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POposting future and new threads in a loop multiple times
 

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  1. POposting future and new threads in a loop multiple times
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    <p>I am writing a program that does calculations in multiple threads and return the result using c++ future, here's a simplified version of my code</p> <pre><code>int main() { int length = 64; vector&lt;std::future&lt;float&gt;&gt; threads(length); vector&lt;float&gt; results(length); int blockLength = 8; int blockCount = length/blockLength; for(int j=0;j&lt;blockCount;j++) { for(int i=0;i&lt;blockLength;i++) { threads[i + j * blockLength] = std::async(func1,i*j); } for(int i=0;i&lt;blockLength;i++) { results[i + j * blockLength] = threads[i].get(); } } </code></pre> <p>the definition of func1 is simplified as follows:</p> <pre><code>float func1(int input) { //calculations... return result; } </code></pre> <p>I would like that the program above does 64 times of calculations, in 8 threads at a time, so that the processor and memory usage would be better at the same time.</p> <p>The program is conceived that it will post <code>blockLength</code> number of threads at a time, and wait till the calculation results are obtained, and proceed to the next loop.</p> <p>the program will post <code>blockLength</code> number of threads for <code>blockCount</code> times, for example, 8 threads for 8 times.</p> <p>but the program is not working, there is always a <code>EXC_BAD_ACCESS</code> exception when the first loop of <code>blockLength</code> threads finishes, besides, the calculation time of each thread is not guaranteed, any thread can run for a long time or finish quickly.</p> <p>Here is a screenshot: <img src="https://i.stack.imgur.com/5iyz0.png" alt="exception at the beginning of a new loop"></p> <p>as is shown above, the CPU usage drops as some of the threads finish, but an exception is thrown as soon as the second loop starts.</p> <p>Would you please point out what is wrong with my usage of <code>future</code>? How can we correct it?</p> <p>Thank you very much!</p>
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    1. USLewen
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      1. POposting future and new threads in a loop multiple times
      1. USGMasucci
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    1. COthreads[i].get() ? What happened to [i + j * blockLength]?
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      1. POposting future and new threads in a loop multiple times
      1. USMartin James
    2. COthanks! that is really a stupid mistake...
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      1. USLewen
    3. CONo problem - I've made thousands, so I'm good at detecting them :)
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      1. USMartin James
 

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