On Thu, Feb 10, 2005 at 12:22:26PM -0000, rajesh ms hit keys to express the following:
>
>
> 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) );})
One question here,
Why is it not
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)offsetof(type,member)) *__mptr = (ptr); \
(type *)( (char *)__mptr - offsetof(type,member) );})
( offsetof() is a macro defined in include/linux/stddef.h, gcc does not support
it ). Is there any reason?
> 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) );
>
> });
>
> 1)What value will be stored in drv? Is there any rule to find this, ( Like last statement ----- My Guess )
This is a gcc extension.
>From info page of gcc,
A compound statement enclosed in parentheses may appear as an expression
in GNU C. This allows you to use loops, switches, and local variables
within an expression.
....
#define max(a,b) ((a) > (b) ? (a) : (b))
But this definition computes either A or B twice, with bad results if
the operand has side effects. In GNU C, if you know the type of the
operands (here let's assume `int'), you can define the macro safely as
follows:
#define maxint(a,b) \
({int _a = (a), _b = (b); _a > _b ? _a : _b; })
For more info, look in the info page of gcc, Statements and Declarations in
Expressions secion.
HTH,
Om.
--
You will gain money by an immoral action.
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