Function Stubbing and Linking

Hi all the C experts,

We are developing the Test Engine for an avionics application.

The library API a.h: contains external definition of function A().
The application app.c: include "a.h", calls function A()
The Test Engine test.c: include "a.h", also calls function A()

We want the application to run on our Test environment, so we want to stub function A() of the library and replace it with our version A() in stub.c. So now both app.c and test.c are calling our re-defined A(). It works ok.
stub.c: include "a.h", defines A().

However, now we just realize we still want test.c to call the original A() of the library, and app.c still calls the stubbed A().
app.c: include "a.h", calls function A() of the stub.c.
test.c: include "a.h", calls function A() of the library.
stub.c: include "a.h", defines A().

Note that app.c is the application to be tested so we cannot make any change to it. We are free to change test.c. What can we change to test.c, or what can we do to the makefile so that the A() in test.c can link to the library?
We really appreciate your help.

If you're using Linux/GCC, there is a linker option (-y IIRC) which allows you to do some fancy renaming of library functions.

If you are doing unit testing of app.c then you need not concern about the code in the fucntion A(). You have to make ensure whether the function is getting called or not which you are doing it by stub.c.

Thanks for your responses.
Your ideas all make sense. It turns out that the makefile in the application is much more complicated than I thought. All the c files are compiled to *.o file, then linked to an executable. This will then be linked to the library to become an image to load in to the hardware target.
I guess this is not possible.