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POStrange concurrency issue with STL / OpenMP in 64 bit builds
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copied!<p>I have an odd issue when I build one of our projects in a 64 bit debug config. It appears to produce some strange behaviour which looks a little like a single iterator is getting incremented multiple times. I've narrowed it down to the following test code:</p> <pre><code>#include <omp.h> #define _HAS_ITERATOR_DEBUGGING 0 #include <vector> #include <set> int main(int argc, const char* argv[]) { std::vector<int> v; for(int j = 0; j < 20; ++j) { v.push_back(j); } #pragma omp parallel for for(int i = 0; i < 100000; ++i) { std::set<int> s; std::vector<int>::const_iterator begin = v.begin(), end = v.end(); s.insert(begin, end); // after this line s should contain the numbers 0-19 if(s.size() != 20) { printf("fail\n"); exit(3); } } return 0; } </code></pre> <p>However, the size test frequently fails which implies that somehow it is not inserting the entire contents of the vector - and a lot of poking at it makes it look rather like the vector iterators are being incremented more than one step at a time. It's quite hard to tell though since it doesn't tend to happen if one breaks in with the debugger.</p> <p>The obvious conclusion to draw would be that it's not threadsafe, but my understanding is that it should be because the only variable modified is <code>s</code> which has local scope. </p> <p>There are quite a few things which will fix the immediate problem:</p> <ul> <li>remove the parallel for</li> <li>throw a critical section around the <code>insert()</code> call</li> <li><code>#define HAS_ITERATOR_DEBUGGING 1</code></li> <li>replace the single insert() call with a manual loop and insert each item individually (this is basically what that function does internally, but the problem definitely goes away when I do it myself)</li> <li>build a 32bit version of the same code</li> <li>build the release version of the same code</li> </ul> <p>This is being compiled under MSVC++ 2008 SP1, with the compiler-supplied STL implementation.</p> <p>Can anyone shed any light on what's going on here? Thanks in advance for any hints - I'm pretty stumped :)</p> <p>Edit: In case it is not clear, I am not looking for a "quick fix" to make this code work; as noted above I know of quite a few of those. What I want to understand is why this problem is occurring in the first place.</p> <p>Edit 2: Code works correctly when compiled with gcc.</p>

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