Proper C preprocessor macro no-op

Question

For debug logging, I have often seen and used something like

#ifdef DEBUG
    #define DLOG(fmt, args...) printf("%s:%d "fmt,__FILE__,__LINE__,args)
#else
    #define DLOG(fmt, args...)
#endif

but in a number of places, I have seen the second #define replaced with

#define DLOG(fmt, args...) do {} while (0)

In particular, there's this answer, and the comment on this other answer to the same question suggests that the problem would be in a situation like

if (condition)
    DLOG("foo");

though my quick test suggests that the resulting semicolon on the line by itself will serve as the no-op statement inside the conditional.

Is one or the other of nothing and do {} while (0) better? If so, why? Is there something else that's even better?

Answer 1

A semicolon by itself has two drawbacks:

• Users of your macro can write it without a semicolon, and the compiler will not complain, and
• Some compilers may issue a warning about a possibly stray semicolon.

The do {} while (0) trick addresses both these concerns:

DLOG("foo") // No semicolon

will trigger an error, and the compiler will not warn you about a "stray" semicolon.

Answer 2

See C #define macro for debug printing for an explanation of why you want a different form of no-op. You want to have the compiler parse the debug printing code even when you aren't using it so that errors do not creep in.

Answer 3

The quick answer is that the do/while method lets you have a multi-statement replacement and still use it as a single statement in an if case like you have in your question. For a single expression replacement, I don't think there's any difference.