Thanks for dcl_dirfiles - I had been using doslib's dos_find function until discovering this - seems faster and also means I don't have to load doslib.
Not sure if this has been reported before and if it has been addressed in the latest releases, but when testing with 64 bit Opendcl and AutoCAD 2010 I get 'no function definition error' for dcl_dirfiles. Everything else works fine and no errors with this function and x32 bit OpenDcl.