Unitdiff scans one or two Free Pascal unit source files and either lists all available identifiers, or describes the differences in identifiers between the two units.
You can invoke unitdiff with an input filename as the only required argument. It will then simply list all available identifiers.
The regular usage is to invoke unitdiff with two arguments:
Invoked like this, it will show the difference in interface between the two units, or list the available identifiers in both units. The output of unitdiff will go to standard output by default.