I'd go with Fletch's solution of using a struct with a type field and a union with the various parameter options. Then pass this as a pointer/array. Maybe something like:

struct data {
    int type;
    union {
       int i;
       char *c;
    } value;
};

int the_function(int argc, struct data *argv)
{
....
}
[download]

which gives you a single signature for all of your calls.

Not exactly clean, but workable...

Michael