Re: [gpsd-dev] Need p/d help with gpsd and or gps HW - no response after gps polling

[Fred Wright]

On Mon, 8 Aug 2016, Steve Lang wrote:

> We found out what was causing the problem. It appears to be user
> error. User was executing:
>
> gpsmon /dev/gpsd0 while gpsd was running.
>
> ls -al /dev/gpsd0
> /dev/gpsd0 -> ttyAMA0
>
> I informed user that 2 programs can not access the same serial port at
> the same time.

Well, actually, the problem isn't that it doesn't work; it's that it
"works" but poorly. :-) Each program gets some of the characters, but
which one gets which characters is determined by the wakeup timing.

If the OS properly understood that the serial driver isn't usefully
shareable, it would just give "device or resource busy" to the second
program.  Unfortunately it doesn't, and the access works just well enough
to appear shareable to someone who isn't looking at the data too closely.

A good reason to run gpsd on a time server is so that one can check on the
GPS status at any time without interfering with NTP.

> Once user stopped doing this the problem stopped,

Ye olde PEBKAC issue. :-)

Fred Wright