Why use apparently meaningless do-while and if-else statements in macros?

Question

In many C/C++ macros I\'m seeing the code of the macro wrapped in what seems like a meaningless do while loop. Here are examples.

#define FOO(X         


        

Answer 1

I don't think it was mentioned so consider this

while(i<100)
  FOO(i++);

would be translated into

while(i<100)
  do { f(i++); g(i++); } while (0)

notice how i++ is evaluated twice by the macro. This can lead to some interesting errors.