Initializing a thread local variable

Question

Is there a mechanism for a library supplying a thread local variable to register a constructor function for it?

I'd like to have my library supply a thread-local struct which should be initialized on thread creation with dynamically obtained data.

If the struct was just global but not thread-local, I'd have a function marked with gcc's __attribute__((__constructor__)) initialize it, but these constructors don't retrigger when a new thread is created.

Answer 1

No, thread creation does not invoke any constructors. This is a good thing; automatic invocation of constructors would not scale in a potentially large application where most threads have nothing to do with your library code and will never call it.

Instead, you need to either have your library code that uses the thread-local object construct it lazily on the first library call in the new thread, or require the calling application to call an initialization function explicitly in threads that will use it. The first option is generally a lot better and the performance impact should not even be measurable; accessing thread-local storage in a library takes longer than a predictable branch:

static _Thread_local int init_done;
if (!init_done) ...