What if thread exits before calling pthread_join

Question

I have a small code

void *PrintHello(void *threadid)
{
   cout<<"Hello"<<endl;
   pthread_exit(NULL);
}

int main ()
{
   pthread_t threads_id;
   pthread_create(&threads_id, NULL, PrintHello, NULL);
   int i=0;
   for(;i<100;i++){cout<<"Hi"<<endl;}   
   pthread_join(threads_id,NULL);
   return 0;
}

I am joining the thread sometime after creation. What will happen if the main tries to join a thread which already exited?

Mcclimans

The pthread_join() function waits for the thread specified by thread to terminate. If that thread has already terminated, then pthread_join() returns immediately. The thread specified by thread must be joinable. – Karakul

Clearly explain in the manual page: If that thread has already terminated, then pthread_join() returns immediately. – Danforth

Answer 1

What will happen if the main tries to join a thread which already exited?

The join operation will immediately finish and return.

Pinhole

Answer 2

Technically there are multiple possible behaviours.

If you join a thread shortly after it dies, the handle may still be valid and pthread_join shall return immediately.

If the thread fully ended before call to pthread_join, the thread_t handle no longer valid and it should return failure with ESRCH.

In very very very extreme case if you join a thread that has been dead long time ago, such handle could be reused and you are joining a different thread.

TL;DR: Do proper synchonizations (e.g. exit flags) and check for returned errors.

Religionism

If the thread is joinable, the ID will not (and can't) be reused until somebody joins it. So in the contrary, not joining a joinable thread is bad, because the resources that are reserved for the thread will never be freed. – Danforth