Question

Write a multithreaded program in C using the pthread library and dynamic memory(malloc) that multiplies two matrices together. The numbers in the matrices must be read in from a text file. The program should also check if the two matrices are capable of being multiplied together.

The amount of threads used has to be dynamic. The user should be able to choose how many threads they wish to use using the command line.

Finally, the result must be stored in a text file.

Answer 1

The following program can be used to implement the given problem in C++.

// CPP Program to multiply two matrix using pthreads 
#include <bits/stdc++.h> 
using namespace std; 
int step_i = 0; 
  
void* multi(void* arg) 
{ 
    int core = step_i++; 
  
    // Each thread computes 1/4th of matrix multiplication 
    for (int i = core * ma1 / 4; i < (core + 1) * ma1 / 4; i++)  
        for (int j = 0; j < mb1; j++)  
            for (int k = 0; k < mb2; k++)  
                matC[i][j] += matA[i][k] * matB[k][j]; 
} 
  
// Driver Code 
int main() 
{
    fstream file;
    string word, filename;
    
    filename = "example.txt";
    file.open(filename.c_str());
    
    cout<<"Enter dimensions of the first matrix :";
    int ma1, ma2, mb1, mb2;
    cin>>ma1>>ma2;
    
    cout<<"Enter dimensions of the second matrix :";
    cin>>mb1>>mb2;
    
    int matA[ma1][ma2];
    int matB[mb1][mb2];
    
    
    if(ma2 != mb1)
    {
        cout<<"The matrices can't be multiplied.";
        return 0;
    }
    
    int matC[ma1][mb2];
    int MAX_THREAD = ma2;
    
    for(int i = 0; i<ma1; i++)
    {
        for(int j = 0; j<ma2; j++)
        {
            file>>word;
            matA[i][j] = word;
        }
    }
    
    for(int i = 0; i<mb1; i++)
    {
        for(int j = 0; j<mb2; j++)
        {
            file>>word;
            matB[i][j] = word;
        }
    }
  
    // Displaying matA 
    cout << endl 
         << "Matrix A" << endl; 
    for (int i = 0; i < ma1; i++) { 
        for (int j = 0; j < ma2; j++)  
            cout << matA[i][j] << " "; 
        cout << endl; 
    } 
  
    // Displaying matB 
    cout << endl 
         << "Matrix B" << endl; 
    for (int i = 0; i < mb1; i++) { 
        for (int j = 0; j < mb2; j++)  
            cout << matB[i][j] << " ";         
        cout << endl; 
    } 
  
    // declaring four threads 
    pthread_t threads[MAX_THREAD]; 
  
    // Creating four threads, each evaluating its own part 
    for (int i = 0; i < MAX_THREAD; i++) { 
        int* p; 
        pthread_create(&threads[i], NULL, multi, (void*)(p)); 
    } 
  
    // joining and waiting for all threads to complete 
    for (int i = 0; i < MAX_THREAD; i++)  
        pthread_join(threads[i], NULL);     
  
    // Displaying the result matrix 
    cout << endl 
         << "Multiplication of A and B" << endl; 
    for (int i = 0; i < ma1; i++) { 
        for (int j = 0; j < mb2; j++)  
            cout << matC[i][j] << " ";         
        cout << endl; 
    } 
    return 0; 
}