CODEWITHSUNDEEP
c++multithreadingpromisefuturepackaged-task
dikson231
dikson231

Reputation: 201

c++ future.get() is not blocking

According to what's written about future in "the c++ programming language 4th edition"
§5.3.5.1 page 120:

If the value isn’t there yet, our thread is blocked until it arrives.

Meaning that get() is a blocking method.

Later in §5.3.5.2 page 122 packaged_task is explained, and the following code sample is given:

double accum(double* beg, double* end, double init)
// compute the sum of [beg:end) starting with the initial value init
{
    return accumulate(beg,end,init);
}

double comp2(vector<double>& v)
{
    using Task_type = double(double*,double*,double); // type of task
    packaged_task<Task_type> pt0 {accum}; // package the task (i.e., accum)
    packaged_task<Task_type> pt1 {accum};
    future<double> f0 {pt0.get_future()}; // get hold of pt0’s future
    future<double> f1 {pt1.get_future()}; // get hold of pt1’s future
    double* first = &v[0];
    thread t1 {move(pt0),first,first+v.size()/2,0}; // start a thread for pt0
    thread t2 {move(pt1),first+v.size()/2,first+v.size(),0}; // start a thread for pt1
    // ...
    return f0.get()+f1.get(); // get the results
}

This makes sense, since according to the quote the following program won't return until the 2 threads in comp2() function finished even without calling join() on them since the thread that called comp2() will wait for the futures until they get() their value:

int main()
{
    vector<double> v {1.1, 8.3, 5.6};
    double res = comp2(v);
    return 0;
}

Unfortunately, this doesn't happen like I thought, and I less I call join() on the 2 threads in comp2(), a run-time error will be thrown.

Could someone explain me what am I missing here and why get() doesn't block ?

Upvotes: 3

Views: 4067

Answers (1)

Leonardo
Leonardo

Reputation: 1844

I've debugged your code with gdb, your runtime error is happening in std::thread destructor. You have to either detach or join them before they're destructed, e.g:

t1.detach();
t2.detach();

inside comp2().

This will net your expected behaviour.

Upvotes: 1

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