do { ... } while (0) — what is it good for?

It is a way to simplify error checking and avoid deep nested if's. For example:

do {
  // do something
  if (error) {
    break;
  }
  // do something else
  if (error) {
    break;
  }
  // etc..
} while (0);

It's the only construct in C that you can use to #define a multistatement operation, put a semicolon after, and still use within an if statement. An example might help:

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

if (condition)
    FOO(x);
else // syntax error here
    ...;

Even using braces doesn't help:

#define FOO(x) { foo(x); bar(x); }

Using this in an if statement would require that you omit the semicolon, which is counterintuitive:

if (condition)
    FOO(x)
else
    ...

If you define FOO like this:

#define FOO(x) do { foo(x); bar(x); } while (0)

then the following is syntactically correct:

if (condition)
    FOO(x);
else
    ....

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.