How to pass matrix as paramter in pthread_create() function?

Question

I have just started with O.S programming and was exploring threads. I want to multiply 2 matrices and get their product using the pthread_create(), pthread_join() and pthread_exit() functions. However, pthread_create() takes input as void* whereas, I want to pass an int** parameter.

I already tried:

1. Passing m3 [my resultant matrix] in the pthread_create() function by typecasting it as (void*) and then typecasting it back to (int**) in my threadMultiply function but that didnt work
2. Putting m1,m2,m3 as global variables but that too gave me errors.

I am very confused and don't know how to approach this anymore. Any help would be much appreciated, thanks

#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>

int** alloc(int, int);
void display(int**, int, int);
void* threadMultiply(void* para);

int main()
{
    int r1,c1,r2,c2,r3,c3;  // rows and columns of each matrix
    int **m1, **m2, **m3;   // all 3 matrices

    // took inputs for r1,c1,r2,c2
    m1=alloc(r1,c1);
    printf("Enter the %d elements of the first matrix\n", r1*c1);
    for(int i=0;i<r1;i++)
        for(int j=0;j<c1;j++)
            scanf("%d", &m1[i][j]);
    m2=alloc(r2,c2);
    printf("Enter the %d elements of the second matrix\n", r2*c2);
    for(int i=0;i<r2;i++)
        for(int j=0;j<c2;j++)
                scanf("%d", &m2[i][j]);
    display(m1, r1, c1);
    display(m2, r2, c2);
        
    if(c1 != r2)
    {
        printf("\nMatrices cannot be multiplied, check dimensions");
        return 0;
    }
        
    r3 = r1;
    c3 = c2;
    m3=alloc(r3,c3);
    int MAX_THREADS = r3*c3;
    pthread_t threads[MAX_THREADS];
 
    // Creating threads. 
    for (int i = 0; i < MAX_THREADS; i++) 
    {
        int *p;
        pthread_create(&threads[i], NULL, threadMultiply, (void*)(p));  //variable 'i' is of type int however function takes parameter of type void* so we have to do type-casting
    }
     
    // joining and waiting for all threads to complete
    for (int i = 0; i < MAX_THREADS; i++)
        pthread_join(threads[i], NULL);   
        
        printf("\nThe resultant matrix is:");
        display(m3, r3, c3);
        
return 0;
}

int** alloc(int row, int col)
{
    //dynamic memory allocation for first 2 matrices
    int **m=0;
    m=(int**)malloc(row*sizeof(int*));

    for(int i=0;i<row;i++)
    {
        *(m+i)=(int*)malloc(col*sizeof(int));
    }
    
return m;
}

void *threadMultiply(void *para)
{
    int i,j,k;
    for(i=0;i<r1;i++)
    {
        for(j=0;j<c2;j++)
        {
            m3[i][j] == 0
                for(k=0;k<c1;k++)                  
                    m3[i][j]+=m1[i][k] * m2[k][j];
        }
    }
    printf("thread finished ...");
    pthread_exit(NULL);
}