Too Busy For Words - the PaulWay Blog

I happened across a the Kernel Newbies Wiki, a sensible attempt to encourage people new to kernel programming to get started, even if they're not going to be diving into hardcore device driver writing immediately. Reading the FAQ, I found a reference to the question "Why do so many #defines in Kernel files use a do { <code> } while 0; construct?" This piqued my interest.

The basic reason why is that if you have a define like:

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

And you have use it in code such as:

if (x > y) 
    exch(x,y);
else
    x = 0;

Then this gets expanded to:

if (x > y)
    { int t; t = x; x = y; y = t; }; # End of if statement here
else # dangling else
    x = 0;

Now, yes, this is bad. But to me it's bad because the programmer has fallen for C's lazy statement syntax and assumed that the 'then' clause of the if statement is always going to be a single statement. This type of construct fails regularly because if you have to add some other statement to that if/then/else statement, like:

if (x > y) 
    exch(x,y);
    call_foobar(x,y);
else
    x = 0;

Then you've fallen for the same trap. Once again, the exch() call completes the if statement and the call_foobar() call is executed unconditionally. Indenting in this case is worse than a sham, it actively deceives the programmer into thinking that the logic will work when it won't. Of course, if the programmer had initially written:

if (x > y) {
    exch(x,y);
} else {
    x = 0;
}

Then it would all make sense, the #define would work, the extra statements added in would work. What's a couple of extra characters and an extra brace line, compared to many hours hunting down an elusive bug? Lazy programming is good in certain circumstances, but it doesn't excuse bad style or the hubris of thinking you'll never have to revise or debug this piece of code.

I know I'll have already put some people offside by commenting on this coding practice. But if and when I ever have to write something for the kernel I'll be using a sensible amount of braces, thank you.