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ULT Thread IDs

Question Text​

Why do the thread IDs returned by threads_create() start from 2 and not 1? Why is this necessary.

Question Answers​

  • Because ID 1 is associated with the main thread. This is an implementation detail and can be omitted.
  • Because ID 1 belongs to the main thread. This is needed in order to associate a ucontext_t with the main thread as well, so the main thread can also be run.

  • Because the underlying kernel thread is assigned ID 1. This is mandatory in order for the OS's scheduler to run this thread.

  • Because libult.so first creates a pool of threads from which it threads_create() retrieves the threads it returns.

Feedback​

The threads_create() function calls init_first_context(), which, in turn, calls tcb_new(), thus creating the first context associated with the main thread (the one calling threads_create() the first time). Without this, the scheduler in libult.so wouldn't be able to run the main thread.