how to save a thread stack before it exits?

Question

I'm working on a multi-threading project and I need to make a copy of a thread's stack at some point in code (I need a pointer to that). because I'm going to need that pointer after this thread is exited (and it's stack is freed). It would also do the job if I could somehow tell pthread_exit() not to free the stack of thread!

PS: The reason behind this is that I want to use setcontext(ucontext_t*) later when this thread is actually dead.

Answer 1

As was mentioned, this may be a case of the XY problem. However, the solution is to use a pthread_mutex_t in each thread.

The function creating the thread would create the pthread_mutext_t and pass it to the thread.

pthread_t tid;
pthread_mutext_t mutex;

pthread_mutex_init(&mutex, NULL);
pthread_mutex_lock(&mutex);

pthread_create(&tid, NULL, fun, &mutex);

// do stuff

pthread_mutex_unlock(&mutex);

The thread function would be as follows...

void fun(void* arg) {

  pthread_mutex_t* mutex = (pthread_mutex_t*) arg;
  pthread_mutex_lock(mutex);

  // do stuff
}

If you need to do this asynchronously, you can register a signal handler for SIGUSR1 and SIGUSR2 and use pause to unschedule the thread.

signal(SIGUSR1, on_usr1);
signal(SIGUSR2, on_usr2);

void on_usr1(int sig) {

  pause();
}

void on_usr2(int sig) {
}

Then use pthread_kill to raise the signal to the thread...

pthread_kill(tid, SIGUSR1);

Answer 2

As the commenters have said, saving a thread stack to restore it later is going to be really hard. If what you need is to prevent the thread to be called, you can try and "pause" it with a lock. See an example here.