##__VA_ARGS__ not swallowing comma when zero args under C99

Question

I\'d like to use a macro like the following:

#define x(...) y(a,##__VA_ARGS__,b)

To expand like so:

x();   ->   y(a,b);
x(1)         


        

Answer 1

As you describe, the following pattern works fines.

#define x(a,...) y(a,##__VA_ARGS__)

After I run the following code snip, I guess ##__VA_ARGS__ will always swallow comma when the prefix args exists(such as _ in the e_x)

#define x(...) y(a,c, ##__VA_ARGS__, b)
#define e_x(_, ...) y(a, c, ##__VA_ARGS__, b)

x();
x(a);
x(a, b);

e_x();
e_x(a);
e_x(a, b);

You can get the result:

y(a,c,, b);
y(a,c,a, b);
y(a,c,a, b, b);

y(a, c, b);
y(a, c, b);
y(a, c, b, b);

Above sample works fine in my computer(both macos and linux) .