Re: [gpsd-dev] Minimum working example for libgps

[Gary E. Miller]

Yo Hal!

On Tue, 07 Nov 2017 13:50:12 -0800
Hal Murray <address@hidden> wrote:

> address@hidden said:
> >> A "main loop" makes sense if you have a typical GUI program.  
> > Or any long lasting program.  The program has to be looping/sleeping
> > somewhere to be ready for callbacks, user input, or gpsd input.   

> Consider a main loop that gets input from X, Y, and Z.  What are the 
> constraints needed to make a main loop work?

None that I see, from just that requirement.

> If all the APIs have a poll option, you can sleep for a short time
> when all have no input.

Or not sleep.

> If one API has a timeout, you can wait there yet get back often
> enough to poll the others.

Yes, the API timeout is almost equivalent to a main loop sleep.

And all still in one main loop.  So I miss the point?

> > I'll bet he does have a loop, he has to be waiting somewhere for
> > the thread to get back to him.  Adding another thread is just an
> > overcomplication.  
> 
> In a thread friendly environment, that code is pretty simple.  Yes,
> it's more work if you don't have any threads yet.

And you still need a loop in the gpsd reading thread.  So nothing saved.

> > Wherever he is sitting, waaiting for work, that is a loop.  Just
> > poll gpsd in there.   
> 
> If you have a multi-threaded program, none of the read/do loops may
> be the main one.  Consider a server with one thread per client.

It happens, but rare and fragile.     And you still have loops.

The OP said no loops.

> An alternate way of looking at things is that the OP doesn't like the
> API that gpsd provides.

Hard to say: not like, or not understand?

> The gpsd model is that it gives you all the
> data it gets and you have to pull often enough to keep the buffer
> from clogging.  An alternative model is that it digests everything,
> saves the latest values, and you can call in (no blocking) and get
> the latest/current value of X.

Yeah, a request last data mode might be interesting, but then gpsd,
or libgpsd, gets a lot more complicated as it needs to save a lot
of extra state variables.

> Many years ago, I was on the fringe of various API wars.  Should the
> API be push, callback, pull, or...?  We got pretty good at writing
> tiny adapter modules.  They are simple in a thread friendly
> environment.  I think the common case was a buffer, a thread, and
> less than a page of code.

If you think you can do it in one page of code, I'd like to see it.

RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
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