Question

I have a question concerning this code which I want to run on QNX:

class ConcreteThread : public Thread { public:     ConcreteThread(int test)     {         testNumber = test;     }      void *start_routine()      {          for(int i = 0; i < 10; i++)         {             sleep(1);             cout << testNumber << endl;         }        }  private:     int testNumber; };     class Thread  { public:     Thread(){};      int Create()     {         pthread_t m_id;         return pthread_create(&m_id, NULL, &(this->start_routine_trampoline), this);     }  protected:     virtual void *start_routine() = 0;  private:      static void *start_routine_trampoline(void *p)     {         Thread *pThis = (Thread *)p;         return pThis->start_routine();     } }; 

Now, when I run this code without the sleep in *start_routine, it will simply print the number 10 times, before continuing on to the next line of code (sequential instead of parallel). However, when I use a sleep like in the code, it doesn’t print any numbers at all and simply goes on to the next line of code. Why doesn’t sleep work and how can I make a thread like this work, instead of running sequential?

Answer 1

Note 1: If you only have 1 processor the code can only be done sequentially no matter how many threads you create. Each thread is given a slice of processor time before it is swapped out for the next threads.

Note 2: If the main thread exits pthreads will kill all child threads before they have a chance to execute.

Now to answer you questions:

Without the sleep. The thread once started has enough time in the single slice it was given to execute the loop 10 times completely.

With the sleep: Your worker thread is going to sleep for a full second. So your main thread has time to do a lot of work. If the main thread exits in this time the worker will be killed.

I would make the following changes:

//  Remove the Create() method //  Put thread creation in the constructor. //  Make the thread variable part of the object  pthread_t m_id;  Thread() {     if (pthread_create(&m_id, NULL, &(this->start_routine_trampoline), this) != 0)     {         throw std::runtime_error('Thread was not created');     } }  // Make sure the destructor waits for the thread to exit. ~Thread() {     pthread_join(m_id); } 

If you go and look at boost threading library. you will find that all the little mistakes like this have already been taken care of; Thus making threading easier to use.

Also note. That using a static may work but it is non portable. This is because pthread’s is a C library and is thus expecting a function pointer with a C ABI. You are just getting lucky for your platform here. You need to define this as a function and declare the ABI by using extern ‘C’

// This needs to be a standard function with C Interface. extern 'C' void *start_routine_trampoline(void *p) { }