2020-09-11 01:15:25 +03:00
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===========================
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The QEMU Object Model (QOM)
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===========================
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2020-09-23 19:21:39 +03:00
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.. highlight:: c
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The QEMU Object Model provides a framework for registering user creatable
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types and instantiating objects from those types. QOM provides the following
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features:
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2020-10-03 05:54:20 +03:00
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- System for dynamically registering types
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- Support for single-inheritance of types
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- Multiple inheritance of stateless interfaces
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2020-09-23 19:21:39 +03:00
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.. code-block:: c
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:caption: Creating a minimal type
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#include "qdev.h"
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#define TYPE_MY_DEVICE "my-device"
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// No new virtual functions: we can reuse the typedef for the
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// superclass.
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typedef DeviceClass MyDeviceClass;
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typedef struct MyDevice
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{
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DeviceState parent;
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int reg0, reg1, reg2;
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} MyDevice;
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static const TypeInfo my_device_info = {
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.name = TYPE_MY_DEVICE,
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.parent = TYPE_DEVICE,
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.instance_size = sizeof(MyDevice),
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};
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static void my_device_register_types(void)
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{
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type_register_static(&my_device_info);
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}
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type_init(my_device_register_types)
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In the above example, we create a simple type that is described by #TypeInfo.
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#TypeInfo describes information about the type including what it inherits
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from, the instance and class size, and constructor/destructor hooks.
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Alternatively several static types could be registered using helper macro
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DEFINE_TYPES()
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.. code-block:: c
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static const TypeInfo device_types_info[] = {
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{
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.name = TYPE_MY_DEVICE_A,
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.parent = TYPE_DEVICE,
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.instance_size = sizeof(MyDeviceA),
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},
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{
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.name = TYPE_MY_DEVICE_B,
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.parent = TYPE_DEVICE,
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.instance_size = sizeof(MyDeviceB),
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},
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};
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DEFINE_TYPES(device_types_info)
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Every type has an #ObjectClass associated with it. #ObjectClass derivatives
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are instantiated dynamically but there is only ever one instance for any
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given type. The #ObjectClass typically holds a table of function pointers
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for the virtual methods implemented by this type.
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Using object_new(), a new #Object derivative will be instantiated. You can
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cast an #Object to a subclass (or base-class) type using
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object_dynamic_cast(). You typically want to define macro wrappers around
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OBJECT_CHECK() and OBJECT_CLASS_CHECK() to make it easier to convert to a
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specific type:
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.. code-block:: c
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:caption: Typecasting macros
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#define MY_DEVICE_GET_CLASS(obj) \
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OBJECT_GET_CLASS(MyDeviceClass, obj, TYPE_MY_DEVICE)
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#define MY_DEVICE_CLASS(klass) \
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OBJECT_CLASS_CHECK(MyDeviceClass, klass, TYPE_MY_DEVICE)
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#define MY_DEVICE(obj) \
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OBJECT_CHECK(MyDevice, obj, TYPE_MY_DEVICE)
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Class Initialization
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====================
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Before an object is initialized, the class for the object must be
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initialized. There is only one class object for all instance objects
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that is created lazily.
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Classes are initialized by first initializing any parent classes (if
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necessary). After the parent class object has initialized, it will be
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copied into the current class object and any additional storage in the
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class object is zero filled.
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The effect of this is that classes automatically inherit any virtual
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function pointers that the parent class has already initialized. All
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other fields will be zero filled.
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Once all of the parent classes have been initialized, #TypeInfo::class_init
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is called to let the class being instantiated provide default initialize for
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its virtual functions. Here is how the above example might be modified
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to introduce an overridden virtual function:
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.. code-block:: c
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:caption: Overriding a virtual function
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#include "qdev.h"
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void my_device_class_init(ObjectClass *klass, void *class_data)
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{
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DeviceClass *dc = DEVICE_CLASS(klass);
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dc->reset = my_device_reset;
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}
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static const TypeInfo my_device_info = {
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.name = TYPE_MY_DEVICE,
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.parent = TYPE_DEVICE,
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.instance_size = sizeof(MyDevice),
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.class_init = my_device_class_init,
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};
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Introducing new virtual methods requires a class to define its own
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struct and to add a .class_size member to the #TypeInfo. Each method
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will also have a wrapper function to call it easily:
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.. code-block:: c
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:caption: Defining an abstract class
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#include "qdev.h"
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typedef struct MyDeviceClass
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{
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DeviceClass parent;
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void (*frobnicate) (MyDevice *obj);
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} MyDeviceClass;
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static const TypeInfo my_device_info = {
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.name = TYPE_MY_DEVICE,
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.parent = TYPE_DEVICE,
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.instance_size = sizeof(MyDevice),
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.abstract = true, // or set a default in my_device_class_init
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.class_size = sizeof(MyDeviceClass),
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};
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void my_device_frobnicate(MyDevice *obj)
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{
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MyDeviceClass *klass = MY_DEVICE_GET_CLASS(obj);
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klass->frobnicate(obj);
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}
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Interfaces
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==========
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Interfaces allow a limited form of multiple inheritance. Instances are
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similar to normal types except for the fact that are only defined by
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their classes and never carry any state. As a consequence, a pointer to
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an interface instance should always be of incomplete type in order to be
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sure it cannot be dereferenced. That is, you should define the
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'typedef struct SomethingIf SomethingIf' so that you can pass around
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``SomethingIf *si`` arguments, but not define a ``struct SomethingIf { ... }``.
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The only things you can validly do with a ``SomethingIf *`` are to pass it as
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an argument to a method on its corresponding SomethingIfClass, or to
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dynamically cast it to an object that implements the interface.
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Methods
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=======
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2020-10-03 05:54:22 +03:00
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A *method* is a function within the namespace scope of
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2020-09-23 19:21:39 +03:00
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a class. It usually operates on the object instance by passing it as a
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strongly-typed first argument.
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If it does not operate on an object instance, it is dubbed
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2020-10-03 05:54:22 +03:00
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*class method*.
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2020-09-23 19:21:39 +03:00
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Methods cannot be overloaded. That is, the #ObjectClass and method name
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uniquely identity the function to be called; the signature does not vary
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except for trailing varargs.
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2020-10-03 05:54:22 +03:00
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Methods are always *virtual*. Overriding a method in
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2020-09-23 19:21:39 +03:00
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#TypeInfo.class_init of a subclass leads to any user of the class obtained
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via OBJECT_GET_CLASS() accessing the overridden function.
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The original function is not automatically invoked. It is the responsibility
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of the overriding class to determine whether and when to invoke the method
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being overridden.
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To invoke the method being overridden, the preferred solution is to store
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the original value in the overriding class before overriding the method.
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This corresponds to ``{super,base}.method(...)`` in Java and C#
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respectively; this frees the overriding class from hardcoding its parent
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class, which someone might choose to change at some point.
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.. code-block:: c
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:caption: Overriding a virtual method
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typedef struct MyState MyState;
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typedef void (*MyDoSomething)(MyState *obj);
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typedef struct MyClass {
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ObjectClass parent_class;
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MyDoSomething do_something;
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} MyClass;
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static void my_do_something(MyState *obj)
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{
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// do something
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}
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static void my_class_init(ObjectClass *oc, void *data)
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{
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MyClass *mc = MY_CLASS(oc);
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mc->do_something = my_do_something;
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}
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static const TypeInfo my_type_info = {
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.name = TYPE_MY,
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.parent = TYPE_OBJECT,
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.instance_size = sizeof(MyState),
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.class_size = sizeof(MyClass),
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.class_init = my_class_init,
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};
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typedef struct DerivedClass {
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MyClass parent_class;
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MyDoSomething parent_do_something;
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} DerivedClass;
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static void derived_do_something(MyState *obj)
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{
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DerivedClass *dc = DERIVED_GET_CLASS(obj);
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// do something here
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dc->parent_do_something(obj);
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// do something else here
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}
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static void derived_class_init(ObjectClass *oc, void *data)
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{
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MyClass *mc = MY_CLASS(oc);
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DerivedClass *dc = DERIVED_CLASS(oc);
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dc->parent_do_something = mc->do_something;
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mc->do_something = derived_do_something;
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}
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static const TypeInfo derived_type_info = {
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.name = TYPE_DERIVED,
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.parent = TYPE_MY,
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.class_size = sizeof(DerivedClass),
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.class_init = derived_class_init,
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};
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Alternatively, object_class_by_name() can be used to obtain the class and
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its non-overridden methods for a specific type. This would correspond to
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``MyClass::method(...)`` in C++.
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The first example of such a QOM method was #CPUClass.reset,
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another example is #DeviceClass.realize.
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Standard type declaration and definition macros
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===============================================
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A lot of the code outlined above follows a standard pattern and naming
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convention. To reduce the amount of boilerplate code that needs to be
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written for a new type there are two sets of macros to generate the
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common parts in a standard format.
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A type is declared using the OBJECT_DECLARE macro family. In types
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which do not require any virtual functions in the class, the
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OBJECT_DECLARE_SIMPLE_TYPE macro is suitable, and is commonly placed
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in the header file:
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.. code-block:: c
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:caption: Declaring a simple type
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2020-10-03 05:54:24 +03:00
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OBJECT_DECLARE_SIMPLE_TYPE(MyDevice, my_device,
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MY_DEVICE, DEVICE)
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2020-09-23 19:21:39 +03:00
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This is equivalent to the following:
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.. code-block:: c
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:caption: Expansion from declaring a simple type
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2020-10-03 05:54:21 +03:00
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typedef struct MyDevice MyDevice;
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typedef struct MyDeviceClass MyDeviceClass;
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2020-09-23 19:21:39 +03:00
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2020-10-03 05:54:21 +03:00
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G_DEFINE_AUTOPTR_CLEANUP_FUNC(MyDeviceClass, object_unref)
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2020-09-23 19:21:39 +03:00
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2020-10-03 05:54:21 +03:00
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#define MY_DEVICE_GET_CLASS(void *obj) \
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OBJECT_GET_CLASS(MyDeviceClass, obj, TYPE_MY_DEVICE)
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#define MY_DEVICE_CLASS(void *klass) \
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OBJECT_CLASS_CHECK(MyDeviceClass, klass, TYPE_MY_DEVICE)
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#define MY_DEVICE(void *obj)
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OBJECT_CHECK(MyDevice, obj, TYPE_MY_DEVICE)
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2020-09-23 19:21:39 +03:00
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2020-10-03 05:54:21 +03:00
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struct MyDeviceClass {
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DeviceClass parent_class;
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};
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2020-09-23 19:21:39 +03:00
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The 'struct MyDevice' needs to be declared separately.
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If the type requires virtual functions to be declared in the class
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struct, then the alternative OBJECT_DECLARE_TYPE() macro can be
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used. This does the same as OBJECT_DECLARE_SIMPLE_TYPE(), but without
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the 'struct MyDeviceClass' definition.
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To implement the type, the OBJECT_DEFINE macro family is available.
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In the simple case the OBJECT_DEFINE_TYPE macro is suitable:
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.. code-block:: c
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:caption: Defining a simple type
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2020-10-03 05:54:21 +03:00
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OBJECT_DEFINE_TYPE(MyDevice, my_device, MY_DEVICE, DEVICE)
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2020-09-23 19:21:39 +03:00
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This is equivalent to the following:
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.. code-block:: c
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:caption: Expansion from defining a simple type
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2020-10-03 05:54:21 +03:00
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static void my_device_finalize(Object *obj);
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static void my_device_class_init(ObjectClass *oc, void *data);
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static void my_device_init(Object *obj);
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static const TypeInfo my_device_info = {
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.parent = TYPE_DEVICE,
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.name = TYPE_MY_DEVICE,
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.instance_size = sizeof(MyDevice),
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.instance_init = my_device_init,
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.instance_finalize = my_device_finalize,
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.class_size = sizeof(MyDeviceClass),
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.class_init = my_device_class_init,
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};
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static void
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my_device_register_types(void)
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{
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type_register_static(&my_device_info);
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}
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type_init(my_device_register_types);
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2020-09-23 19:21:39 +03:00
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This is sufficient to get the type registered with the type
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system, and the three standard methods now need to be implemented
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along with any other logic required for the type.
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If the type needs to implement one or more interfaces, then the
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OBJECT_DEFINE_TYPE_WITH_INTERFACES() macro can be used instead.
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This accepts an array of interface type names.
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.. code-block:: c
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:caption: Defining a simple type implementing interfaces
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2020-10-03 05:54:21 +03:00
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OBJECT_DEFINE_TYPE_WITH_INTERFACES(MyDevice, my_device,
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MY_DEVICE, DEVICE,
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{ TYPE_USER_CREATABLE },
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{ NULL })
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2020-09-23 19:21:39 +03:00
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If the type is not intended to be instantiated, then then
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the OBJECT_DEFINE_ABSTRACT_TYPE() macro can be used instead:
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.. code-block:: c
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:caption: Defining a simple abstract type
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2020-10-03 05:54:24 +03:00
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OBJECT_DEFINE_ABSTRACT_TYPE(MyDevice, my_device,
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MY_DEVICE, DEVICE)
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2020-09-23 19:21:39 +03:00
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API Reference
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-------------
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2020-09-11 01:15:25 +03:00
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.. kernel-doc:: include/qom/object.h
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