Re: [Libusb-devel] reading from a barcode scanner that acts like a keyboard

[Peter S. <stu...@cd...>]

On Tue, Oct 31, 2006 at 08:08:04PM +0800, Brice Rebsamen wrote:
> Hello,
> 
> I have a USB barcode scanner that acts like a keyboard: whenever a
> barcode is read it sends the code where the keyboard would normally
> write. I am writing some code in C to read from the scanner. This is
> my first application using libusb, and it is the first time I am
> working with a USB device.

USB is great in many ways, very flexible, but it may be unintuitive
to newcomers.

The keyboard-equivalent bytes sent to applications are not sent in
the clear over the bus. They're sent in blocks of data called
reports, which are specific to the USB HID device class.

Long story short there's lots of code to support this device already
and I'm not sure libusb is the best way to go. Please read recent
discussion on the list about why trying to read files from a USB
memory key with libusb is not the best way to go. Granted this is
less complicated, but the principle is the same.

What requirements do you have on portability? If you're writing a
Linux-only application, perhaps using the other end of the existing
HID/Input software in the kernel is better? Or even change your
application so that it doesn't matter which human interface device is
used to enter the barcode number. This can be a feature, think torn
barcode with numbers below intact. It will depend on your usage
scenario and requirements. USB is a lot like an expert consultant
- if you know exactly what you want it can all be incredibly
rewarding for everyone involved. If you're not sure of what you want
or fail to express it sufficiently clearly, all you get is more
problems.

If you need portability, depending on *_np() is a no-no.
(np is short for non-portable)

As for re-attaching, I'm not sure that's possible.

Another option may be the hiddev kernel interface, which also lets
the kernel hid driver remain attached. There's a libhid too, but
unfortunately it doesn't use hiddev yet. I just need some free time..
;)


//Peter