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Macros using do{}while(0);

Posted by Strainu | Filed under C

If you ever had the chance to look in the Linux kernel sources, you might have seen macros defined like this:

This basicly means that the code is executed exactly once, so the first idea is that do{}while(0); is useless.
In fact, there are a number of reasons for writing the macros this way:

• Empty statements give warnings from the compilers, so rather than writing
  #define FOO

  you might want to write

  #define FOO do{}while(0)
• It gives you a basic block in which to declare local variables. You could simply use curly brackets, but this could cause serious problems in conditional statements. Let’s take the following example:
  #define exch(a,b) {int t; t = a; a = b; b = t;}

  You then use the macro in the following code:

  if(a[i]<a [i+1])
      exch(a[i],a[i+1]);
  else
      ready = 1;

  This is what the copiler will get:

  if(a[i]<a [i+1]){
      int t;
      t = a[i];
      a[i] = a[i+1];
      a[i+1] = t;
  };
  else
      ready = 1;

  This is interpreted as an if statement whithout else and you will get an error like “else without matching if”. However, if you write the macro like this:

  #define exch(a,b) do{int t; t = a; a = b; b = t;}while(0)

  the code will behave as expected.

You can find more informations about this in the Kernelnewbies FAQ.

Tags: C/C++, kernel