On 2012-06-06 13:28, walter harms wrote:
Am 06.06.2012 13:20, schrieb Ian Abbott:
On 2012-06-06 10:49, Dan Carpenter wrote:
On Wed, Jun 06, 2012 at 11:45:43AM +0200, walter harms wrote:
Am 06.06.2012 11:28, schrieb Ian Abbott:
On 2012-06-05 12:01, Dan Carpenter wrote:
On Tue, Jun 05, 2012 at 12:06:10PM +0200, walter harms wrote:
Hi all,
the patch is fine with me but i have a few basic questions:
Why the (void *) at all ? it returns a name what is a const char *.
We're really returning a pointer to a private struct, it's just that
the first element on the struct always has to be a pointer to char *.
To be pedantic, it's really returning a pointer to some member of type
'const char *' within a private struct. To make life easier for
themselves, those drivers make that the first member of the private
struct so a void pointer to the member is also a void pointer to the
struct.
I do not like it. It is confusing.
Yep.
But at least the confusion is concentrated in a single place! From the
individual drivers' point of view it's fairly simple (as long as it puts
the 'const char pointer to board name' member at the top of its private
data structure).
Sometimes you can not write "clean" code but a few lines of comments are
then a must.
Are these struct comedi_driver *driv of different size every time ?
Nope.
It's the private board information data structures for each driver that
are different sizes, but the size is passed in the comedi comedi_driver
'offset' member along with the 'board_name' and 'num_names' members. Not
all drivers set those members and just use the 'driver_name' to match
any supported device, but I digress.
If i can believe the comments in the structure offset is the distance to the next
name /* offset in bytes from one board name pointer to the next */
and the name is not the first entry it is the second.
This board name pointer is in a driver-specific, board information
struct. All of them make it the first member of the driver-specific
struct for convenience, although they don't need to - they could use
offsetof() or container_of() to convert the pointer to the member to a
pointer to the containing struct. The comedi core does not care about
the driver-specific struct but needs to be told how to access the board
names in an initialized array of these structs.
That means any program assuming that this code returns a (struct comedi_driver *)
has to do some correction or it will be off-bye-one sizeof(struct comedi_driver *).
Please correct me if i am wrong:
I have no clue how dev->board_ptr is used, if it works it is fine with me.
If the low-level driver initializes 'board_name', 'offset' and
'num_names' to something non-zero in its 'struct comedi_driver' then
'dev->board_ptr' will point to one of the board name pointers in
initialized array of driver-specific, board information structs when the
driver's attach() hook is called. If 'num_names' is 0, 'dev->board_ptr'
will be NULL when the attach() hook is called. In either case, the
low-level driver is free to change the value of 'dev->board_ptr' for its
own purposes, although generally it will be pointed to some element of
its array of board information structs.
--
-=( Ian Abbott @ MEV Ltd. E-mail: <abbotti@xxxxxxxxx> )=-
-=( Tel: +44 (0)161 477 1898 FAX: +44 (0)161 718 3587 )=-
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