How to parallelize a function, that uses some variable

Question

I have a function, that generates random URL, and try to download the file.

void tryURL()
{
    randURL.clear();
    for (unsigned short i = 0; i < urlLength; i++)  {
    randURL = randURL + (alphabet[(int)((double)rand() / (RAND_MAX + 1) * alphabetLength)]);
    }
    wcout << randURL << endl;
    HRESULT getImg = URLDownloadToFile(NULL, LPCWSTR( (beginURL + randURL + endURL).c_str() ), LPCWSTR( (randURL + L"/" + endURL).c_str() ), 0, NULL);
    if (SUCCEEDED(getImg))
    {
    wcout << L"Success" << endl;
    }
}

If I execute this function normally, It's work fine:

tryURL();
0ybOAt
tryURL();
lTPKaR
tryURL();
Ivi05m
...

But, I need to run this function repeatedly times at that moment. I tried this:

thread threads[10];

for (int i = 0; i < 10; ++i) {
    threads[i] = thread(tryURL);
}

for (int i = 0; i < 10; ++i) {
    threads[i].join();
}

And it always returns me same value

0ybOAt0ybOAt
0ybOAt

0ybOAt0ybOAt

0ybOAt
0ybOAt0ybOAt
0ybOAt
0ybOAt

And even endl don't show's up sometimes.

How can I fix it? I think, it broken because always using same variable randURL, but I don't know how to evade this.

Vittorio Romeo · Accepted Answer · 2017-11-19T16:36:14.620

2

Instead of using the same variable, make tryURL return the URL:

// assuming that URLs are strings
std::string tryURL() { /* ... */ }

Then, create a vector of std::future<std::string> to represent the asynchronous computations that will return URLs:

std::vector<std::future<std::string>>> futures;
futures.reserve(10);

for (int i = 0; i < 10; ++i) 
{
    futures.emplace_back(std::async(std::launch::async, tryURL));
}

Finally, consume the URLs in the main thread:

for(auto& f : futures) 
{
    consume(f.get());
}

edited Nov 19 '17 at 16:36

answered Nov 18 '17 at 21:32

Vittorio Romeo

90,666
33
258
416

Can't get this code working on VC17. launch_async replaced with launch::async, but consume command not found. Can you fix it? – megapro17 Nov 19 '17 at 14:57
@megapro17: `consume` is just an example. That's where your logic should go. – Vittorio Romeo Nov 19 '17 at 16:36

score 0 · Answer 2 · answered Nov 18 '17 at 23:44

0

Other questions on StackOverflow provide some clues:

A simple fix for the rand() issue is to generate the URL:s in the main thread and pass them to the thread.

void tryURL(std::wstring URL)
...
// TODO: Set randURL to a random URL
threads[i] = thread(tryURL, randURL);

answered Nov 18 '17 at 23:44

Niklas Angare

71
3

ok, I fixed the rand. but my program runs only it 4 threads. I can't get why. I tried more, but only 4 threads at one time only executed. Also, I have 4 cores CPU. for (int i = 0; i < 100; ++i) { thr[i] = thread(tryURL); } – megapro17 Nov 21 '17 at 16:24

How to parallelize a function, that uses some variable

2 Answers2