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Is it possible to iterate over arguments in variadic macros?

Asked 2009-12-09T07:35:04.787
98

I was wondering if it is possible to iterate over arguments passed to a variadic macro in C99 or using any GCC extensions ?

For e.g. is it possible to write a generic macro that takes a structure and its fields passed as arguments and prints offset of each field within the structure ?

Something like this:

struct a {
    int a;
    int b;
    int c;
};

/* PRN_STRUCT_OFFSETS will print offset of each of the fields 
   within structure passed as the first argument.
*/

int main(int argc, char *argv[])
{
    PRN_STRUCT_OFFSETS(struct a, a, b, c);

    return 0;
}
c foreach c99 c-preprocessor variadic
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1 Answer

5

Maybe use the varargs as an array initializer, and iterate over countof(array)? i.e. sizeof(array)/sizeof(array[0]). The array could potentially be a C99 anonymous array.

I can't think of another way to iterate over the var-args of a macro, since I don't know how to do anything to the text of each var-arg element. The var-arg part might as well be a single argument that has commas in it, for all you can do to it with CPP, AFAIK.

But here's my idea for iterating over var-args:

#define countof(a) ( sizeof(a)/sizeof((a)[0]) )
#define MACRO(fd, format, ...) do { int ar_[] = { __VA_ARGS__ }; \
for(int i=0; i<countof(ar_) ; ++i){ \
    fprintf(fd, format, ar_[i]); \
} } while(0)
answered 2009-12-09T08:40:29.773

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