Calling a function when thread is exiting in PThreads or Windows - c++

Question

I am creating a C++ library for both Linux (with PThreads) and Windows (with their built-in WinThreads) which can be attached to any program, and needs to have a function called when the thread is exiting, similar to how atexit works for processes.
I know of pthread_cleanup_push and pthread_cleanup_pop for pthreads, but these do not work for me since they are macros that add another lexical scope, whereas I want to declare this function the first time my library is called into, and then allow the program itself to run its own code however it needs to. I haven't found anything similar in Windows whatsoever.
Note that this doesn't mean I want an outside thread to be alerted when the thread stops, or even that I can change the way the thread will be exited, since that is controlled by the program itself, my library is just attached, along for the ride.
So the question is: What is the best way, in this instance, for me to have a function I've written called when the thread closes, in either Windows or Linux, when I have no control over how the thread is created or destroyed?
For example in main program:
void* threadFunc(void* arg)
{
printf("Hello world!\n");
return NULL;
}
int main(int argc, char** argv)
{
int numThreads = 1;
pid_t* pids = NULL;
pids = (pid_t*) calloc(sizeof(pid_t), numThreads);
pthread_create(&ntid, NULL, threadFunc, &nVal);
pthreads[0] = ntid;
pthread_join(pthreads[0], NULL);
return 0;
}
In library:
void callMeOnExit()
{
printf("Exiting Thread!\n");
}
I would want for callMeOnExit to be called when the thread reaches return NULL; in this case, as well as when the main thread reaches the return 0;. Wrapping pthread_exit would work for other cases, and could be a solution, but I'd like a better one if possible.
If anyone has any ideas on how I might be able to do this, that would be great!

Answer 1

So after a few code reviews, we were able to find a much more elegant way to do this in Linux, which matches both what Windows does with Fibers (as Neeraj points out) as well as what I expected to find when I started looking into this issue.
The key is that pthread_key_create takes in, as the second argument, a pointer to a destructor, which is called when any thread which has initialized this TLS data dies. I was already using TLS elsewhere per thread, but a simple store into TLS would get you this feature as well to ensure it was called.

Answer 2

Change this:
pthread_create(&ntid, NULL, threadFunc, &nVal);
into:
struct exitInformData
{
void* (CB*)(void*);
void* data;
exitInformData(void* (cp*)(void*), void* dp): CB(cp) data(dp) {}
};
pthread_create(&ntid, NULL, exitInform, new exitInformData(&threadFunc, &nVal));
Then Add:
void* exitInform(void* data)
{
exitInformData* ei = reinterpret_cast<exitInformData*>(data);
void* r = (ei.CB)(ei.data); // Calls the function you want.
callMeOnExit(); // Calls the exit notification.
delete ei;
return r;
}

Answer 3

For Windows, you could try Fls Callbacks. They FLS system can be used to allocate per thread (ignore the 'fiber' part, each thread contains one fiber) storage. You get this callback to free the storage, but can do other things in the callback as well.

Answer 4

I found out that this has already been asked, although the solution given then may not be the same as what you want...
Another idea might be to simply extend from the pthread_t class/struct, and override the pthread_exit call to call another function as you want it to, then call the superclass pthread_exit