c – Why do do {} while (0)?

Question:

I have noticed this code more than once.

#if FUSE_TRACE
#define TRACE(x...) fprintf(stderr,x)
#else
#define TRACE(x...) do {} while (0)
#endif

Why so, and not

#if FUSE_TRACE
#define TRACE(x...) fprintf(stderr,x)
#else
#define TRACE(x...)
#endif

As I understand it, do {} while (0) translated into a NOP instruction, but why is it here?

Answer:

Macros are a subtle thing. Imagine a situation, someone wrote a code (a conditional example) (but it’s better not to write this precisely because of this).

for (int i = 0;i < 10; i++)
 TRACE("ok"), s+=i;
do_any();

Everything will work correctly here. In release mode, if we use it as you suggest

#define TRACE(x...)

The following will happen, the code is converted to

if (something) 
    , s+=i;
do_any();

And it won't compile.

Also won't compile

expr? TRACE("y") : TRACE("n");

However, using the macro #define TRACE(x...) do {} while (0) has similar problems. I personally use the following macro for such purposes:

#define TRACE(x...) ((void)0)

Or I use a separate function (not a macro) to display the logs.