1. Multithreading Tutorial
Professor: Shu-Ching Chen
TA: Hsin-Yu Ha

2. What is a Thread?
An independent stream of instructions that can be scheduled to run
A path of execution
int a, b;
int c;
a = 1;
b = a + 2;
c = 3;
Sequentially execute
c = 5;
CPU
Multi-Thread
Single-Thread

3. Program v.s. Process v.s. Thread
An execution file stored in the harddrive
Process
An execution file stored in the Memory
Thread
An execution path of part of the process
HDD
Memory
Program
Process
Thread

4. Why is thread? Parallel execution Shared resources
Easier to create and destroy than processes (100X)
Easy porting to multiple CPUs

5. Compiling a pthreads program
Return Result :

6. Creating threads (1) pthread_create() Return 0 if OK, nonzero on error
Four Arguments
Thread : A thread identifier
Attr : A pointer to a thread attribute object
Start_routine : A pointer to the function the thread is execute
Arg : The argument to the function

7. Creating threads (2)
Single variable

8. Creating threads (3)
Several variables

9. Terminating Threads (1)
pthread_exit() Return 0 if OK, nonzero on error
Four ways of terminating a thread
The thread returns from its starting routine
The thread makes a call to the pthread_exit subroutine
The thread is canceled by another thread
The entire process is terminated

10. Terminating Threads (2)
Example of pthread_exit()

11. Terminating Threads (3)
pthread_join() Return 0 if OK, nonzero on error
Wait from other threads to terminate by calling it

12. Terminating Threads (4)

13. Other Thread functions
pthread_self()
It returns the unique, system assigned thread ID of the calling thread
pthread_detach() Return 0 if OK, nonzero on error
It can be used to explicitly detach a thread

14. Synchronizations
Join
Semaphore
Mutexes
Barrier

15. Synchronization - Semaphore (1)
A separate POSIX standard does define them
Important functions
sem_init : Initialize a new semaphore
sem_destroy : deallocate an existing semaphore
sem_wait :
sem_post

16. Synchronization - Semaphore (2)

17. Synchronization - Mutexes (1)
Mutexes are used
to prevent data inconsistencies due to operations by multiple threads upon the same memory area performed at the same time
to prevent race conditions where an order of operation upon the memory is expected

18. Synchronization - Mutexes (2)

19. Synchronization - Mutexes (3)

20. Synchronization – Barrier (1)
It is used when some parallel computations need to "meet up" at certain points before continuing.
Pthreads extension includes barriers as synchronization objects
(available in Single UNIX Specification)
Enable by #define _XOPEN_SOURCE 600 at start of file
Initialize a barrier for count threads
int pthread_barrier_init(pthread_barrier_t *barrier,
const pthread_barrier attr_t *attr, int count);
Each thread waits on a barrier by calling
int pthread_barrier_wait(pthread_barrier_t *barrier);
Destroy a barrier
int pthread_barrier_destroy(pthread_barrier_t *barrier);

21. Synchronization – Barrier (2)

22. References POSIX Threads Programming Pthreads primer
Pthreads primer
POSIX thread (pthread) Tutorial