Using pthread to print 2-d array

Question

So i have an assignment that says I have create a 2-d array[5][12] with random values between 1-99. Then using pthreads, I have to either add 1 or subtract 1 to each element in the array and then print the results and divide the process into either 2, 3, or 4 threads. The number of threads depends on the what the user inputs on the command line. I have code that compiles and runs. However, my desired output is only printed when the number 3 is entered. Could you guys tell me where I went wrong in my code? I'm having trouble understanding pthreads to begin with.

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <ctype.h>
#include <pthread.h>
#include <iostream>

using namespace std;
int list[5][12];
int rows = 5;
int cols = 12;
int threadc;

void *threadf(void *arg)
{
    int x = (int) arg;
    for(int i = (x*60)/threadc; i < ((x+1) * 60)/threadc; i++)
    {
        for(int j = 0; j < 12; j++)
        {
            if (list[i][j] % 2 == 0)
                list[i][j] += 1;
            else
                list[i][j] -= 1;
        }
    }
}

void cArray()
{
    srand(time(NULL));
    for(int i = 0; i < 5; i++)
    {
        for(int j = 0; j < 12; j++)
        {
            list[i][j] = rand() % 99 + 1;
        }
    }

}

void pArray(int list[][12], int rows, int cols)
{
    cout << "\n";
    for(int i = 0; i < rows; i++)
    {
        for(int j = 0; j < cols; j++)
        {
            cout << list[i][j] << " ";
        }
        cout << "\n";
    }
 }

int main(int argc, char *argv[])
{
    if(argc != 2) exit(0);
    threadc = atoi(argv[1]);
    assert(threadc >= 2 && threadc <=4);
    pthread_t *thread;
    thread = new pthread_t [threadc];
    if(thread == NULL)
        exit(0);
    cArray();
    cout << "2-d Array: ";
    pArray(list, rows, cols);
    int t;
    for(int i = 0; i < threadc; i++)
    {
        t = pthread_create(&thread[i], NULL, threadf, (void *)i);
        if (t != 0)
            return 1;
    }
    for(int i = 0; i < threadc; i++)
    {
        t = pthread_join(thread[i], NULL);
        if(t != 0)
            return 1;
    }  
    cout << "Modified 2-d Array: ";
    pArray(list, rows, cols);
    return 0;
}

Answer 1

Somehow thread_join returns an error code(22 in my case) when the number of threads created is 2. If you remove the return statement in the second loop, your program prints the final output.

for(int i = 0; i < threadc; i++)
{
    t = pthread_join(thread[i], NULL);
    if (t != 0) 
        return 1; // <- your program works if you comment out this.


}

According to this link: http://minirighi.sourceforge.net/html/errno_8h.html 22 is EINVAL which means 'thread is un-joinable'.

Since you care about the return value of pthread_join, I would advise to add a successful termination function (pthread_exit(NULL);) at the end of your thredf. Also, to avoid buffer overrun as mentioned by @user4581301, you could pass a pointer to the data.

So thredf would be like

void *threadf(void *arg)
{
    cout << endl;
    int x = *((int*)arg); // <- NOTICE HERE!
    for(int i = (x*60)/threadc; i < ((x+1) * 60)/threadc; i++)
      //...
    }
    pthread_exit(NULL); // <- NOTICE HERE!
}

And the main:

int main(int argc, char *argv[])
{
    if(argc != 2) exit(0);
    threadc = atoi(argv[1]);
    assert(threadc >= 2 && threadc <=4);
    pthread_t *thread;
    thread = new pthread_t [threadc];
    int *data = new int[threadc]; // <- NOTICE HERE!

    if(thread == NULL)
        exit(0);
    cArray();
    cout << "2-d Array: ";
    pArray(list, rows, cols);
    int t;
    for(int i = 0; i < threadc; i++)
    {
        data[i] = i;
        //                                                NOTICE HERE!
        t = pthread_create(&thread[i], NULL, threadf, (void *)(&data[i])); 
        if (t != 0)
            return 1;
    }
    // ...