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POBoost Threads - Racing
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copied!<p>I'm playing around with some Boost threads, and I've written the following code:</p> <pre><code>#include <boost/thread.hpp> #include <iostream> volatile int threads = 0; const int iterations = 200; volatile char output[iterations*4]; volatile int k = 0; void w() { threads++; for (int i = 0; i < iterations; i++) { k++; output[k] = '0'; } } void y() { threads++; for (int i = 0; i < iterations; i++) { k++; output[k] = '1'; } } int main(int argc, char* argv[]) { boost::thread a(&w); boost::thread b(&w); boost::thread c(&y); boost::thread d(&y); while (k <= iterations * 4) {} for (int i=0; i < k; i++) { std::cout << output[i]; } return 0; } </code></pre> <p>What I don't understand is why the completion of these threads appeared to wait on each other, this is shown in the output:</p> <pre><code>000000000000000000000000000000000000000011111111 </code></pre> <p>I was under the impression the output would be in random order, with statements output similar to the following (expected output):</p> <pre><code>00100000111001000000010001111100000000011111111 </code></pre> <p>Instead they follow, all 0, then all 1 (or vice versa).</p> <p>Why is this? At this point it seems more like the threads are stacked in a random order, but still joined; ie waiting on each other before executing. I actually expected the threads++ code to possibly run at the same time in some threads, leaving them with the some thisthread value.</p> <p>Disclaimer: I'm very new to Boost:threads, simply playing around, I understand race conditions are very dangerous (may lead to undefined behavior), this is just putting my hand in the fire so I can understand it.</p>

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