Why do {...} while (0); in macros?

There are a couple of reasons:

• (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 \
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();