Initial thread

When a process is created, one thread is automatically created. This thread is called the initial thread.

It ensures the compatibility between the old processes with a unique implicit thread and the new multithreaded processes. The initial thread has some special properties, not visible to the programmer, that ensure binary compatibility between the old single-threaded programs and the multithreaded operating system. It is also the initial thread that executes the main routine in multithreaded programs.