Thread locals don't fully solve the problem. They work well if you immediately call the closure, but what if you want to store the closure and call it later?

    #include <stdlib.h>
    #include <string.h>
    #include <stddef.h>

    typedef int (*comp)(const void *untyped_left, const void *untyped_right);

    thread_local int in_reverse = 0;

    __attribute__((noinline))
    int compare_impl(const void *untyped_left, const void *untyped_right, int in_reverse) {
        const int* left = untyped_left;
        const int* right = untyped_right;
        return (in_reverse) ? *right - *left : *left - *right;
    }

    comp make_sort(int direction) {
        in_reverse = direction;
        int compare(const void *untyped_left, const void *untyped_right) {
            return compare_impl(untyped_left, untyped_right, in_reverse);
        }
        return compare;
    }

    int main(int argc, char* argv[]) {

        int list[] = { 2, 11, 32, 49, 57, 20, 110, 203 };

        comp normal_sort = make_sort(0);
        comp reverse_sort = make_sort(1);

        qsort(list, (sizeof(list)/sizeof(*list)), sizeof(*list), normal_sort);
            
        return list[0];
    }

Because we create `reverse_sort` between creating `normal_sort` and calling it, we end up with a reverse sort despite clearly asking for a normal sort.

The answer is you wrap it and you don’t return until the thing stored in the thread local is not needed

So we can't return the closure?

Then it's clearly only half a solution.

The example I gave above should work fine in any language with first-class closures.

> The closure problem can be neatly described by as “how do I get extra data to use within this qsort call?”

    _Thread_local struct {
      void *data;
      int (*compare)(const void *a, const void*, void*);
    } _qsort2_closure ; 

    static int _qsort2_helper(const void *a, const void *b) {
        return _qsort2_closure.compare(a, b, _qsort2_closure.data);
    }

    void qsort2(void *base, size_t elements, size_t width, int (*compare)(const void *a, const void*, void*), void *userData) 
    {
        _qsort2_closure.data = userData;
        _qsort2_closure.compare = compare;
        qsort(base, elements, width, _qsort2_helper);
    }

you also need to restore the _qsort2_closure when done. But again you are reinventing dynamic scoping with all its advantages and disadvantages.

> you also need to restore the _qsort2_closure when done

No I do not. It will reassigned next call.

> But again you are reinventing dynamic scoping

No. I’m not reinventing anything. I’m using the existing feature of thread local variables.

The usage of such is entirely an implementation detail of qsort2 with the exception of recursion.

Dynamic scoping typically refers to defining variables which have scope outside of their call stack. No usage of this API requires it.

Can you just try to learn something new?

This is the classic lexical vs dynamic scoping. Dynamic scoping works great until it doesn't.

Don’t use C then? It sounds like you want JavaScript, Python, or Lisp.

Once again, the caller of the API does not declare any variables so there is no dynamic scoping.

Yep. Thread locals are probably faster than the other solutions shown too.

It’s confusing to me that thread locals are “not the best idea outside small snippets” meanwhile the top solution is templating on recursion depth with a constexpr limit of 11.

The method of having static variables to store state in functions is used heavily in ANSI C book. It’s honestly a beautiful technique when used prudently.

reentrancy.

It doesn’t store state for later. It’s literally impossible to tell it’s happening.

Imagine a comparison function that needs to call sort() as part of its implementation. You could argue that's probably a bad idea, but it would be a problem for this case.

(You could solve that with a manually maintained stack for the context in a thread local, but you'd have to do that case-by-case)

That is true. It can be protected against with assert.

I think the times you need to do this are few. And this version is much more pruden.