Thread locals do solve the problem. You create a wrapper around the original function. You set a global thread local user data, you pass in a function which calls the function pointer accepting the user data with the global one.

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.

reentrancy.