If you ever had the chance to look in the Linux kernel sources, you might have seen macros defined like this:
#define foo(params) \
do{ \
//instructions \
}while(0)
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]
This is what the copiler will get:
if(a[i]
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.
[…] You can use the same code in userspace programs by replacing printk with printf. And just in case you’re wondering what’s with the empty do-while, you might want to take a look at this older article. […]
[cc lang=”text”]a = a + b; b = a – b; a = a – b; [/cc]
😉 no more
int t;True. However, this wasn`t really the point of the program 🙂 Besides that, your code has a small overflow problem 🙂 Say a = b = 3.000.000.000 … Then a+b will give you an overflow. An alternative solution might be:
[cc lang=”c”]x = x ^ y; y = x ^ y; x = x ^ y;[/cc]