[sane-devel] sane_init(), sane_get_devices() and SANE_Authorization_Callback()

[m. allan noah]

2010/5/26 Kåre Särs <kare.sars at iki.fi>:
> Hi,
>
> I have a couple of somewhat related problems regarding  sane_get_devices(),
> sane_init() and SANE_Authorization_Callback().
>
> 1) sane_get_devices() does not update the available devices list without
> calling sane_init(). I asked about this some time ago and got the answer that
> it was a bug. With the version in (K)Ubuntu Lucid and the pashazz-sane PPA, I
> still have the problem that the scanner is not found if turned on after
> sane_init(). I use the epson2 and v4l backends. With epson2 I get a crash if
> the scanner is on the first time but is turned off before the second
> sane_get_devices().
>
> Do you have any hints on where to start digging?

Many backends do not properly re-scan for devices. Those backends
should be fixed. Which version of sane-backends is this?

> 2) With  sane_get_devices() I can get the vendor, model, type and backend
> names for the available scanners. Is there a way to get the same information
> about the currently opened device? I do not find an API for that.  Have I
> missed something? The problem is that I can specify a backend like "test" and
> sane_open() will open either "test:0" or "test:1", but I do not know which one
> and I do not get the vendor, model and type strings.

Correct. This is a deficiency in the sane API. You have to keep the
device list, and open a device by the given name. If you ask a backend
for the NULL device, it is impossible to know which one you will get.

> 3) libksane can be used in multi threaded applications and at the moment I'm
> making sure that sane_exit() is only called when the last instance exits, as
> sane_exit() will invalidate the other running instances. Should I do the same
> for sane_init()?. So far it has not resulted in problems, but when I tried a
> workaround for the  sane_get_devices() problem I noticed that I got unwanted
> behavior.

sane-backends is not designed to be thread safe, as only one function
(sane_cancel) is explicitly required to be reentrant. So, arrange your
app to only call sane from one thread. But, more to your question- you
should only call sane_init once. If you call it again (particularly
without calling sane_exit first), I would not be surprised to hear of
bad things happening.

> 4) Again in a multi threaded application there might be two scanners that
> require authentication. The resource string that is provided to
> SANE_Authorization_Callback() contains the backend name and a md5 hash, but it
> does not contain the same ":libusb:001:004" extra info type of sting that
> sane_get_devices() returns for the backend.

Which backend?

> How should I identify the different scanners with the same backend? Or is the
> authorization restricted to the backend with the same authentication for all
> the scanners with the same backend?

Personally, I'd probably skip the auth callback...

allan
-- 
"The truth is an offense, but not a sin"