Data Type Size Differences

One of the most common errors when porting a C driver core to the I/O Kit is assuming that a data type in C will have the same size as a data type with a similar name in C++. This is not always the case.

A good rule is to always make sure you use exactly the same data type (with the same name) on both sides when passing data between the C and C++ portions of your driver. This can save you lots of headaches and unnecessary debugging in the future.

Handling C to C++ Callbacks

C code cannot call class methods directly. This poses a bit of a problem if you are writing code where a C++ method needs to be passed as a callback into your C core code.

To get around this issue, you should create an intermediate relay function within a C++ file. This function is a standard C function, but compiled using the C++ compiler. This function can then be called by C functions and easily make calls into C++ class methods.

The following rules must be followed:

• The C++ function must be declared static.

• One of its argument must be a pointer to an instance of your class.

• Calls to class functions must then be made with respect to that pointer.

For example, you might write a relay function that looks like this:

static int org_mklinux_iokit_swim3_driver_dosomething(

    org_mklinux_iokit_swim3_driver *self, int something_arg)

{

    dIOLog(DEBUG_GENERAL, "dosomething: enter (0x%x)\n",

        (unsigned int)self);

    self->dosomething(something_arg);

}

For your callback, you would need to pass two things into the C core code: the address of the class instance (the value of the this pointer), and the address of this function. After that, the C code can call the class function dosomething by doing something like this:

ret = (*funcptr)(classptr, argument);

where funcptr is a pointer to the relay function, classptr is a pointer to the class instance whose dosomething method is to be called, and argument is the argument to be passed to the dosomething method.