Populating an array with values using multithreads

Question

Im suppose to use threads to populate an array with values (value is the same for all elements of array for each thread, but different threads have different values). In this code I come across a problem where when i set the start and no of elements that the thread has to populate in the array, it doesnt get set properly for the first thread, the rest seem to work fine.. could someone help me figure out where im going wrong, im suppose to do this without locks, help would be much appreciated!! Example no of threads = 2 Size of array = 3 the first thread will populate the first 2 elements and the second thread will populate the last 3 elements..

    #include <pthread.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <iostream>
    using namespace std;

int* array;

struct variables
{
   int start;
   int elements;
   int value;
};

void* runner(void *param)
{
   //int e=atoi((char*)param);
   variables* a=(struct variables*)param;
   cout<<"start="<<a->start<<endl;
   cout<<"elements"<<a->elements<<endl;
   cout<<"value"<<a->value<<endl;
   for(int i=a->start, j=0; j<a->elements; i++, j++ )
   {    
       array[i]=a->value;  
   }
}

int main()
{
    pthread_t tid[10];
    int noOfThreads, sizeOfArray;
    cout<<"No of threads (between 1-20): ";
    cin>>noOfThreads;
    cout<<"size of array: ";
    cin>>sizeOfArray;

    array = new int[sizeOfArray];
    int e=sizeOfArray/noOfThreads;

    //int start=0;

    for(int i=0,j=0; i<noOfThreads; i++)
    { 
        variables v;
        cout<<"i="<<i<<endl;
        v.start=(e*j);
        cout<<"v.start="<<v.start<<endl;
        cout<<"j="<<j<<endl;
        if(i==(noOfThreads-1))
    {
            e=sizeOfArray-(e*(noOfThreads-1));
            cout<<"e="<<e<<endl;
        }
        cout<<"Enter the value for Thread "<<i+1<<":";
        cin>>v.value;
        v.elements=e;
        pthread_create(&tid[i], NULL, runner,&v);
        j++;
    }

    for(int i=0; i<noOfThreads; i++)
    {
        pthread_join(tid[i], NULL);
    }

    for(int i=0; i<noOfThreads; i++)
    {
        cout<<"tid="<<tid[i]<<endl;
    }

    cout<<"printing array"<<endl;
    for(int i=0; i<sizeOfArray; i++)
    {
        cout<<array[i]<<" ";
    }
}

mugiseyebrows · Accepted Answer · 2013-12-07 18:52:26Z

1

You create variables v on stack, pass pointer to thread, v goes out of scope on loop iteration, you reallocate [same] memory and rewrite start elements and value, all threads will have same pointer (same structure), i believe, unless they exit faster than v expired. Put this in runner: cout << params to check if i'm right.

answered Dec 7, 2013 at 18:52

mugiseyebrows

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3 Comments

Ben Voigt Over a year ago

Yes, he should use an array for thread start arguments just like for the tid array.

user2949783 Over a year ago

but if it is a pointer to v, how do i cast it in runner?? can you help me with how i should modify the code pls!!

user2949783 Over a year ago

i tried it with an array to pass the arguments, still same problem!!

mugiseyebrows · Accepted Answer · 2013-12-07 19:30:09Z

#include <pthread.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <iostream>
    using namespace std;

int* array;

struct variables
{
   int start;
   int elements;
   int value;
};

void* runner(void *param)
{
   //int e=atoi((char*)param);
   variables* a=(struct variables*)param;
   cout<<"start="<<a->start<<endl;
   cout<<"elements="<<a->elements<<endl;
   cout<<"value="<<a->value<<endl;
   for(int i=a->start, j=0; j<a->elements; i++, j++ )
   {    
       array[i]=a->value;  
   }
}

int main()
{
    pthread_t tid[10];
    int noOfThreads, sizeOfArray;
    cout<<"No of threads (between 1-20): ";
    cin>>noOfThreads;
    cout<<"size of array: ";
    cin>>sizeOfArray;

    array = new int[sizeOfArray];

    variables* v = new variables[noOfThreads];
    int e=sizeOfArray/noOfThreads;
    for (int i=0;i<noOfThreads;i++) {
        v[i].start = e*i;
        if(i==(noOfThreads-1)) {
            v[i].elements=sizeOfArray-(e*(noOfThreads-1));
        } else {
            v[i].elements=e;
        }
        cout<<"Enter the value for Thread "<<i+1<<":";
        cin>>v[i].value;
    }

    for(int i=0; i<noOfThreads; i++) {
        pthread_create(&tid[i], NULL, runner,&v[i]);
    }

    for(int i=0; i<noOfThreads; i++)
    {
        pthread_join(tid[i], NULL);
    }

    for(int i=0; i<noOfThreads; i++)
    {
        cout<<"tid="<<tid[i]<<endl;
    }

    cout<<"printing array"<<endl;
    for(int i=0; i<sizeOfArray; i++)
    {
        cout<<array[i]<<" ";
    }
}

You're welcome. Also, don't forget to fix this: tid[10]; "No of threads (between 1-20): "

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