What is a 'thunk'? (in the context of re-entrant sort functions, eg: qsort_r)

Question

Having already checked What is a 'thunk'?, I'm still not sure why the extra argument passed to the re-entrant qsort_r would be called thunk.

Eg: (BSD style qsort_r)

void qsort_r(void *base_, size_t nmemb, size_t size, void *thunk,
             int (*compar)(void *, const void *, const void *));

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To be clear, I know what the argument is for, and how to use it, just not why its called thunk (in the BSD's qsort_r at least).

Answer 1

The name seems to be from the original proposed implementation by Diomidis Spinellis

The implementation shows that it is just used as an opaque data block, passed through qsort_r and back to your compare function.

It seems to somewhat match the 3rd concept in this answer.

a mapping of machine data from one system-specific form to another, usually for compatibility reasons

But really, it just seems like a misleading name. I usually think of thunks as containing blocks of code. In this case it is just a container for context.