1. Copyright ©: Nahrstedt, Angrave, Abdelzaher
Threads (part II)
Copyright ©: Nahrstedt, Angrave, Abdelzaher

2. Copyright ©: Nahrstedt, Angrave, Abdelzaher
Pthread Operations
POSIX function
description
pthread_create
create a thread
pthread_detach
set thread to release resources
pthread_equal
test two thread IDs for equality
pthread_exit
exit a thread without exiting process
pthread_kill
send a signal to a thread
pthread_join
wait for a thread
pthread_self
find out own thread ID

3. Exiting and Cancellation
Copyright ©: Nahrstedt, Angrave, Abdelzaher
Exiting and Cancellation
Question:
If a thread calls exit(), what about other threads in the same process?
When does a process terminate?

4. Exiting and Cancellation
Copyright ©: Nahrstedt, Angrave, Abdelzaher
Exiting and Cancellation
Question:
If a thread calls exit(), what about other threads in the same process?
A process terminates when:
it calls exit directly
one of its threads calls exit
it returns from main()
it receives a termination signal
In any of these cases, all threads of the process terminate.

5. Copyright ©: Nahrstedt, Angrave, Abdelzaher
Thread Exit
void pthread_exit(void *value_ptr);
Common uses:
Return value is often a pointer to a malloc’d struct (memory must be free’d by joining thread)
Can I return the address of a variable on the stack? 5 5

6. Thread Exit: pthread_exit or return?
Copyright ©: Nahrstedt, Angrave, Abdelzaher
Thread Exit: pthread_exit or return?
When a thread is done, it can return from its first function (the one used by pthread_create) or it can call pthread_exit
An implicit call to pthread_exit() is made when a thread other than the thread in which main() was first invoked returns from the start routine that was used to create it. The function’s return value shall serve as the thread’s exit status. (from man pages) 6 6

7. Copyright ©: Nahrstedt, Angrave, Abdelzaher
Cancel that thread!
One thread can request that another exits with pthread_cancel
int pthread_cancel(pthread_t thread);
The pthread_cancel returns after making the request.
The cancellation action in the target thread shall run asynchronously with respect to the calling thread returning from pthread_cancel().
Quiz: Do you see any danger by canceling a thread at an arbitrary point of its execution?

8. Copyright ©: Nahrstedt, Angrave, Abdelzaher
Cancel that thread!
Quiz: Do you see any danger by canceling a thread at an arbitrary point of its execution?
 Asynchronous cancellation immediately terminates another thread wherever it is in its execution sequence. This is dangerous and should be avoided (think about the case when a thread locks a shared resource and then it is cancelled while still inside its critical section) 8 8

9. Copyright ©: Nahrstedt, Angrave, Abdelzaher
Cancel that thread!
Cancellation is the mechanism by which a thread can terminate the execution of another thread.
Depending on its settings, the target thread can then either ignore the request, honor it immediately, or defer it till it reaches a cancellation point.
Threads are always created by pthread_create with cancellation enabled and deferred. That is, the initial cancellation state is PTHREAD_CANCEL_ENABLE and the initial type is PTHREAD_CANCEL_DEFERRED. 9 9

10. Copyright ©: Nahrstedt, Angrave, Abdelzaher
Cancel that thread!
When a thread eventually honors a cancellation request, it performs as if pthread_exit(PTHREAD_CANCELED) has been called at that point; hence, the thread stops executing with the return value PTHREAD_CANCELED.
void pthread_testcancel(void) is a deferred cancellation point
Other functions can also act as deferred cancellation points like blocking operations (such as waiting on a mutex lock or an I/O operation) 10 10

11. Zombies, Thread Detach & Join
Copyright ©: Nahrstedt, Angrave, Abdelzaher
Zombies, Thread Detach & Join
Call pthread_join() or pthread_detach() for every thread that is created joinable
so that the system can reclaim all resources associated with the thread
Failure to join or to detach threads memory and other resources will leak until the process ends

12. Copyright ©: Nahrstedt, Angrave, Abdelzaher
Detaching a Thread
int pthread_detach(pthread_t threadid);
Indicate that system resources for the specified thread should be reclaimed when the thread ends
If the thread has already ended, resources are reclaimed immediately
This routine does not cause the thread to end
Threads are detached
after a pthread_detach() call
after a pthread_join() call
if a thread terminates and the PTHREAD_CREATE_DETACHED attribute was set on creation

13. How to make a Thread Detached
Copyright ©: Nahrstedt, Angrave, Abdelzaher
How to make a Thread Detached
void *processfd(void *arg);
int error;
int fd
pthread_t tid;
if (error = pthread_create(&tid, NULL, processfd, &fd)) {
fprintf(stderr, "Failed to create thread: %s\n", strerror(error)); }
else if (error = pthread_detach(tid)){
fprintf(stderr, "Failed to detach thread: %s\n", strerror(error));

14. How a thread can detach itself before terminating
Copyright ©: Nahrstedt, Angrave, Abdelzaher
How a thread can detach itself before terminating
void *runnger(void *arg) { …
/* return NULL after detaching itself */
if (!pthread_detach( pthread_self()) )
return NULL; }

15. “Waiting” on a Thread: pthread_join()
Copyright ©: Nahrstedt, Angrave, Abdelzaher
“Waiting” on a Thread: pthread_join()
int pthread_join(pthread_t thread, void** retval);
pthread_join() is a blocking call on non-detached threads
It indicates that the caller wishes to block until the thread being joined exits
You cannot join on a detached thread, only non-detached threads (detaching means you are NOT interested in knowing about the thread’s exit)
It is a cancellation point

16. Copyright ©: Nahrstedt, Angrave, Abdelzaher
Pthread_join
int error;
void *exitcodep;
pthread_t tid; …
if (error = pthread_join(tid, &exitcodep)){
fprintf(stderr, "Failed to join thread: %s\n", strerror(error)); }
else {
fprintf(stderr, "The exit code was %d\n", (int)exitcodep);