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Waiting for a thread to terminate

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• Joinable and detached threads
  • As many parallel applications often have interspersed sequential and parallel steps, where the exection of the sequential step must wait until all threads in the parallel step has finished execution; you often find that threads must sometimes WAIT on each other.

  • The crucial difference between joinable and detached threads is precisely this wiating property

  • Detached threads run independently from other threads and cannot wait for other threads.

    The waiting on other threads is accomplished by using the function pthread_join() (more later).

  • Joinable threads may call pthread_join() to wait on another thread.

  • The default value is joinable, you do NOT want to change this (unless you are going to provide some kind of barrier synchronization routine yourself....)

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• Waiting for worker threads to finish....
  • You have seen it in some examples before, I just want to spell out the effect of the function pthread_join() explicitly.

  • When a thread A calls the function

    Thread A invokes: pthread_join( Tid, &status ); Tid = ID of a thread status = variable to receive the exit state of thread. (&status will pass the variable by reference)

    Then the following will happen:
    • The calling thread (A) will wait until the target thread (Tid) completes (= exits)

      The thread Tid completes if:

      the function (that thread Tid executes) returns       or          thread Tid executes the pthread_exit( ) function

    • Note that thread Tid must be JOINABLE

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    Example: waiting for 1 thread (Hello World)

    • Program:

      #include <pthread.h> /* ================================== Thread prints "Hello World" ================================== */ void *worker(void *arg) { cout << "Hello World !" << endl; return(NULL); /* Thread exits (dies) */ } /* ================================================= MAIN: create a trhead and wait for it to finish ================================================= */ int main(int argc, char *argv[]) { pthread_t tid; /* --------------------- Create threads --------------------- */ if ( pthread_create(&tid, NULL, worker, NULL) ) { cout << "Cannot create thread" << endl; exit(1); } cout << "Main now waits for thread tid to finish...." << endl ; pthread_join(tid, NULL); exit(0); }

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    • Example Program: (Demo above code)                                                
      • Prog file: click here

      How to run the program:

      Right click on link and save in a scratch directory To compile:   gcc -pthread HelloWorld.c To run:          ./a.out

      Better demo program:

      Compile: gcc -o Join -pthread Join.c Run: Join

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  • Example 2: waiting for multiple threads to complete

    int main(int argc, char *argv[]) { int i, num_threads; thread_t tid[100]; /* Thread ID used for thread_join() */ int param[100]; /* Parameters for threads */ num_threads = ... (number of threads to create); /* -------------------------- Create threads --------------------------- */ for (i = 0; i < num_threads; i = i + 1) { param[i] = ... ; // Initialize parameter for thread i if ( pthread_create(&tid[i], &attr, worker, & param[i]) != 0 ) { printf("Cannot create thread\n"); exit(1); } } /* ---------------------------------- Wait for ALL threads to finish ---------------------------------- */ for (i = 0; i < num_threads; i = i + 1) pthread_join(tid[i], NULL); }

    Previous example: thread01.C

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   g++ -pthread thread01.C To run:          ./a.out