Hi,
On 2022-08-06 12:04, Olivier Valentin wrote:
I propose that we change the function signature of the functions and the
callback to include a (void *) context pointer:
SANE_Status
sanei_usb_find_devices (SANE_Int vendor, SANE_Int product, void *context,
SANE_Status (*attach) (SANE_String_Const dev, void *context));
void
sanei_usb_attach_matching_devices (const char *name, void *context,
SANE_Status (*attach) (const char *dev, void *context));
Hi Ralph,
Your change may impact proprietary backends (Brother, Canon, ...).
Thierry
Would it be possible to rely on the relative position of `const char *dev`
inside the device structure by making use of `container_of()` ?
Olivier
The name that is provided comes from a private structure in the sanei*()
library and so I don't think that the wider structure it is really
accessible to the backend.
In this situation, my backend is iterating through a list of predefined,
supported scanners, each with USB vendor and product values. If a match
is found, the callback is triggered, but the backend doesn't have any
context to determine which of the supported scanner structures it
relates to. In my case, all of the useful information is in there and
the new device structure that the callback creates needs to have a
pointer to that original structure.
What I am doing now, is using the USB vendor and product values that I
can get from sanei_usb_get_vendor_product_byname() to search again
through the list of supported scanners to get it back again.
Other backends typically stick that pointer into a global variable for
the callback function to refer to.
I prefer the first because I instinctively feel that using globals to
pass context information to the callback feels sucky, although of course
the callback is also creating a new device structure and adding it to a
global list. That's not really avoidable but for some reason it doesn't
feel as hacky as the global context variable.
For the moment, I will stick with one of the regular solutions.
Cheers,
Ralph
