Today Return Quiz First release of final project template Multithreading Options Qt socket buffers between threads Tomorrow: Quiz Interthread communication in Qt SE-2811 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder 1 SE3910 Week 8, Class 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 HAND if you want to use it.) [1] pthreads 2

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. a-threads-vs-pthreads SE Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 3

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

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

Threading – pthreads (updated with links) JavaPthreads Object o; o.notify(); phread_cond_t c = PTHREAD_COND_INITIALIZER; pthread_cond_broadcast(c); o.wait();pthread_cond_wait(c,m); o.notify();phtread_cond_signal(c); o.notifyAll();phtread_cond_broadcast(c); SE Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 6 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.”

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

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

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. ” -to-really-truly-use-qthreads-the-full-explanation/ I have successfully used this approach. SE-2811 Dr.Yoder 9

Qt’s connect method enum enum SE-2811 Dr.Yoder 10

Qt Connection types ConstantValueDescription Qt::AutoConnection 0 (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.lives in Qt::Direct Connection 1 The slot is invoked immediately when the signal is emitted. The slot is executed in the signalling thread. Qt::Queued Connection 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. Qt::Blocking QueuedConnection 3 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. Qt::Unique Connection 0x80 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.QObject::connect SE-2811 Dr.Yoder 11

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 // what happens? (For Java? For Qt?) Click stop // what happens? (For Java? For Qt?) SE-2811 Dr.Yoder 12

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

Possibly Fun Very simple C++ wrapper for pthreads cs345/lectures/CS345-Lecture-09.pdf cs345/lectures/CS345-Lecture-09.pdf notify/wait – example comparison with Java and pthreads Compares create for Java, pthreads, and Win32 threads adsII.pdf adsII.pdf SE Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 14

Certainly fun (Dr. Taylor’s Reviews) ointers ointers Function Pointers C/C++/Java Object-Oriented C SE Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 15

Other references s/02-14.jpg s/02-14.jpg Simple pthread chart From m0/index.jsp?topic=%2Fapis%2Fusers_75. htm 1m0/index.jsp?topic=%2Fapis%2Fusers_75. htm IBM example of using pthread_cond_init SE Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 16

Standards 9/xsh/pthread.h.html 9/xsh/pthread.h.html pthead.h 9/xsh/systypes.h.html 9/xsh/systypes.h.html systypes.h – for xxxx_t SE Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 17

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

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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 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder 21

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

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