Introduction to Pthreads: Concurrent Programming in C

1. Pthreads & Concurrency implementation

2. Acknowledgements • The material in this tutorial is based in part on: • POSIX Threads Programming , by Blaise Barney

3. History Advantages Content Pthreads Overview

4. History • Different versions of threads • each hardware vendor created own version • IEEE POSIX 1003.1c Standard • C programming types and procedure calls

5. Advantages • Potential program performance gains

6. Advantages • More efficient inter-thread communication • Threaded applications • Overlap CPU work and I/O • Priority/Real-time scheduling • Asynchronous event handling

7. The Pthreads API • Thread management • Create, detach, join • Set/query thread attributes • Mutex • Mutual Exclusion • Condition variables • Address communications between threads • Non-mutex synchronization • Read/write locks • Barriers

8. The Pthreads API • Naming conventions

9. Create & Terminate • Program: Hello World Threads Management

10. Creation • Routine • pthread_create( thread, attr, start_routine, arg)

11. Termination • Four ways for a Pthread to terminate • Returns from start_routine • Call pthread_exit routine • Cancel by another thread through pthread_cancel routine • exit call

12. Example: Hello World • Program Requirements • Print “Hello World” through standard output • Use two threads to finish the task, one should print “Hello”, the other should print “World” • Terminate all related threads after finishing the printing task

13. Example: Hello World #include <stdio.h> #include <pthread.h> ……

14. Example: Hello World void print_message_function( void *ptr ) { char *message; message = (char *) ptr; printf("%s ", message); }

15. Example: Hello World main() { pthread_t thread1, thread2; char *message1 = "Hello"; char *message2 = "World"; …… }

16. Example: Hello World main() { pthread_t thread1, thread2; char *message1 = "Hello"; char *message2 = "World"; …… }

17. Example: Hello World main() { pthread_t thread1, thread2; char *message1 = "Hello"; char *message2 = "World"; …… }

18. Example: Hello World main() { …… pthread_create(&thread1, pthread_attr_default, (void*)&print_message_function, (void*) message1); pthread_create(&thread2, pthread_attr_default, (void*)&print_message_function, (void*) message2); exit(0); }

19. Example: Hello World main() { …… pthread_create(&thread1, pthread_attr_default, (void*)&print_message_function, (void*) message1); pthread_create(&thread2, pthread_attr_default, (void*)&print_message_function, (void*) message2); exit(0); }

20. Example: Hello World main() { …… pthread_create(&thread1, pthread_attr_default, (void*)&print_message_function, (void*) message1); pthread_create(&thread2, pthread_attr_default, (void*)&print_message_function, (void*) message2); exit(0); }

21. Example: Hello World main() { …… pthread_create(&thread1, pthread_attr_default, (void*)&print_message_function, (void*) message1); pthread_create(&thread2, pthread_attr_default, (void*)&print_message_function, (void*) message2); exit(0); }

22. Example: Hello World main() { …… pthread_create(&thread1, pthread_attr_default, (void*)&print_message_function, (void*) message1); pthread_create(&thread2, pthread_attr_default, (void*)&print_message_function, (void*) message2); exit(0); }

23. Example: Hello World main() { …… pthread_create(&thread1, pthread_attr_default, (void*)&print_message_function, (void*) message1); pthread_create(&thread2, pthread_attr_default, (void*)&print_message_function, (void*) message2); exit(0); }

24. Example: Hello World main() { …… pthread_create(&thread1, pthread_attr_default, (void*)&print_message_function, (void*) message1); pthread_create(&thread2, pthread_attr_default, (void*)&print_message_function, (void*) message2); exit(0); }

25. Example: Hello World Thread 1 default thread Thread 2

26. Example: Hello World Thread 1 default thread Thread 2

27. Example: Hello World Thread 1 default thread Thread 2

28. Example: Hello World Thread 1 default thread Thread 2

29. Example: Hello World Thread 1 default thread Thread 2

30. Example: Hello World Thread 1 default thread Thread 2 Printf Printf

31. Example: Hello World Thread 1 default thread Thread 2 Printf Exit Printf

32. Example: Hello World Thread 1 standard output Thread 2 Thread 1 default thread Thread 2 Return

33. Example: Hello World void print_message_function( void *ptr ) { char *message; message = (char *) ptr; printf("%s ", message); } main() { …… pthread_exit(); //exit(0); }

34. Example: Hello World void print_message_function( void *ptr ) { char *message; message = (char *) ptr; printf("%s ", message); pthread_exit(); } main() { …… pthread_exit(); //exit(0); }

35. Passing parameters to threads • The pthread_create() routine permits the programmer to pass one argument to the thread start routine. • For cases where multiple arguments must be passed, this limitation is overcome by creating a structure which contains all of the arguments, and then passing a pointer to that structure. • All arguments must be passed by reference and cast to (void *).

36. Joining and detaching threads • pthread_join(threadid,status) • blocks the calling thread until the specified threadid thread terminates. • target thread's termination return status placed in status if specified in the target thread's call to pthread_exit(). • A joining thread can match one pthread_join() call. • It is a logical error to attempt multiple joins on the same thread. • pthread_detach(threadid, status) • to explicitly detach a thread • pthread_attr_setdetachstate(attr,detachstate) • pthread_attr_getdetachstate(attr, detachstate)

37. Create & Destroy • Lock & Unlock • Program: Bounded Buffer Mutex

38. Mutex • Mutual Exclusion • Act as a “lock” to prevent race conditions • Typical sequence of use • create, initialize • lock, unlock • destroy • Losers in the competition for the mutex variable have to block

39. Creating & Destroying • Type: pthread_mutex_t • Initializing • Statically • pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER • Dynamically • pthread_mutex_init (mutex, attr) • pthread_mutex_attr_tattr specifies protocol for priorities, priority ceiling, and process sharing • Destroying • pthread_mutex_destroy – to delete a mutex object • pthread_mutex_attr_destroy – to delete a mutexattr object

40. Lock & Unlock • Lock • pthread_mutex_lock (mutex) • blocks calling thread if mutex already locked • pthread_mutex_trylock(mutex) • returns with “busy” code if already locked • Unlock • pthread_mutex_unlock (mutex) • Error • mutex is released • mutex owned by others