The GPS preview is a GTK based project to create a usable GUI tool with the simple output (no maps). It is built and tested around the gllin driver but should work with any other driver via pipe at /tmp/nmeaNP. It detects GPRMC and GPGGA messages and constantly shows the following data on the screen:
- Speed in kilometers per hour (converts from knots).
- Curved distance from the initially marked point (uses advanced IERC 2003 geodetic reference to convert degrees into kilometers).
- Number of the satellites.
- Time (UTM, not a local time).
The source code of this project is available in the project page (http://projects.openmoko.org/projects/gpv/), under LGPL. The project uses GTK framework and C programming language.
The project organization
The project releases the ready to run .ipk files and the packed plain source files (without the build environment). The build environment (as well as the newest version of the source code) can be checked from CVS (the only module is gpv): the anonymous access is open. The build environment is not included in each source distribution as it tends to grow above the currently set 3 Mb limit.
Inside the project
The most interesting part of this project was to keep the external driver thread and GUI thread sufficiently happy together. We cannot just read from the pipe in a loop as this blocks the GUI repainting! Instead, the code schedules the GTK timer, and then it needs to check if any input from the pipe is available - again, without blocking the current thread for too long. This is done using select function which can check the given stream for the availability of input, passing the certain time-out duration. This time-out seems also tricky: if the value or 0.1 s or less is passed on the Neo, function never reports any input present. However too large values block GUI in the input check step. The code now has a kind of adaptation, gradually increasing the duration of this check if there is no input available for a while.
The GPS Preview program (gpv) is released either in .ipk file or in the form of the source code. The released installer still needs the gllin to run, but, if needed, can start it itself.