do { … } while (0) — what is it good for? [duplicate]

Answer 1

It helps to group multiple statements into a single one so that a function-like macro can actually be used as a function. Suppose you have:

#define FOO(n)   foo(n);bar(n)

and you do:

void foobar(int n) {
  if (n)
     FOO(n);
}

then this expands to:

void foobar(int n) {
  if (n)
     foo(n);bar(n);
}

Notice that the second call bar(n) is not part of the if statement anymore.

Wrap both into do { } while(0), and you can also use the macro in an if statement.