IOUserClientMethodArgumentscompletionvalueisalwaysNULL

在IOUserClient的completion方法中使用setAsyncReference64方法，将completion的值传递到IOUserClientMethodArguments的asyncReference字段中，如下所示：

IOReturn MyUserClient::foo(uint32_t type, IOUserClientMethodArguments* arguments,
	vm_size_t argumentsSize)
{
    // perform some action
    
    // set the async reference for the completion
    OSAsyncReference asyncRef;
    asyncRef.asyncInvocation = arguments->asyncReference;
    asyncRef.asyncReference = NULL;
    arguments->setAsyncReference64(asyncRef);
    
    return kIOReturnSuccess;
}

void MyUserClient::fooCompletion(void* target, void* parameter, IOReturn status, UInt32 bufferSize)
{
    // retrieve the async reference and completion object
    IOUserClientMethodArguments* arguments = (IOUserClientMethodArguments*)parameter;
    OSAsyncReference asyncRef = *(OSAsyncReference*)arguments->asyncReference;
    OSObject* completion = (OSObject*)asyncRef.asyncReference;
    
    // perform completion with async reference values
    if (completion != NULL) {
        completion->retain();
        completion->release();
    }
}