Can anyone explain how to use the File Descriptor Source module in
gnuradio-companion?
I am trying to write a flow-graph to read data from an arbitrary file,
chosen via the GUI at run time? This must be possible, right? I can't
see how to do it.
I don't have time to write a full explanation, but note that you want the File Source block, not the File Descriptor Source block. The File Descriptor Source is only really useful to read from standard input or for use in more complex programs than you can write with GRC.
The File Source block takes a string for the filename. If you create a Variable block with a string value, you can enter the ID of the Variable instead of a filename, and then change that Variable using GUI in the usual fashion. I haven't tested this, but that's how it should work.