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POtest "pthread_create" function behaviour
 

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  1. POtest "pthread_create" function behaviour
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    <p>I am new in multi-threaded programming and I have a question about "pthread_create" behaviour this is the code : </p> <pre><code>#include &lt;pthread.h&gt; #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #define NTHREADS 10 #define ARRAYSIZE 1000000 #define ITERATIONS ARRAYSIZE / NTHREADS double sum=0.0, a[ARRAYSIZE]; pthread_mutex_t sum_mutex; void *do_work(void *tid) { int i, k=0,start, *mytid, end; double mysum=0.0; mytid = (int *) tid; start = (*mytid * ITERATIONS); end = start + ITERATIONS; printf ("Thread %d doing iterations %d to %d\n",*mytid,start,end-1); for (i=start; i &lt; end ; i++) { a[i] = i * 1.0; mysum = mysum + a[i]; } sum = sum + mysum; } int main(int argc, char *argv[]) { int i, start, tids[NTHREADS]; pthread_t threads[NTHREADS]; pthread_attr_t attr; for (i=0; i&lt;NTHREADS; i++) { tids[i] = i; pthread_create(&amp;threads[i], NULL/*&amp;attr*/, do_work, (void *) &amp;tids[i]); } /* Wait for all threads to complete then print global sum */ /* for (i=0; i&lt;NTHREADS; i++) { pthread_join(threads[i], NULL); }*/ printf ("Done. Sum= %e \n", sum); sum=0.0; for (i=0;i&lt;ARRAYSIZE;i++){ a[i] = i*1.0; sum = sum + a[i]; } printf("Check Sum= %e\n",sum); } </code></pre> <p>the result of execution is : </p> <pre><code>Thread 1 doing iterations 100000 to 199999 Done. Sum= 0.000000e+00 Thread 0 doing iterations 0 to 99999 Thread 2 doing iterations 200000 to 299999 Thread 3 doing iterations 300000 to 399999 Thread 8 doing iterations 800000 to 899999 Thread 4 doing iterations 400000 to 499999 Thread 5 doing iterations 500000 to 599999 Thread 9 doing iterations 900000 to 999999 Thread 7 doing iterations 700000 to 799999 Thread 6 doing iterations 600000 to 699999 Check Sum= 8.299952e+11 </code></pre> <p>all thread are created and the execution is not sequential (remove pthread_join), but the function do_work is executed in order and depend on thread. it means iterations 0 to 99999 are done by thread 0 and iterations 100000 to 199999 are done by thread 1 etc ...</p> <p>the question is here why for example iterations 0 to 99999 is not done by thread 2 ? </p>
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    1. COYou don't use the mutex you have defined. You should use the mutex to access global variables. Also, you pass the `&tids[i]` which has been assigned `i`. YOur `start` and `end` are based on this input. Hence, the output
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    2. COhttps://computing.llnl.gov/tutorials/pthreads/
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