C variadic macro with two named parameters

Question

I want to use a variadic macro but it appears to be designed to only treat the first parameter specially. I want the first two parameters to be named and the rest not, like so:

#define FOO(AA,BB,...) AA->BB(AA,##...)

FOO(mystruct,funcname,123)

However this is not working with LLVM. Am I doing something wrong, or is there a limitation to how the variadic macro works?

UPDATE

The correct answer is, use ##VA_ARGS instead of ##...

There are some webpages that claim that "..." is valid but at least with the MacOS llvm it is not.

What is it that you intend to achieve here? What is the expected expansion of that macro? Do you know what ## even does? What is the error message you got? — Antti Haapala
– Antti Haapala, Commented Dec 30, 2018 at 17:01

Antti Haapala · Accepted Answer · 2018-12-30 17:10:53Z

1

The macro arguments are not expanded with ... in the macro expansion - how could they, because then you couldn't have a macro that used ellipsis in the expansion. Instead it will be available as a special parameter __VA_ARGS__.

With this, the following program

#define FOO(AA,BB,...) AA->BB(AA, __VA_ARGS__)

FOO(mystruct,funcname,123)
FOO(mystruct,funcname,123,456)

will be preprocessed to

The ## is a token-pasting operator. It will make a single preprocessing token out of 2 parts. , ## ... attempts to make a preprocessing token ,.... It is not a valid C token, and that is why Clang will report

<source>:3:1: error: pasting formed ',...', an invalid preprocessing token

edited Dec 30, 2018 at 17:10

answered Dec 30, 2018 at 17:05

Antti Haapala

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4 Comments

popoe Over a year ago

I need to use ##__VA_ARGS__. Your solution covers the >=1 params cases but not 0.

popoe Over a year ago

I don't know why I was seeing webpages claiming that "..." would work. I used to have the original C programming book-- that would have been a better source of truth than blogs.

Antti Haapala Over a year ago

@popoe that's a GNU extension

rici Over a year ago

@popoe: variadic macros aren't described at all in K&R, because the original C didn't have variadic macros. They were added in C99, although they were already implemented as a gcc extension. Afaik, ... has never been used to indicate where varargs should be expanded. At least, not in C.

Petr Skocik · Accepted Answer · 2018-12-30 18:15:03Z

... macro arguments are pasted into macro bodies with __VA_ARGS__. The problem is how to allow for it to be empty. If it is empty, you'll usually want to comma before it erased and you can use the GNU ##__VA_ARGS__ extension to achieve that.

#define FOO(AA,BB,...) AA->BB(AA,##__VA_ARGS__) /*GNU extension*/
FOO(mystruct,funcname) //warning with -pedantic
FOO(mystruct,funcname,123)

The above, however, will trigger warnings if compiled with -pedantic. If you want your macro usable without warnings at -pedantic, you could perhaps achieve that by swapping the first two arguments in the macro definition.

#define FIRST(...)  FIRST_(__VA_ARGS__,)
#define FIRST_(X,...) X
#define BAR_(CallExpr,...) CallExpr(__VA_ARGS__)
#define BAR(BB,/*AA,*/...) BAR_(FIRST(__VA_ARGS__)->BB,__VA_ARGS__)

BAR(funcname,mystruct) //no warning
BAR(funcname,mystruct,123)

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