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NAME

     kobj - a kernel object system for FreeBSD

SYNOPSIS

     #include <sys/param.h>
     #include <sys/kobj.h>

     void
     kobj_class_compile(kobj_class_t cls);

     void
     kobj_class_compile_static(kobj_class_t cls, kobj_ops_t ops);

     void
     kobj_class_free(kobj_class_t cls);

     kobj_t
     kobj_create(kobj_class_t cls, struct malloc_type *mtype, int mflags);

     void
     kobj_init(kobj_t obj, kobj_class_t cls);

     void
     kobj_delete(kobj_t obj, struct malloc_type *mtype);

     DEFINE_CLASS(name, kobj_method_t *methods, size_t size);

DESCRIPTION

     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.

     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.

     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.

     The simplest way to create a kernel object is to call kobj_create() with
     a suitable class, malloc type and flags (see 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 by
     zeroing the memory and installing a pointer to the class’ method dispatch
     table.  Objects created in this way should be freed by calling
     kobj_delete().

     Clients which would like to manage the allocation of memory themselves
     should call kobj_init() with a pointer to the memory for the object and
     the class which implements it.  It is also possible to use 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.

     The functions kobj_class_compile(), kobj_class_compile_static() and
     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.
     If a class is to be used before malloc(9) is initialised, then
     kobj_class_compile_static() should be called with the class and a pointer
     to a statically allocated kobj_ops structure before the class is used to
     initialise any objects.

     To define a class, first define a simple array of kobj_method_t.  Each
     method which the class implements should be entered into the table using
     the macro 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 DEFINE_CLASS() can then
     be used to initialise a kobj_class_t structure.  The size argument to
     DEFINE_CLASS() specifies how much memory should be allocated for each
     object.

HISTORY

     Some of the concepts for this interface appeared in the device framework
     used for the alpha port of FreeBSD 3.0 and more widely in FreeBSD 4.0.

AUTHORS

     This manual page was written by Doug Rabson.