In C function declaration 'params, ...)' equal to 'params...)'?

Question

all In glusterfs's functions, there is one as follows NOTES: the whole define in stack.h

//libglusterfs/src/glusterfs/stack.h

#define STACK_WIND(frame, rfn, obj, fn, params...)                             \
    STACK_WIND_COMMON(frame, rfn, 0, NULL, obj, fn, params)

#define STACK_WIND_COMMON(frame, rfn, has_cookie, cky, obj, fn, params...)     \
    do {                                                                       \
        ...  \
        next_xl_fn(_new, obj, params);                                         \
        THIS = old_THIS;                                                       \
    } while (0)

I did not google search anything about explaining such declaring methods, any idea will be appreciated?

Answer 1

params... as part of function-like macro is a GNU extension before C standardized ellipsis as part of arguments of function-like macros.

See https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html#Variadic-Macros

Nowadays, prefer __VA_ARGS__.

In C function declaration 'params, ...)' equal to 'params...)'?

No, they are not equal in function-like macros. The first one needs to use __VA_ARGS__ to reference variadic arguments and use __VA_OPT__ or ##__VA_ARGS__ to remove leading comma. In the params... then params is straight up replaced by all the arguments including commas.

Doing just ellipsis , ...) and using __VA_ARGS__ would be equal to , params...) and using params.

In normal functions params... as part of parameter-list is just invalid.