pthreads multi-threaded matrix multiplication - c++

Question

I'm currently trying to write a C++ program with pthreads.h for multi-threaded matrix multiplication.
I'm trying to create the threads as follows
int numthreads = (matrix[0].size() * rsize2);//Calculates # of threads needed
pthread_t *threads;
threads = (pthread_t*)malloc(numthreads * sizeof(pthread_t));//Allocates memory for threads
int rc;
for (int mult = 0; mult < numthreads; mult++)//rsize2
{
struct mult_args args;
args.row = mult;
args.col = mult;
cout << "Creating thread # " << mult;
cout << endl;
rc = pthread_create(&threads[mult], 0, multiply(&args), 0);
}
This then creates threads that execute my multiply function which is coded as follows
void *multiply(int x, int y)
{
int oldprod = 0, prod = 0, sum = 0;
cout << "multiply";
for(int i = 0; i < rsize2; i++)//For each row in #ofrows in matrix 2
{
prod = matrix[x][i] * matrix2[i][y];//calculates the product
sum = oldprod + prod; //Running sum starting at 0 + first product
oldprod = prod; //Updates old product
}
My error lies in my multiply function. I'm trying to find a compatible way to pass in an x and y coordinate for each thread so it knows specifically which summation to calculate but i'm not sure how to do this in a way that is acceptable for the pthreads_create() function.
Update:
I know that I have to use a struct to accomplish this
struct mult_args {
int row;
int col;
};
but I can't get the multiply function to accept the struct

Answer 1

You will have to modify your multiply function so that it takes a single void* parameter. To do this, you will need to make a struct to store x and y and pass a pointer to it in pthread_create.
struct multiply_params
{
int x;
int y;
multiply_params(int x_arg, int y_arg) noexcept :
x(x_arg), y(y_arg)
{}
};
// ...
for (int mult = 0; mult < numthreads; mult++)
{
cout << "Creating thread # " << mult;
cout << endl;
multiply_params* params = new multiply_params(1, 0);
rc = pthread_create(&threads[mult], 0, multiply, (void*) params);
}
Then in your multiply function, rewrite it like this, taking a single void* parameter which will be the pointer of multiply_params which we passed into pthread_create. You have to cast this argument from void* so we can access its fields.
void* multiply(void* arg)
{
multiply_params* params = (multiply_params*) arg;
int x = params->x;
int y = params->y;
delete params; // avoid memory leak
// ...
}