C preprocessor name definition

## does not expand its operands to their actual value. Try compiling the code below with -E option:

#define VAR ((int*)0xffff)
#define VAR_HANDLER VAR ## _handler

int VAR_HANDLER = VAR; // just to see the result

int
main (void)
{
  ;
}

The output is:

int VAR_handler = ((int*)0xffff);

int
main (void)
{
  ;
}

With that said, I don't think that doing this is a good idea. I'd recommend sticking to the most simple way of doing this — without macros.

To expand on the correct answer by @Moses, when the ## operator is present in a so-called "macro replacement list", there's two scenarios given some a ## b:

• Either a and b are function-like macro parameters, in which case they are expanded before token concatenation.
• Or a and b is something else, in which case the pre-processor applies the ## operator to the sequences of pre-processing tokens 'a' and 'b' respectively, not considering if those form valid pp-tokens that may be expanded.

For this reason, you can use the code somewhat as-is. However, MY_TIMER becomes problematic because once the preprocessor starts expanding macros, it goes all the way and expands TC0 too. We can solve that with a work around, moving the _ from the _Handler etc into the MY_TIMER macro:

#define MY_TIMER          TC0 ## _

And then we need some helper macros to expand MY_TIMER before token concatenation, we can't do MY_TIMER ## _Handler or we'd just end up with MY_TIMER_Handler with no connection to TC0:

#define CONCAT(a,b) a ## b
#define EXPAND_CONCAT(a,b) CONCAT(a,b)

#define MY_TIMER          TC0 ## _
#define MY_TIMER_HANDLER  EXPAND_CONCAT(MY_TIMER, Handler)

Full example:

#include <stdio.h>

typedef struct
{
  int tc0_stuff [123];
} Tc;
#define TC0 ((Tc       *)0x42003000UL)

#define CONCAT(a,b) a ## b
#define EXPAND_CONCAT(a,b) CONCAT(a,b)

#define MY_TIMER          TC0 ## _
#define MY_TIMER_HANDLER  EXPAND_CONCAT(MY_TIMER, Handler)

void TC0_Handler (void)
{
  printf("handling mysterious stuff");
  TC0->tc0_stuff[0] = 456;
}

int main (void)
{
  MY_TIMER_HANDLER();
}

If you don't need the previous value, you may undefined it (and at least emit some warning):

#ifdef TC0
#undef TC0
#warning "TC0 already defined, beware."
#endif

#define MY_TIMER          TC0
#define MY_TIMER_HANDLER  MY_TIMER ## _Handler
#define MY_TIMER_IRQ      MY_TIMER ## _IRQn

If you need both, it is not possible; you then have to find non conflicting symbol names.

As we known nothing about the use cases, it not possible to help you more.