Is it a good idea to use varargs in a C API to set key value pairs?

Question

I am writing an API that updates a LOT of different fields in a structure.

I could help the addition of future fields by making the update function variadic:

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Answer 1

Why not have one arg, an array. Even better, a pointer to an array.

struct field {
  int val;
  char* name;
};

or even...

union datatype {
  int a;
  char b;
  double c;
  float f;
// etc;
}; 

then

struct field {
  datatype val;
  char* name;
};

union (struct* field_name_val_pairs, int len);

ok 2 args. I lied, and thought a length param would be good.