C macro to only change function calls and not function definitions - c

Question

I have a very similar question to: Macro and function with same name
I want a macro to only change the function call and not the function itself
#define original_function(a, b) replace_function(a, b)
uint64_t replace_function(int a, int b) {
// some functionality
}
uint64_t original_function(int a, int b) {
// some functionality
}
uint64_t main(uint64_t argc, uint64_t* argv) {
original_function(10, 20);
}
this would result in changing the original function definition
#define original_function(a, b) replace_function(a, b)
uint64_t replace_function(int a, int b) {
// some functionality
}
uint64_t replace_function(int a, int b) {
// some functionality
}
uint64_t main(uint64_t argc, uint64_t* argv) {
original_function(10, 20);
}
I cannot do this (which is the answer in the linked question)
uint64_t (original_function)(int a, int b) {
// some functionality
}
//...
And I cannot put the definition after the function
uint64_t original_function(int a, int b) {
// some functionality
}
#define original_function(a, b) replace_function(a, b)
// ...
Is it somehow possible to define in the macro to only change function calls and not function definitions?
Or can I somehow define a macro to rename only the original function (not the call) so it won't be affected?
Edit:
I can only change the macro not the code after that...

Answer 1

You can "pattern match" the line that the definition is on using the C preprocessor. To do this in a useful fashion, first you need an indirect SECOND macro:
#define SECOND(...) SECOND_I(__VA_ARGS__,,)
#define SECOND_I(A,B,...) B
...then it's useful to have an indirect GLUE:
#define GLUE(A,B) GLUE_I(A,B)
#define GLUE_I(A,B) A##B
...and finally, "selectively blue paint" original_function onto itself if and only if it appears on a specific line (this approach requires figuring out which line it's on, but I would suggest not being picky with this strange request):
#define original_function(a,b) \
SECOND(GLUE(EXCEPT_FOR_LINE_,__LINE__),replace_function)(a,b)
So this macro will expand original_function with two arguments with replace_function, given that the first constructed token is not defined. But if that first token is defined, then the token expands first, after which the second argument for that is selected. Which means, you get to do this:
#define EXCEPT_FOR_LINE_6 ,original_function
...and if original_function is actually defined on line 6 (and not called on line 6), then the final expansion looks like the original... on line 6; and on all other lines it will generate a call to replace_function.
This example uses this technique on your code. And as a bonus, all of the macros above in this example are defined on the command line (per your reflected intended usage in the comments).