Scope of macros in C?

C Preprocessor works top to bottom irrespective of function calls. It is effective from that point (line) in whatever the file that macro is defined, until corresponding undef or till end of the translation unit.

So, your code would become,

# define i 20
               // from now on, all token i should become 20
void fun();
int main()
{
  printf("%d",i);   // printf("%d",20);
  fun();
  printf("%d",i);   // printf("%d",20);
  return 0;
}
void fun()
{
#undef i
              // from now on, forget token i
#define i 30
              // from now on, all token i should become 30
}

Your second code would become,

# define i 20
               // from now on, all token i should become 20
void fun()
{
#undef i
               // from now on, forget i
#define i 30
               // from now on, all token i should become 30
}
int main()
{
  printf("%d",i);    //  printf("%d",30);
  fun();
  printf("%d",i);    // printf("%d",30);
  return 0;
}