How to ask for the name of something? [duplicate]

My question was retitled:

What is this function called?

Originally my title was:

How is this function called?

I still think that my version was correct; I always state such questions this way. Which one is really correct?

What do you call...? should be answered by a noun.

Q. What do you call your dog? A. Lucky

How do you call...? should be answered by an action.

Q. How do you call your dog? A. By whistling

How is this function called? is a different question from What is this function called? The latter question is asking for a name of the function, whereas the former question is asking for the proper way to call the function, which might require specific arguments, etc.

A literal translation of the German "Wie heißt das Buch?" is "How is the book called?" but an idiomatic English translation would be "What is the name of that book?" I mention this because your other answers have taken the cue from "called" to give you constructions with "called". When asking what something is "called" it's about a more general property. "What do you call the man who brings your mail?" The mailman. "What is the name of the man who brings your mail?" Bob.