Question

I recently found this GCC macro:

#define max(a,b) \
   ({ typeof (a) _a = (a); \
       typeof (b) _b = (b); \
     _a > _b ? _a : _b; })

I didn’t realize before I saw this code, that a block of code {...} can somehow return value in C.
1) Could you give me a hint how this works?

Though, I usually was able to achieve the same result by abusing the comma operator:

#define max(a,b) \
    (typeof (a) _a = (a), \
     typeof (b) _b = (b), \
     (_a > _b ? _a : _b)) 

or if it was only for side-effect I would use do { ... } while(0)

2) What is the preferred way of doing this?

Answer 1

The ({ ... }) construct is a gcc extension.

So is the typeof operator.

A MAX macro (note the conventional use of all-caps) is easy enough to write:

#define MAX(a, b) ((a) > (b) ? (a) : (b))

It does evaluate one of its arguments more than once, so you shouldn’t invoke it as, for example, MAX(x++, y--). The use of all-caps serves to remind the user that it’s a macro, not a function, and to be careful about arguments with side effects.

Or you can write a function (perhaps an inline one) for each type.