CODEWITHSUNDEEP
multithreadingc++11mutexdeadlock
user3798283
user3798283

Reputation: 467

Not able to force sequential execution of thread in C++? DeadLock?

I am trying to read words from a string thread wise . Means one thread reads one word and when all the words finished then all the threads should also exit peacefully . In this example there are 11 words in the string and 4 threads are operating on that string . But the program is getting hanged while running . I am not able to identify the issue . I tried recursion as well but that didn't work and got hanged . 

#include <iostream>
#include <mutex>
#include <sstream>
#include <thread>
#include <chrono>
#include <condition_variable>
using namespace std;
stringstream s("Japan US Canada UK France Germany China Russia Korea India Nepal");
int count = 0;
string word;
condition_variable cv;
mutex m;
int i = 0;
bool check_func(int i,int k)
{
    return i == k;
}
void print(int k)
{
  while(count < 11)   // As there are 11 words
  {
     unique_lock<mutex> lk(m);
     int z = k;
     cv.wait(lk,[&]{return check_func(i,z);});            // Line 33
     s >> word;
     cout<<word<<" ";
     i++;
     cv.notify_all();
     count++;
  }
   return;
}
int main() 
{
    thread threads[4];
    for(int i = 0; i < 4; i++)
       threads[i] = thread(print,i);
    for(auto &t : threads)
       t.join();
    return 0;
}

Upvotes: 2

Views: 267

Answers (1)

paddy
paddy

Reputation: 63471

You never notified the condition variable. It never woke up. All threads are waiting for something to happen. Notify at the end of your print function:

void print(int k)
{
   unique_lock<mutex> lk(m);
   int z = k;
   cv.wait(lk,[&]{return check_func(i,z);});            // Line 33
   cout<<"Thread no. "<<this_thread::get_id()<<" parameter "<<k<<"\n";
   i++;
   cv.notify_all();  // Wake up all waiting threads - the correct one will continue.
}

You also need to initialize the global variable i to zero, or you will have undefined behaviour.

Upvotes: 4

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