[fpc-pascal] When are used units recompiled?

[Tom Verhoeff]

In a programming course that I teach, some students are using Lazarus+FPC
instead of Delphi.  I made available two precompiled units, that is, the
*.ppu and *.o files.  However, when the students tried to compile their
programs using these units, the compiler complained that it could not
find the *.pas files.  When compiling with -va (verbose all), it became
apparent that the compiler disliked something about the *.ppu files.
(Sorry, I didn't get the exact message from the students.)

Can someone explain to me under what circumstances FPC will (attempt to)
recompile a unit for which *.ppu and *.o are already available?
I couldn't find this in the documentation.

Versions: I used FPC 2.0.1 to compile the units; the students
used FPC 2.0.2 to compiler their programs.

Thanks,

	Tom
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