cca is called from the command line.
In the following examples, foo.c is the following:
main(){
#ifndef FOO
#define FOO 1
int foo_temp = FOO;
printf("FOO = %d\n", foo_temp);
#else
int bar;
scanf("%d", &bar);
#endif /* FOO */
printf("Hello, world!\n");
#ifdef N
#if N == 42
printf("The ultimate answer is 42\n");
#elif N == 13
printf("Boy are you unlucky.\n");
#else
printf("This number isn't very interesting.\n");
exit();
#ifndef TIME
printf("You are out of TIME.\n");
#endif /* TIME */
#endif /* N == 42 */
#else
#define N 7
int n = 7;
printf("We all know n = %d\n", n);
#endif /* N */
}