Re: [Bluez-users] Automatic Object Pushing when Devices Enter Range

[Marcel Holtmann <marcel@holtmann.org>]

Hi Cris,

> > I'd really like is a way to register or specify a script to be called
> > whenever a device comes within range.  I'd expect the script to be passed the
> > address of the device, allowing me to interrogate it regarding it's
> > capabilities, and so forth.
> > 
> > The alternative is, of course, to call "hcitool scan" every few seconds,
> > compare with a log of the known or seen ID's, and process from there, but as
> > we were all taught in grammar school CS, polling is bad mm'kay.
> 
> The way bluetooth allows to discover nearby devices will not allow to
> avoid some sort of polling: One device needs to be in inquiry scan
> mode, the other needs to do an inquiry; one needs to be trying to
> receive an inquiry signal, the other must send it.
> 
> It depends a bit on the volume of traffic you expect here. If you
> don't expect ever more than a few devices per hour and can accept not
> to send the file to more than one device at the same time, you can use
> the 'inq' command to hcitool. At least the last time I checked, the
> 'scan' command to hcitool is misnamed. It's reall only an inquiry with
> a subsequent request for the remote name (more work to do for
> bluetooth and thus more time lost for data traffic); hcitool 'scan'
> has nothing to do with a `scan' in bluetooth terminology, which is the
> listening of the device for inquiry or page attempts. Anyway, unless
> you want to log those `userfriendly' names, it's useless in an
> unattended situation. In this case of very low and limited traffic,
> you could write this using as a script, but you are likely to run into
> problems in the area of authentication (pin codes) for the way bluez
> handles things.
> 
> As volume of traffic grows, the bluez becomes inappropriate:
> Bluetooth's basic concepts are event-oriented and designed for small
> and/or embedded devices. Both are defeated by bluez: The library locks
> in a select loop while waiting for the answering event (rather than
> let the application do this, which could handle other things in the
> meanwhile), and the use of dbus is certainly not for small devices
> (just look at the huge amount of libraries pulled in for installing
> dbus). You can get around the lock by forking each command, but then,
> you need to do some IPC which is much more complicated than really
> needed. In the end, a solution for some a bit more serious traffic
> will require you to write this in a language like C. Either you use
> the lower level functions in the bluetooth library, avoiding that
> loop, or you write it all by yourself from scratch (my choice). But in
> this situation, you still might want to consider using two (or more)
> bluetooth radios, using one only in periodic inquiry mode, as you can
> not detect a nearby device while transmitting the file.

the use of two radios for these kind of things is always a good idea and
what you want is periodic inquiry. And to be quite clear. The only sane
way of doing this is over the D-Bus API which has a really clean
interface to the inquiry and periodic inquiry commands. It will take
care of everything if the blocking inquiry() from the library is not
your favorite choice. Writing everything from scratch is a total waste
of time.

The D-Bus is not as bad everybody tries to tell you. For the hcid for
example we only need the additional D-Bus library:

# ldd /usr/sbin/hcid
        libdbus-1.so.3 => /usr/lib/libdbus-1.so.3 (0x00002ae5ac039000)
        libbluetooth.so.2 => /usr/lib/libbluetooth.so.2 (0x00002ae5ac16f000)
        libc.so.6 => /lib/libc.so.6 (0x00002ae5ac285000)
        libnsl.so.1 => /lib/libnsl.so.1 (0x00002ae5ac4c6000)
        /lib64/ld-linux-x86-64.so.2 (0x00002ae5abf1c000)

Regards

Marcel



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