the device
framework used for the alpha port of
.Fx 3.0
and more widely in
.Fx 4.0 .
.Sh AUTHORS
This man page was written by
.An Doug Rabson .