Item — HN Reader

This is a good example. Without trampolines, this could look like this (Godbolt: https://godbolt.org/z/nK5fqMxjs).

    int main() {
        struct Point {
            int x, y;
        } points[3] = {
            { 3, 1 },
            { 2, 2 },
            { 5, 7 }
        };

        struct Point target = { 4, 5 };

        long dsq(const struct Point *p) {
            long dx = p->x - target.x, dy = p->y - target.y;
            return dx*dx + dy*dy;
        }

        typedef typeof(dsq) dsq_f;

        int compare(const void *p, const void *q, void *data) {
           wide(dsq_f) *dsq = data;
           long a = CALL(*dsq, (p)), b = CALL(*dsq, (q));
           return (a > b) - (a < b);
        }

        qsort_r(points, 3, sizeof(struct Point), compare, &CLOSURE(dsq_f, dsq));

        for (int i = 0; i < 3; ++i)
            printf("%d,%d\n", points[i].x, points[i].y);
    }

There are slightly different ways how to define the helper macros, I am still experimenting a bit. Here you could avoid the typedef if defined differently. But ideally, there would be native language support that avoids these macros.

Well, if you're already using qsort_r, what's the point of using nested functions, if you can have a context pointer with the target?

And if you're not using qsort_r, but reaching for _Thread_local, the target can be _Thread_local instead of dsq.

Fair. If you only have one object to access such as the target pointer, then it probably makes not much difference with void-pointer based APIs such as qsort_r (for new APIs it would add type safety). Where it removes more boilerplate code is when you have several such objects and would have to create an extra data structure to access them.

Or without qsort_r, you could use a thread local variable:

    typedef typeof(dsq) dsq_f;
    _Thread_local static wide(dsq_f) wdsq;
    wdsq = CLOSURE(dsq_f, dsq);

https://godbolt.org/z/3e157c6b1