In this demo we intend to show that we can use a syscall to terminate a thread and release the resources owned by it.
To illustrate the mechanism, we must introduce the following:
thread_wait(insrc/user/thread.c)thread_cancel(insrc/user/thread.c)
This function sets the state of the current thread to waiting and yields.
Since the scheduler will not pick a thread in waiting state, the thread will remain idle until another thread changes its state.
When we call this function, it will cancel the thread specified by the tid.
If tid is given as CURRENT_THREAD, the current thread will be terminated. The scheduler will choose another thread to run.
If we apply cancel() to a valid thread, the memory space it uses will be released. Then we can create another thread using the released memory space.
When we call the function to cancel an invalid thread, it will return false. No thread will be terminated.
In the beginning, the setup() function creates a thread called cancel_self which cancels itself after it starts to run.
When the scheduler chooses setup thread to run again, it creates another 3 threads.
The tid of each thread and their corresponding routines are listed below:
| tid | routine | usage |
|---|---|---|
| 1 | work | thread to be cancelled |
| 2 | canceller | execute cancelling |
| 3 | work | thread to be cancelled |
The setup thread then sets its state to waiting and yields. The scheduler chooses thread 1 to run next. It prints out the message "foo" and yields.
The canceller thread starts by checking that you can't cancel an invalid thread, then cancelling thread 3. Then canceller adds a new thread. We will place the new thread into space that used to store the thread we cancelled. As a result, the tid of the new thread will be 3. We cancel the new thread 3 when we end up checking.
After that, the canceller thread terminates thread 1. We see that its state is cancelled when we use thread_join() to join thread 1.
Finally, the canceller cancels thread 0 (which is still in the waiting state) and returns. The demo now ends because there are no valid threads left.