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:

update(FIELD_NAME1, 10, FIELD_NAME2, 20);

then later add FIELD_NAME3 with out changing any existing calls:

update(FIELD_NAME1, 10, FIELD_NAME2, 20, FIELD_NAME3, 30);

words of wisdom please?

Answer 1

Generally, no.

Varargs throws out a lot of type-safety - you could pass pointers, floats, etc., instead of ints and it will compile without issue. Misuse of varargs, such as omitting arguments, can introduce odd crashes due to stack corruption, or reading invalid pointers.

For instance, the following call will compile and result in crashes or other odd behavior:

UpdateField(6, "Field1", 7, "Field2", "Foo");

The initial 6 is how many parameters to expect. It will convert the string pointer "Foo" to an int to put in Field2, and it will try to read and interpret two other parameters that aren't present, which will probably cause a crash here from dereferencing stack noise.

I believe the implementation of varargs in C is a mistake (given today's environment - it probably made perfect sense in 1972.) The implementation is you pass a bunch of values on the stack and then the callee will walk the stack picking up parameters, based on its interpretation of some initial control parameter. This type of implementation basically screams for you to make a mistake in what might be a very difficult to diagnose way. C#'s implementation of this, passing an array of objects with an attribute on the method, is just must saner, albeit not directly mappable into the C language.