Multithreading for loop in C++

Question

I've got the following code and I'm attempting to add threading so it executes faster but I'm really stumped. I basically want different threads to execute different iterations of the for loop to speed up the process since I'll be pinging a lot of urls. However, I wasn't sure how to do that or if it was possible so I did what is seen below but I get an error at "thread doStuff(exec" right at the exec part that says there is no instance of constructor "std::thread::thread" matches the argument list. How can I apply threading to this situation when the value of the parameter for the "exec" function is changing at every iteration of the for loop?

On a side note, if you couldn't tell, I need to put the result of the "exec" function into index i of the vector "work".

EDIT: The last function is supposed to look incomplete because I deleted code that was irrelevant to this problem.

vector<string> check(vector<string> url)
{

    vector<string> work;
    for (int i = 0; i < url.size(); i++)
    {
        string ping = "ping ";
        ping.append(url[i]);
        thread doStuff(exec,work.push_back(exec(ping.c_str())));

        cout << work[i] << endl;
    }

    return work;
}
std::string exec(const char* cmd) {
    string timeOut = "Request timed out";
    string reply = "Reply from";
    std::array<char, 128> buffer;
    std::string result;
    std::shared_ptr<FILE> pipe(_popen(cmd, "r"), _pclose);
    if (!pipe) throw std::runtime_error("popen() failed!");
    while (!feof(pipe.get())) {
        if (fgets(buffer.data(), 128, pipe.get()) != nullptr)
            result += buffer.data();
    }
return result;

Threading is a wasps nest of potential (and hard to find) bugs if you don't know what you are doing (in most cases; expert only territory). Step 1) learn C++ really well. Step 2) learn about synchronization, priority inversion, atomic operations, deadlocking and much more. Step 3) attempt threads. — Jesper Juhl
– Jesper Juhl, Commented Jun 19, 2018 at 15:51
@JesperJuhl, Re, "learn C++ really well." And just sweep out King Augeas' stables on your way. Learning all of C++ really well is a monumental task. One could be qualified to undertake the study of threads after learning a subset of C++ really well. E.g., One need not know how to design a template library in order to start learning about threads. — Solomon Slow
– Solomon Slow, Commented Jun 19, 2018 at 16:53

Tim Johns · Accepted Answer · 2018-06-19 16:09:06Z

3

Since you're using Visual Studio, you might want to consider parallel_for. Also, C++17 introduced the Parallelism TS, which is a work in progress attempt at standardizing parallel algorithms. This blog post gives a nice introduction to what features are available, and how you can try them out today.

edited Jun 19, 2018 at 16:09

answered Jun 19, 2018 at 15:54

Tim Johns

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9 Comments

Cole Over a year ago

I just tried using parallel_for but when it hits ping.append(domain[i]) it triggers a breakpoint in dbgrptt.cpp.

Tim Johns Over a year ago

Hmm, I'm not sure what caused that breakpoint, but looking at your code, I suspect you will have another problem with using parallel_for with your current implementation. You modify the work vector inside the for loop. If you changed that to a parallel_for loop, then you would have multiple threads modifying the same vector. This would need to be synchronized (either with a mutex or a thread-safe vector).

Tim Johns Over a year ago

Can you provide more details about the breakpoint? I imagine there's a backtrace?

Cole Over a year ago

Oh I see. I wish I could give you more details about the breakpoint but it refuses to tell me because it can't find dbgrptt.cpp. However, I tested to make sure it was a problem with accessing the vector by putting string hi = domain[i] and that caused the same breakpoint. How exactly would I go about adding in the mutex?

Cole Over a year ago

Weird...it says the size of the url vector is 0...I'm not sure why I didn't catch that before.

|

SaurabhS · Accepted Answer · 2018-06-19 17:02:53Z

2

You can use std::async in for loop

auto handle = std::async(std::launch::async,exec,ping.c_str());
work.push_back(handle.get());

That should work.

answered Jun 19, 2018 at 17:02

SaurabhS

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3 Comments

Cole Over a year ago

I get no compilation errors with this but it seems to be running no faster than before.

SaurabhS Over a year ago

Your program with std::thread (no return) is running faster than before?

Cole Over a year ago

Well I was never able to get std::thread to work but when I use std::async it runs at the same speed.

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