Why use "while(0)"?

Question

In Linux code I saw some macros with:

do
{

}while(0) 

Is there a reason? Because there seems to be no logic in a repeat loop where the code repeats only once.

Answer 1

It probably does not exist because it is little needed and it can be solved in this way. Anyway this construction is used essentially in macros (I think you already knew this by tags original of the question).

It serves to group multiple commands without causing errors when the preprocessor expands code.

Example:

#define MACRO(x) do { funcao(x); x++; } while (0)

Can be used in:

if (a > 0)
    MACRO(a);
else
    a++;

This would cause error in the same use:

#define MACRO(x) { funcao(x); x++; }

Because it would be expanded to:

if (a > 0)
    { funcao(x); x++; };
else
    a++;

It would work if you hadn’t used the semicolon in the macro line, but nobody does it. And it would be a mistake to make the programmer know that this macro should be used like this.

This is even worse:

#define MACRO(x) funcao(x); x++

Expand to:

if (a > 0)
    funcao(x); x++;
else
    a++;

I put in the Github for future reference.

It would obviously destroy the use of if as imagined and the else would be loose, generating a syntax error.

In some cases it is possible to use this in normal code, but it is rare to be useful and readable.

Answer 2

It is also a way to avoid if’s greatly grouped and simplify error checking:

do {
  // faça algo
  if (error) {
    break;
  }
  // faça alguma outra coisa
  if (error) {
    break;
  }
  // etc..
} while (0);