Thread life cycle

thread-life-cycle

Init

Thread creation thread_create() puts a thread into its init state and allocates and initialises per-thread data structures.
Once that is done, thread creation code puts the thread in ready state by adding the thread to the ready list (set of runnable threads that are waiting their turn to use a processor).

A thread in ready state is available to be run but is not currently running.
Its TCB is on the ready list, and the values of its registers are stored in its TCB.
At any time, the scheduler can cause a thread to transition from ready to running state, by copying its register values from its TCB to a processor’s registers.

A thread in running state is running on a processor. At this time, its register values are stored on the processor rather than in the TCB.
A running thread can transition to the ready state in two ways:
- The scheduler can preempt a running thread and move it to the ready state by saving the thread’s registers to its TCB and switching the processor to run the next thread on the ready list.
- A running thread can voluntarily relinquish the processor and go from running to ready by calling thread_yield().

A thread in the waiting state is waiting for some event.
While the scheduler can move a thread in the ready state to the running state, a thread in the waiting state cannot run until some action by another thread moves it from waiting to ready.
The TCB of a waiting thread is stored on the waiting list of some synchronisation variable associated with the event. When the required event occurs, the operating system moves the TCB from the synchronisation variable’s waiting list to the scheduler’s ready list, transitioning the thread from waiting to ready.

A thread in finished state never runs again.
The system can free some or all of its state for other use. Some systems may store finished threads in a finished list.

One way to understand the states is to consider where a thread’s TCB and registers are stored.

State of Thread	Location of TCB	Location of Registers
`init`	Being Created	TCB
`ready`	Ready List	TCB
`running`	Running List	Processor
`waiting`	Waiting List of a Synchronisation Variable	TCB
`finished`	Finished List / Deleted	TCB / Deleted