On Thu, Feb 10, 2005 at 12:22:26PM -0000, rajesh ms wrote:
> ?
> Hi all,
>
> I have got a question.
>
> I was going through pci_Driver.c. Then i found
>
>
> drv = to_pci_driver(dev->driver);
>
> and
> drv is of type struct pci_driver*
>
> #define to_pci_driver(drv) container_of(drv,struct pci_driver, driver)
>
> #define container_of(ptr, type, member)
> ({ \
> const typeof( ((type *)0)->member ) *__mptr = (ptr); \
> (type *)( (char *)__mptr - offsetof(type,member) );})
>
>
>
> Then i substituted for all parameters and got this
>
> drv = ({
>
> const struct device_driver *__mptr = (dev->driver);
> (struct pci_driver*)( (char *)__mptr - offsetof(struct pci_driver,driver) );
>
> });
Yeah, fun isn't it :)
> 1)What value will be stored in drv? Is there any rule to find this,
It will be a pointer to the pci_driver structure.
> 2)We are declaring const struct device_driver pointer and substracting
> the offset value (struct device_driver * is a member of struct
> pci_driver )and getting the address of struct pci_driver which may
> contain some data that is irrelevent to struct pci_driver. If this is
> the return value whether this will cause problems? How this is dealt
> in the code.
Look at the definition of struct pci_driver. It contains a struct
device_driver. When passed a pointer to a struct device_driver, the
code "knows" that it is safe to treat it as a part of the struct
pci_driver. So it moves backwards in memory to get the real pointer to
the struct pci_driver and everyone is happy.
This convention is used a _lot_ in the driver model code in the kernel
to allow for "inheritance" in C.
Hope this helps,
greg k-h
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