C++ and mouthwash

Having a problem figuring out an error when I compile an object.

I have a .c file that I'm not supposed to modify, but need to use a function from inside a .h+.cpp object file.  The error has to deal with scope as the compiler (g++) complains.

I understand that normally I should use objectname::function, but since the function I need to use is contained outside the object, I'm not sure what the scope is.

Also, I've been trying to compile it like this:
g++ hurr.c derp.h derp.cpp -o derp.o

Perhaps this isn't correct but I'm not sure of another way while retaining the .c file and not simply copying its contents.

>>11
Yes, derp.h is included in derp.cpp

The contents are not too important as they can work without the hurr.c's function, but I still need to somehow use the hurrfunction.  This is my only error right now, and my googling has turned up a lot of almost-but-still-not related results.

Perhaps my google-fu is weak, but I just can't find much of anything on correctly calling a function from an outside, unchangeable .c file within a c++ object file.  I find loads of results for calling within a main, but an object does not have a main, and those methods just seem to not work.

When I try to use the extern stuff, I get this error:
error: expected unqualified-id before string constant

Not sure what this means...