Named parameters using a struct.

    struct calculate_args {
        int x, y;
        enum {add=0, sub} operator;
    };

    int calculate_func(struct calculate_args args) {
        if (args.operator == add)
            return args.x + args.y;
        else
            return args.x - args.y;
    }

    #define calculate(...) calculate_func((struct calculate_args){__VA_ARGS__})

Now you can combine positional and named parameters or omit them.

    calculate(1, 3); // 4
    calculate(8, 3, .operator=sub); // 5
    calculate(.operator=sub, .y=7); // -7

Works very well in cases where there is a lot of parameters that default to 0. Keep in mind that you still need to know how structs work and you lose compile-time error detection.