What is the purpose of using do {...} while (0) in macros?

Thedo/while(0) pattern seen in many if not most macros in the Linux kernel and elsewhere has a specific purpose: It is the only construct in C that lets you define macros thatalways work the same way, so that a semicolon after your macro always has the same effect,regardless of how the macro is used (with particularly emphasis on the issue of nesting the macro in an if without curly-brackets).

Let's consider an example:

#define foo(x)  bar(x); baz(x)

Say you used it like this:

foo(wolf);

This expands to:

bar(wolf); baz(wolf);

So far, so good. That does what you intended. But let's consider usage like this:

if (!feral)     foo(wolf);

This expands to:

if (!feral)     bar(wolf); baz(wolf);

Which is, if sanely formatted:

if (!feral)     bar(wolf); baz(wolf);

That likely isn't what you intended. As shown, it isn't possible to write multistatement macros that do the right thing in all situations. You can't make macros behave like functions—withoutdo/while(0).

Let's revisit our original macro, wrapped in do/while(0):

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

Now, this statement is functionally equivalent to the former. The do ensures the logic inside the curly-brackets executes, thewhile(0) ensures that happens but once. Same as without the loop. So what's different? Let's look at using the macro. Again:

if (!feral)     foo(wolf);

Which now becomes:

if (!feral)     do { bar(wolf); baz(wolf); } while (0);

Which is the same as:

if (!feral) {     bar(wolf);     baz(wolf); }

You might rejoin, why not just wrap the macro in curly-brackets? Why also have thedo/while(0) logic?

This is why. Consider:

#define foo(x)  { bar(x); baz(x); }

And:

if (!feral)     foo(wolf); else     bin(wolf);

This becomes:

if (!feral) {     bar(wolf);     baz(wolf); }; else     bin(wolf);

Note the stray semicolon, which makes the else a dangling else. If you want people to be able to use your macro like a function and put semicolons where they belong, then this doesn't work.