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Hacker's Guide to gpsd
This is not a complicated piece of code. Essentially, it spins in a loop
polling for input from one of three sources:
1) The GPS, connected via serial or USB device.
2) A client making requests over a TCP/IP port.
4) A DGPS server issuing periodic differential-GPS updates.
GPS input updates an internal data structure called gNMEAdata, which has
slots in it for all the data you can get from a GPS. Client commands mine
that structure and ship reports up the socket to the client. DGPS data is
passed through, raw, to the GPS
The program supports multiple GPS types. All are represented by
driver method tables; the main loop knows nothing about the driver
methods except when to call them. At any given time one driver is
active; by default it's the NMEA one.
Each driver may have a trigger string that the NMEA interpreter
watches for. When that string is recognized at the start of a
line, the interpreter switches to its driver. The new driver
initializer method is called immediately.
(The initializer method is called unconditionally each time the device
is opened, if there is no trigger string.)
The trickiest part of the code is the handling of input sources in gpsd.c
itself. It had to tolerate clients connecting and disconnecting at random
times, and the GPS being unplugged and replugged, without leaking file
descriptors; also arrange for the GPS to be open when and only when clients
are active.
That's about all there is to it.
If you're looking for things to hack on, first see the TODO file.
Then, there is some C code in the contrib/ directory that has some
potentially useful techniques in it. Picking a good one to integrate
into gpsd might be a useful thing.
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