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Re: [gpsd-users] Blocked device when using ldattach

From: Eric S. Raymond
Subject: Re: [gpsd-users] Blocked device when using ldattach
Date: Sun, 26 Feb 2012 09:32:54 -0500
User-agent: Mutt/1.5.21 (2010-09-15)

Marc Sowen <address@hidden>:
> I'm trying to use gpsd together with another application which only uses 
> the PPS-informationen provided by the LinuxPPS framework. Because the 
> ldattach process creates a file descripter pointing to my /dev/ttyS0 
> device, gpsd fails to get access:
> ~ # gpsd -n -N -D3 /dev/ttyS0
> gpsd:INFO: launching (Version 3.4)
> gpsd:ERROR: can't create IPv6 socket
> gpsd:INFO: listening on port gpsd
> gpsd:INFO: NTPD ntpd_link_activate: 1
> gpsd:INFO: stashing device /dev/ttyS0 at slot 0
> gpsd:INFO: opening GPS data source type 2 at '/dev/ttyS0'
> gpsd:ERROR: /dev/ttyS0 already opened by another process
> gpsd:ERROR: initial GPS device /dev/ttyS0 open failed
> The reason for that is that the function "fusercount" scans the /proc 
> directory for any process having /dev/ttyS0 open. I would disable the 
> fusercount function for my setup now, but I feel a bit uneasy doing so: Is 
> there any drawback having the ldattach process running on the same device 
> as gpsd? BTW: I start ldattach this way:
> ldattach PPS /dev/ttyS0
> Thanks for your help!
> Regards,
> Marc Sowen
> Research Engineer

I don't know what your attached line discipline is doing, but if it's ever 
reading from the device that could cause the GPS not to see data.  

If all it's doing is watching the PPS for a line transition, you might
in theory be OK but this setup makes me extremely nervous about
unplanned interactions.  GPSD assumes it's in sole charge of the
devices it's managing; violations of that assumption could produce
subtle bugs.
                <a href="";>Eric S. Raymond</a>

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