How does the C preprocessor handle circular dependencies?

Question

I want to know how the C preprocessor handles circular dependencies (of #defines). This is my program:

#define ONE TWO 
#define TWO THREE
#defi         


        

Answer 1

Here's a nice demonstration of the behavior described in rici's and Eric Lippert's answers, i.e. that a macro name is not re-expanded if it is encountered again while already expanding the same macro.

Content of test.c:

#define ONE 1, TWO
#define TWO 2, THREE
#define THREE 3, ONE

int foo[] = {
  ONE,
  TWO,
  THREE
};

Output of gcc -E test.c (excluding initial # 1 ... lines):

int foo[] = {
  1, 2, 3, ONE,
  2, 3, 1, TWO,
  3, 1, 2, THREE
};

_{(I would post this as a comment, but including substantial code blocks in comments is kind of awkward, so I'm making this a Community Wiki answer instead. If you feel it would be better included as part of an existing answer, feel free to copy it and ask me to delete this CW version.)}