What happens if you declare a variable inside a macro?

Question

Suppose I have a macro defined as this:

#define FOO(x,y) \
do {
  int a,b;
  a = f(x);
  b = g(x);
  y = a+b;
} while (0)

When expanding the macro, does GCC "guarantee" any sort of uniqueness to a,b? I mean in the sense that if I use FOO in the following manner:

int a = 1, b = 2;
FOO(a,b);

After, preprocessing this will be:

int a = 1, b = 2;
do {
  int a,b;
  a = f(a);
  b = g(b);
  b = a+b;
} while (0)

Can/will the compiler distinguish between the a outside the do{} and the a inside the do? What tricks can I use to guarantee any sort of uniqueness (besides making the variables inside have a garbled name that makes it unlikely that someone else will use the same name)?

(Ideally functions would be more useful for this, but my particular circumstance doesn't permit that)

Answer 1

If we consider scoping of variables, it is guaranteed that a,b inside the do..while() will be different from the ones defined outside.

For your case, the a,b defined outside will not exist inside the do..while().

There are lots of things to watch out for when using MACROs.