Re: Struct Inheritance in drivers/ata/

[Peter Hamilton <peterghamilton@xxxxxxxxx>]

I see that it works, but I'm not exactly following how.  Maybe I just don't have enough C experience.  But I can't seem to duplicate that functionality.

I understand the way I've normally seen inheritance, where:

struct Child {

  struct Parent p;

  int somethingOnlyForChildren;

}

Or something similar.  Then the Child struct can be casted as a Parent, and since there is no padding before the first piece of data in a struct, every parent attribute is accessed as though the struct were a Parent.

I see how it all lines up in memory, but in the case of the kernel code I can't quite follow it.  Wouldn't everything assigned to 'inherits' be so much farther down in memory?  I feel like I'm missing something.  Is there a special way they all need to be accessed?

- Peter

On Thu, May 26, 2011 at 7:23 PM, Greg KH <greg@xxxxxxxxx> wrote:

On Thu, May 26, 2011 at 02:46:08PM -0600, Peter Hamilton wrote:
> The code in drivers/ata/ uses an implementation of inheritance that I have not
> seen before.  It's only briefly explained in the header file ( include/linux/
> libata.h:885):
>
> /*  
>  * ->inherits must be the last field and all the preceding
>  * fields must be pointers.
>  */
>
> The structs are then initialized with .inherits assigned first:
>
> drivers/ata/sata_nv.c:475
>
> static struct ata_port_operations nv_nf2_ops = {                              
>         .inherits               = &nv_generic_ops,
>         .freeze                 = nv_nf2_freeze,                              
>         .thaw                   = nv_nf2_thaw,                                
> };      
>
>
> Is this actually implementing inheritance?  Why do all preceding fields need to
> be pointers?
>
> As far as I can tell, this style is only found in the ata drivers.  

I think you are right, but more subsystems need to emulate it, it is
very powerful and works very well.  I have been wanting to convert the
usb-serial layer to use the same thing one of these days.

> Could anyone explain how this works?

The code is all there that shows it, but basically the driver is telling
the core to "use this type of functions, but if I set any others, use
them instead."  It's a nicer way of doing inheritance in C than we do in
other places in the kernel where we are a bit more "verbose" in making
it happen.

hope this helps,

greg k-h

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