Multithreading Options in Qt: Quiz on Interthread Communication

In today's class, the focus was on multithreading options in Qt, including understanding the socket buffers between threads. The first release of the final project template was discussed. Tomorrow's agenda includes a quiz on interthread communication in Qt. Dr. X also presented slide design insights. Stay updated with the latest on software engineering and Qt development through our sessions.

• Qt development
• Multithreading
• Interthread communication
• Software engineering
• Slide design

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Presentation Transcript

1. SE3910 Week 8, Class 2 Today Return Quiz First release of final project template Multithreading Options Qt socket buffers between threads Tomorrow: Quiz Interthread communication in Qt SE-2811 1 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder

2. Options for multithreading (review) pthreads POSIX (Linux/Unix standard) threads Can use in C/C++ qthreads Qt Threads Object-Oriented Uses pthreads under the hood, when on POSIX [1] Derek Malloy s GPIO class Uses pthreads under the hood, but is single- threaded Boost (Please discuss this with me BEFORE 2

3. I was asked this question in an interview today . (continued) "When we create a thread with pthread_create() (POSIX Threads), the thread starts on its own. Why do we need to explicitly call start() in Java. What is the reason that Java doesnt start the thread when we create an instance of it." I was blank and interviewer was short of time and eventually he couldnt explain the reason to me. http://stackoverflow.com/questions/5269535/jav a-threads-vs-pthreads SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 3

4. Task States (more essential rambling) Wiki:Process (computing) See also Laplante and Ovaske 4E p. 97 SE-2811 Dr.Yoder 4

5. Threading pthreads (review) Java java.lang.Thread No external jar needed link with -pthread Thread t = new Thread(r) t.start(); interface Runnable { void run(); } t.join(); Object o; synchronized(o) { } /* Garbage coll. */ Pthreads #include <pthread.h> pthread_create(t,r,sr,a) Parameter: void* (*sr) (void *) pthread_join(*t, &p) pthread_mutex_init(m,null) pthread_mutex_lock( ) pthread_mutex_destroy( ) 5

6. Threading pthreads (updated with links) Java Object o; o.notify(); Pthreads phread_cond_t c = PTHREAD_COND_INITIALIZER; pthread_cond_broadcast(c); pthread_cond_wait(c,m); phtread_cond_signal(c); phtread_cond_broadcast(c); o.wait(); o.notify(); o.notifyAll(); See Java coding example NotifyWaitExample Caveat: POSIX threads can wait at condition variables of a greater generality than available in Java, but the corresponding queues may be leaky. http://wimhesselink.nl/pub/whh241b.pdf SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 6

7. Ex: Edit to include notify/wait to interleave these threads. Consider this code- snippet Queue q = synchronized(q) { while(true) { img = getImage(); q.offer(img); } } // on another thread synchronized(q) { while(true) { img = q.poll(); showImage(); } } SE-2811 Dr.Yoder 7

8. Threading qthreads (with corrections) Java qthreads java.lang.Thread #include <QThread> No external jar needed (moc and friends take care of this) Thread t = new Thread(r) t.start(); interface Runnable { void run(); } QMainWindow)) t.join(); that checks if all threads are done. Object o; QMutex synchronized(o) { } /* Garbage coll. */ ???? QThread *t = new QThread; moveToThread(t); // note here QObject (e.g. QWidget (e.g. connect the QThread::finish() signal to a slot Avoid sharing memory entirely see code example 8

9. Useful if you are into Qt slots/signals == events A QThread should be used much like a regular thread instance: prepare an object (QObject) class with all your desired functionality in it. Then create a new QThread instance, push the QObject onto it using moveToThread(QThread*) of the QObject instance and call start() on the QThread instance. That s all. https://mayaposch.wordpress.com/2011/11/01/how -to-really-truly-use-qthreads-the-full-explanation/ I have successfully used this approach. SE-2811 Dr.Yoder 9

10. Qts connect method http://doc.qt.io/qt-5/qobject.html#connect http://doc.qt.io/qt-5/qt.html#ConnectionType- enum SE-2811 Dr.Yoder 10

11. Qt Connection types http://doc.qt.io/qt-5/qt.html#ConnectionType-enum Constant Value Description (Default) If the receiver lives inthe thread that emits the signal, Qt::DirectConnection is used. Otherwise, Qt::QueuedConnection is used. The connection type is determined when the signal is emitted. Qt::AutoConnection 0 Qt::Direct Connection Qt::Queued Connection Qt::Blocking QueuedConnection3 1 The slot is invoked immediately when the signal is emitted. The slot is executed in the signalling thread. 2 The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed inhe t receiver's thread. Same as Qt::QueuedConnection, except that the signalling thread blocks until the slot returns. This connection must not be used if the receiver lives in the signalling thread, or else the application will deadlock. This is a flag that can be combined with any one of the above connection types, using a bitwise OR. When Qt::UniqueConnection is set,QObject::connect() will fail if the connection already exists (i.e. if the same signal is already connected to the same slot for the same pair of objects). This flag was introduced in Qt 4.6. SE-2811 Dr.Yoder Qt::Unique Connection 0x80 11

12. Coding example. Exercise: Both the Java and Qt solutions will behave poorly if I start multiple threads. Predict how each solution will behave if I: Click start Click start // what happens? (For Java? For Qt?) Click stop // what happens? (For Java? For Qt?) SE-2811 Dr.Yoder 12

13. A bit more on the Rate Monotonic Analysis SE-2811 Dr.Yoder 13

14. Possibly Fun Very simple C++ wrapper for pthreads http://www.cs.utexas.edu/users/lavender/courses/ cs345/lectures/CS345-Lecture-09.pdf notify/wait example comparison with Java and pthreads http://wimhesselink.nl/pub/whh241b.pdf Compares create for Java, pthreads, and Win32 threads http://cs.millersville.edu/~csweb/lib/userfiles/9Thre adsII.pdf SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 14

15. Certainly fun (Dr. Taylors Reviews) http://msoe.us/taylor/tutorial/ce2810/functionp ointers Function Pointers http://msoe.us/taylor/tutorial/ce2810/csimilar C/C++/Java http://msoe.us/taylor/tutorial/ce2810/ooc Object-Oriented C SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 15

16. Other references http://granite.sru.edu/~whit/cpsc464/Notes/fig s/02-14.jpg Simple pthread chart From http://granite.sru.edu/~whit/cpsc464/Notes/ch2.html https://pic.dhe.ibm.com/infocenter/iseries/v7r 1m0/index.jsp?topic=%2Fapis%2Fusers_75. htm IBM example of using pthread_cond_init SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 16

17. Standards http://pubs.opengroup.org/onlinepubs/790879 9/xsh/pthread.h.html pthead.h http://pubs.opengroup.org/onlinepubs/790879 9/xsh/systypes.h.html systypes.h for xxxx_t SE-3910 - Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 17

18. Muddiest Point Wait for the slides, or follow this link to answer both questions at once: http://bit.ly/1Mow5a3 SE-2811 Dr.Yoder 18

19. http://bit.ly/1Mow5a3 SE-2811 Dr. Josiah Yoder 19

20. http://bit.ly/1Mow5a3 SE-2811 Dr. Josiah Yoder 20

21. References EBB: Derek Malloy, Exploring Beaglebone, Wiley, 2015 RTS: Laplante and Ovaska, Real-Time Systems Design and Analysis by, Fourth Edition, Wiley, 2012 SE-2811 21 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder

22. Someday soon Late next week? Real-Time Operating Systems What is a RTOS? How does it relate to the rest? OS roles Interrupts and the OS Definitions, Flowchart, Timing diagram Detailed steps Scheduling Task states & pre-runtime vs runtime SE-2811 Dr.Yoder 22

23. Task States Wiki:Process (computing) See also Laplante and Ovaske 4E p. 97 SE-2811 Dr.Yoder 23