Re: open SHM data only after cgps start

[Hans Kurscheidt]

Bonjour Gary,

>

What does not look correct to you?

<

As you can see, the unliked systemd starts gpsd w/ or w/out "-n" at boot. 4me therefore, systemd is not the culprit, but obviously I luck deeper understanding of the gpsd internals. However I would guess that gpsd, once started from systemd would wait for an "open" from some application wanting GPS data, in case that there's no "-n" switch given, regardless if it was started by systemd or from command level. I do not believe that something has still to go via systemd. Systemd has done its job and started gpsd and is out of the picture. In case of "-n" given, gpsd shoots GPS data into the blue, SHM is filled if there's an APP to read it or not. In this case, my APP receives data. If there's no "-n", gpsd appears 2b waiting for an incoming data request. It looks to me that there's something wrong, when it comes to SHM. Tracing in my APP the open statement

if ((gps_open(GPSD_SHARED_MEMORY, NULL, &gps_data)) == -1) {

is always returning 0 (OK), but gpsd does not provide data. So my wild ass guess is that "open" opens the SHM, but gpsd behind is not informed that there's is now somebody waiting for data, and this is IMHO independent from systemd. Only if I start cgps, gpsd gets apparently woken up and data is supplied to everybody wanting, as long as cgps runs. When cgps is closed, my app reads the rest from the /dev/ttyACM0 buffer and then no data is anymore available and the fd to /dev/ttyACM0 is closed.

So where I was caught:

There is for what I can manage very difficult to find and sparse documentation, in most cases confused across different versions. You said "don't make assumptions" but the level of documentation forces it.

I assumed

• that the "-n" switch was for command level. I supposed that starting gpsd at boot by systemd would create a gpsd instance, which could be then woken up, by any kind of "open" statement from APPs. I did and still do believe that there's no difference, if gpsd gets started from command level or from systemd (as long as systemd starts it)
  
• that the installation from source (scons) and the abook describing it would also install/overwrite the  app relevant compiler .h and linker lib and systemd UNIT++ files. Please be aware that building from source (because no uptodate binary is available) is a complex beast, which would take several days just studying all the scripts, in order to fully understand what it does. I saw dozens of screens running over the monitor, impossible to follow, what was going on. I am -excuse me- "just an enduser", may be a dumb one...
  
• that the apt-purge would remove all artifacts of previous installations
• make the linker pointing to the latest lib.

I also asked for a reference manual to better understand the functions called like open, read ..., what they do, constraints e.g. when using SHM must use "-n", don't use systemd, etc., but I found myself scrawling the internet at stackoverflow etc.

To cut a long story short, the "-n" switch must always be given, or otherwise there are cases where the "open" from app level does not make the gpsd deamon provide GPS data.

Thanks for your help anyway, because otherwise I would still be sitting here, tinkering to find data.

Best Regards

hk

Am 05.02.2021 um 22:34 schrieb Gary E. Miller:

> Yo Hans!
>
> On Fri, 5 Feb 2021 20:12:39 +0100
> Hans Kurscheidt <lve0200@gmail.com> wrote:
>
> > not so clear:
> >       
> Welcome to my world.  :-)
>
> > Always fresh boot:
> >
> > w/  "-n"  in /etc/default/gpsd
> >
> > Welcome to Debian Stretch with Armbian Linux 4.19.62-sunxi
> > System load:   0.69 0.22 0.08   Up time:       0 min
> > Memory usage:  16 % of 492MB    IP:            192.168.1.22
> > 192.168.100.1 CPU temp:      53°C
> > Usage of /:    19% of 29G
> >
> > [ 0 security updates available, 2 updates total: apt upgrade ]
> > Last check: 2021-02-05 20:04
> >
> > [ General system configuration (beta): armbian-config ]
> >
> > Last login: Fri Feb  5 19:35:38 2021 from 192.168.1.31
> >
> > hk@orangepizero:~$ systemctl status gpsd
> > ● gpsd.service - GPS (Global Positioning System) Daemon
> >     Loaded: loaded (/etc/systemd/system/gpsd.service; enabled; vendor 
> > preset: enabled)
> >     Active: active (running) since Fri 2021-02-05 20:04:03 CET; 53s
> > ago Main PID: 718 (gpsd)
> >      Tasks: 2 (limit: 4915)
> >     CGroup: /system.slice/gpsd.service
> >             └─718 /usr/sbin/gpsd -N -n /dev/ttyACM0
> >       
> As I do not know systmed(rizzle), I can't comment intelligently on that.
>
> > Feb 05 20:04:03 orangepizero systemd[1]: Started GPS (Global
> > Positioning System) Daemon.
> > hk@orangepizero:~$ ps -ef | grep gpsd
> > nobody     718     1  0 20:04 ?        00:00:00 /usr/sbin/gpsd -N -n 
> > /dev/ttyACM0
> > hk        1673  1619  0 20:05 pts/0    00:00:00 grep gpsd
> >       
> So gpsd is running.
>
> > ----------------------------
> >
> > w/out "-n"  in /etc/default/gpsd
> >
> > Welcome to Debian Stretch with Armbian Linux 4.19.62-sunxi
> > System load:   0.90 0.34 0.12   Up time:       1 min
> > Memory usage:  16 % of 492MB    IP:            192.168.1.22
> > 192.168.100.1 CPU temp:      50°C
> > Usage of /:    19% of 29G
> >
> > [ 0 security updates available, 2 updates total: apt upgrade ]
> > Last check: 2021-02-05 20:07
> >
> > [ General system configuration (beta): armbian-config ]
> >
> > Last login: Fri Feb  5 20:04:43 2021 from 192.168.1.31
> >
> > hk@orangepizero:~$ systemctl status gpsd
> > ● gpsd.service - GPS (Global Positioning System) Daemon
> >     Loaded: loaded (/etc/systemd/system/gpsd.service; enabled; vendor 
> > preset: enabled)
> >     Active: active (running) since Fri 2021-02-05 20:07:32 CET; 1min
> > 2s ago Main PID: 702 (gpsd)
> >      Tasks: 1 (limit: 4915)
> >     CGroup: /system.slice/gpsd.service
> >             └─702 /usr/sbin/gpsd -N /dev/ttyACM0
> >
> > Feb 05 20:07:32 orangepizero systemd[1]: Started GPS (Global
> > Positioning System) Daemon.
> > hk@orangepizero:~$ ps -ef | grep gpsd
> > nobody     702     1  0 20:07 ?        00:00:00 /usr/sbin/gpsd -N 
> > /dev/ttyACM0
> > hk        1655  1621  0 20:08 pts/0    00:00:00 grep gpsd
> >       
> That looks right to me.
>
> > ??
> >       
> What does not look correct to you?
>
> RGDS
> GARY
> ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
> 	gem@rellim.com  Tel:+1 541 382 8588
>
> 	    Veritas liberabit vos. -- Quid est veritas?
>     "If you can't measure it, you can't improve it." - Lord Kelvin
>