How do you test if two #defines are the same with the C preprocessor

Question

I have a C program which has platform-specific defines for access to low-level hardware. On some platforms, two macros point to the same variable, on others they are different:

 //Platform_One.h
 #define FOO_PORT   (io.portA)
 #define BAR_PORT   (io.portB)

 //Platform_Two.h
 #define FOO_PORT   (io.portC)
 #define BAR_PORT   (io.portC)  //same

I have some initializer code that is different based on whether the #defines are the same or not. Conceptually, I'd like code like this:

 callback_struct_t callbacks[] = {
 #if FOO_PORT == BAR_PORT           //unfortunately invalid
     {&FOO_PORT, handle_foo_bar_func},
 #else
     {&FOO_PORT, handle_foo_func},
     {&BAR_PORT, handle_bar_func},
 #endif          
      {0,0}
 };

Is there a reliable way to test at compile time if two arbitrary macros have the same definition?

Answer 1

You cannot compare the preprocessor macros as strings. One possibility would be to put the hardware port address (e.g., via another macro in the platform-specific headers) into the #defines and then compare the addresses.

However, the easiest way might be to do the comparison of addresses in actual code, e.g.:

if (&FOO_PORT == &BAR_PORT) {
    // populate callbacks with handle_foo_bar_func
} else {
    // populate callbacks with handle_foo_func and handle_bar_func
}

While not done in pre-processor, the compiler may be able to optimise away the unused branch since the hardware addresses are likely compile-time constants.