what is the usage of PTHREAD_CREATE_JOINABLE in pthread?

Question

I read some codes as below:

void
mcachefs_file_start_thread()
{
  pthread_attr_t attrs;
  pthread_attr_init(&attrs);
  pthread_attr_setdetachstate(&attrs, PTHREAD_CREATE_JOINABLE);
  pthread_create(&mcachefs_file_threadid, &attrs, mcachefs_file_thread, NULL);
}

Here, what is the usage of setting attrs as PTHREAD_CREATE_JOINABLE? Besides, isn't it the default attribute of a thread created by pthead_create?

Answer 1

Yes, PTHREAD_CREATE_JOINABLE is the default attribute. The purpose is that it allows you to call pthread_join on the thread, which is a function that waits until the thread finishes, and gives you return value if its main routine.

Sometimes, when you're creating a thread to do some background work, it might be a good idea to make sure it has finished before you use its results or move to something else. That's what joinable threads are for.

Answer 2

From posix spec, the default setting of the detach state attribute in a newly initialized thread attributes object is indeed PTHREAD_CREATE_JOINABLE. See for instance http://linux.die.net/man/3/pthread_attr_setdetachstate So you are right: the pthread_attr_setdetachstate line of code is not necessary in your code snippet.