Can someone explain how this code works?

Question

Hi all,
in a C-challange i stumpled across this question:

// begin of code

#include<stdio.h>

// macro definitions:
#define macro(r,a,b) b##a##r
#define CHAOS macro(r,a,b)

// function forward declaritions:
void CHAOS(int n);

// main function
int main(void)
{
bar(4);
bar(2);

return 0;
}

// function definitions:
void CHAOS(int n)
{
printf("%d", n);
}

// end of code

42 is the output.

I debugged the code, because i did not understand how it worked and the function CHAOS() is called at bar(). Why is that?

Answer 1

#define is a pre-processor directive first CHOAS is replaced by macro(r,a,b)
Again we have another #define it replaces macro(r,a,b) by b##a##r
## acts as token pasting operator
So b##a##r=bar
Hence void CHOAS(int n) is void bar(int n)
bar(4);//outputs 4
bar(2);//outputs 2
So,final output is 42