Why do a lot of #defines in the kernel use do { ... } while(0)?

There are a couple of reasons:

• (from Dave Miller) Empty statements give a warning from the compiler so this is why you see #define FOO do { } while(0).

• (from Dave Miller) It gives you a basic block in which to declare local variables.

• (from Ben Collins) It allows you to use more complex macros in conditional code. Imagine a macro of several lines of code like:

  #define FOO(x) \
          printf("arg is %s\n", x); \
          do_something_useful(x);

  Now imagine using it like:

  if (blah == 2)
          FOO(blah);

  This interprets to:

  if (blah == 2)
          printf("arg is %s\n", blah);
          do_something_useful(blah);;

  As you can see, the if then only encompasses the printf(), and the do_something_useful() call is unconditional (not within the scope of the if), like you wanted it. So, by using a block like do { ... } while(0), you would get this:

  if (blah == 2)
          do {
                  printf("arg is %s\n", blah);
                  do_something_useful(blah);
          } while (0);

  Which is exactly what you want.

• (from Per Persson) As both Miller and Collins point out, you want a block statement so you can have several lines of code and declare local variables. But then the natural thing would be to just use for example:

  #define exch(x,y) { int tmp; tmp=x; x=y; y=tmp; }

  However that wouldn't work in some cases. The following code is meant to be an if-statement with two branches:

  if (x > y)
          exch(x,y);          // Branch 1
  else  
          do_something();     // Branch 2

  But it would be interpreted as an if-statement with only one branch:

  if (x > y) {                // Single-branch if-statement!!!
          int tmp;            // The one and only branch consists
          tmp = x;            // of the block.
          x = y;
          y = tmp;
  }
  ;                           // empty statement
  else                        // ERROR!!! "parse error before else"
          do_something();

  The problem is the semi-colon (;) coming directly after the block. The solution for this is to sandwich the block between do and while (0). Then we have a single statement with the capabilities of a block, but not considered as being a block statement by the compiler. Our if-statement now becomes:

  if (x > y)
          do {
                  int tmp;
                  tmp = x;
                  x = y;
                  y = tmp;
          } while(0);
  else
          do_something();

• (from Bart Trojanowski) gcc adds Statement-Expressions which provide an alternative to the do-while-0 block. They provide the above mentioned benefits and are slightly more legible.

  #define FOO(arg) ({         \
             typeof(arg) lcl; \
             lcl = bar(arg);  \
             lcl;             \
      })