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

I assume it's good for inner scope variable declaration and for using breaks (instead of gotos.)

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 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);