Working of the C Preprocessor

Question

How does the following piece of code work, in other words what is the algorithm of the C preprocessor? Does this work on all compilers?

#include 

        

Answer 1

The preprocessor just replaces the symbols sequentially whenever they appear. The order of the definitions does not matter in this case, b is replaced by a first, and the printf statement becomes

printf("%i", a);

and after a is replaced by 170, it becomes

printf("%i", 170);

If the order of definition was changed, i.e

#define a 170
#define b a

Then preprocessor replaces a first, and the 2nd definition becomes

#define b 170

So, finally the printf statement becomes

printf("%i",170);

This works for any compiler.