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PO#pragma omp flush to make exchange data among threads
 

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    <p>Hi writing a very simple example on how to use omp flush to exchange data, in a producer-> consumer way,among threads I have found a funny behavior.</p> <pre><code>int a=-1; int flag=1; int count=0; #pragma omp parallel num_threads(2) { int TID; TID=omp_get_thread_num(); #pragma omp sections { #pragma omp section /////////// Producer { for(int i=0; i&lt;9;i++) { a=i; #pragma omp flush(a) flag=1; printf("Producer a: %d flag:%d TID %d \n",a,flag,TID); while(flag) { #pragma omp flush(flag) } } flag=2; #pragma omp flush(flag) } // end producer #pragma omp section /////////// Consumer { while(1) { count++; flag=0; while(!flag) { #pragma omp flush(flag) } #pragma omp flush(a) printf("Consumer a: %d Flag: %d count %d TID %d \n",a,flag,count,TID); if (flag==2) break; // no more data } // end while(1) }// end consumer }// end sections </code></pre> <p>Using this very simple code will produce an erroneous output: Producer a: 0 flag:1 TID 0<br> Producer a: 1 flag:1 TID 0<br> Consumer a: 1 Flag: 1 count 1 TID 1<br> Producer a: 2 flag:1 TID 0<br> Consumer a: 2 Flag: 1 count 2 TID 1<br> Producer a: 3 flag:1 TID 0<br> Consumer a: 3 Flag: 1 count 3 TID 1<br> Producer a: 4 flag:1 TID 0<br> Consumer a: 4 Flag: 1 count 4 TID 1<br> Producer a: 5 flag:1 TID 0<br> Consumer a: 5 Flag: 1 count 5 TID 1<br> Producer a: 6 flag:1 TID 0<br> Consumer a: 6 Flag: 1 count 6 TID 1<br> Producer a: 7 flag:1 TID 0<br> Consumer a: 7 Flag: 1 count 7 TID 1<br> Producer a: 8 flag:1 TID 0<br> Consumer a: 8 Flag: 1 count 8 TID 1<br> Consumer a: 8 Flag: 2 count 9 TID 1 </p> <p>The error is that the first datum produced a=0 is ignored by the consumer. If I simply invert the order of the sections, letting the producer be thread 1 then everything is ok..... Producer a: 0 flag:1 TID 1<br> Consumer a: 0 Flag: 1 count 1 TID 0<br> Producer a: 1 flag:1 TID 1<br> Consumer a: 1 Flag: 1 count 2 TID 0<br> .... Whats my mistake ?</p> <p>..... After the interesting discussion with Ejd (thanks) the code was edited to:</p> <pre><code>int a=-1; int flag=0; int count=0; #pragma omp parallel num_threads(2) { int TID; TID=omp_get_thread_num(); #pragma omp sections { #pragma omp section /////////// Consumer { while(1) { count++; if (flag) printf("Consumer a: %d Flag: %d count %d TID %d \n",a,flag,count,TID); flag=0; while(!flag) { #pragma omp flush(flag) } if (flag==2) break; // no more data } // end while(1) }// end consumer #pragma omp section /////////// Producer { for(int i=0; i&lt;9;i++) { a=i; printf("Producer a: %d flag:%d TID %d \n",a,flag,TID); flag=1; while(flag) { #pragma omp flush(flag,a) } } flag=2; #pragma omp flush(flag) } // end producer }// end sections </code></pre> <p>That now works nicely. Thanks !</p>
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