[sane-devel] Developers take note: Backend initialization

[m. allan noah]

On Tue, Dec 7, 2010 at 6:04 PM, Olaf Meeuwissen
<olaf.meeuwissen at avasys.jp> wrote:
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> On 2010年12月07日 22:32, m. allan noah wrote:
>> On Tue, Dec 7, 2010 at 12:43 AM, Olaf Meeuwissen
>> <olaf.meeuwissen at avasys.jp> wrote:
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>>> On 2010-12-07 12:17, m. allan noah wrote:
>>>> [...]
>>>> 1. Move calls to sanei_usb_init() to sane_get_devices()
>>>
>>> How does this work when a frontend does not call sane_get_devices() and
>>> immediately tries to sane_open() a USB device?
>>
>> [...]
>>> Backends can safely call sanei_usb_init() in both sane_init() and
>>> sane_get_devices().  They should just move the device discovery logic to
>>> sane_get_devices() and call that from sane_init() if they really still
>>> want to do discovery at sane_init().
>>
>> Actually, since sane_init() calls are usually followed by
>> sane_get_devices(), it makes more sense to do this within sane_open()
>> if no device name is given or if the list is not already loaded. This
>> is the technique I used for the fujitsu backend.
>
> The problem is with "usually".  Backend maintainers have no guarantee
> from the spec that frontends will do so.

Certainly, but the 'usually' means than most of the time, the backend
would search for devices in sane_init, and do it again a millisecond
later in sane_get_devices.

>
> In the epkowa backend I also call sane_get_devices() in sane_open() if
> it hasn't been called yet but that's just because I'm too lazy to:
>  - decide what the first available device is in case of no device name
>  - bother with validating the device name properly (it's easier to see
> whether it's in the list)

If its lazy but it works, it's not lazy :)

So, lets consider my original request amended with this later
suggestion- watch out for the case of a front-end which does not call
sane_get_devices.

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