C – Advantages of do { } while(0) Versus ({ }) in a Macro

The do ... while and if ... else are there to make it so that a semicolon after your macro always means the same thing. Let's say you had something like your second macro.

#define BAR(X) f(x); g(x)

Now if you were to use BAR(X); in an if ... else statement, where the bodies of the if statement were not wrapped in curly brackets, you'd get a bad surprise.

if (corge)
  BAR(corge);
else
  gralt();

The above code would expand into

if (corge)
  f(corge); g(corge);
else
  gralt();

which is syntactically incorrect, as the else is no longer associated with the if. It doesn't help to wrap things in curly braces within the macro, because a semicolon after the braces is syntactically incorrect.

if (corge)
  {f(corge); g(corge);};
else
  gralt();

There are two ways of fixing the problem. The first is to use a comma to sequence statements within the macro without robbing it of its ability to act like an expression.

#define BAR(X) f(X), g(X)

The above version of bar BAR expands the above code into what follows, which is syntactically correct.

if (corge)
  f(corge), g(corge);
else
  gralt();

This doesn't work if instead of f(X) you have a more complicated body of code that needs to go in its own block, say for example to declare local variables. In the most general case the solution is to use something like do ... while to cause the macro to be a single statement that takes a semicolon without confusion.

#define BAR(X) do { \
  int i = f(X); \
  if (i > 4) g(i); \
} while (0)

You don't have to use do ... while, you could cook up something with if ... else as well, although when if ... else expands inside of an if ... else it leads to a "dangling else", which could make an existing dangling else problem even harder to find, as in the following code.

if (corge)
  if (1) { f(corge); g(corge); } else;
else
  gralt();

The point is to use up the semicolon in contexts where a dangling semicolon is erroneous. Of course, it could (and probably should) be argued at this point that it would be better to declare BAR as an actual function, not a macro.

In summary, the do ... while is there to work around the shortcomings of the C preprocessor. When those C style guides tell you to lay off the C preprocessor, this is the kind of thing they're worried about.

Just a guess as to why they might suggest using

do { ... } while(0,0)

over

do { ... } while(0)

Even though there's no behavior difference and should be no runtime cost difference between the two.

My guess is that the static analysis tool complains about the while loop being controlled by a constant in the simpler case and doesn't when 0,0 is used. The customer's suggestion is probably just so they don't get a bunch of false positives from the tool.

For example I occasionally come across situations where I want to have a conditional statement controlled by a constant, but the compiler will complain with a warning about a conditional expression evaluating to a constant. Then I have to jump through some hoops to get the compiler to stop complaining (since I don't like to have spurious warnings).

Your customer's suggestion is one of the hoops I've used to quiet that warning, though in my case it wasn't controlling a while loop, it was to deal with an "always fails" assertion. Occasionally, I'll have an area of code that should never execute (maybe the default case of a switch). In that situation I might have an assertion that always fails with some message:

assert( !"We should have never gotten here, dammit...");

But, at least one compiler I use issues a warning about the expression always evaluating to false. However, if I change it to:

assert( ("We should have never gotten here, dammit...", 0));

The warning goes away, and everybody's happy. I'm guessing that even your customer's static analysis tool would be, too. Note that I generally hide that bit of hoop jumping behind a macro like:

#define ASSERT_FAIL( x) assert( ((x), 0))

It might be nice to be able to tell the tool vendor to fix the problem, but there might be legitimate cases where they actually do want to diagnose a loop being controlled by a constant boolean expression. Not to mention the fact that even if you convince a tool vendor to make such a change, that doesn't help you for the next year or so that it might take to actually get a fix.