Dynamic Matrix Multiplication with Pthreads - c

Question

I'm a beginner with Thread Programming and C in general and I'm trying to figure out how to do a simple Matrix Multiplication with Pthreads. I want to create a thread for every column and put the results in a Result Matrix. I'm trying to do it dynamicly, which means the user is allowed to use an input as a size to create two n x n matrices.
My code right now, excluding filling the matrix and reading the size n is the following:
#include <pthread.h>
#include <stdio.h>
#include<stdlib.h>
typedef struct Matrix {
int line, col, size;
double (*MA)[];
double (*MB)[];
double (*MC)[];
} Matrix;
void *multiply(void *arg) {
Matrix* work = (Matrix*) arg;
int s, z;
s = work->col;
z = work->line;
work->MC[0][0] = 0.0.//can't use MC, MB, MA here!!
return 0;
}
int main() {
Matrix* m;
//read size and set it to int size (miissing here, does work)
double MA[size][size], MB[size][size], MC[size][size];
int i, j;
//filling the matrices (missing here, does work)
pthread_t threads[size];
for (i = 0; i < size; i++) {
m = malloc(sizeof(Matrix*));
m->size = size;
m->col = i;
pthread_create(&threads[i], NULL, multiply, m);
}
for (i = 0; i < size; i++) {
pthread_join(threads[i], NULL);
}
return 0;
}
The problem is, that I cant use neither MA, MB nor NC(:= the result) in the multiply method with something like its shown in the code.
I just get the error "invalid use of array with unspecific bounds" even though I declared all three of them in the main method.
Do I understand anything wrong here or how can I fix that? I tried to adapt a example of my lecture where a thread for every element will be created.
Thanks in advance!

Answer 1

Just about the error:
work->MC[0][0] = 0.0.//can't use MC, MB, MA here!!
MC was declared as double (*MC)[] and you try to use it as a two dimensional array like you had declared it double MC[N]{M]. You can use a two (or more) dimensional array like you did if and only if the first dimension was fixed or if you alloc it row by row.
So your program could be:
#include <pthread.h>
#include <stdio.h>
#include<stdlib.h>
typedef struct Matrix {
int line, col, size;
double MA[][];
double MB[][];
double MC[][];
} Matrix;
void *multiply(void *arg) {
Matrix* work = (Matrix*) arg;
int s, z;
s = work->col;
z = work->line;
work->MC[0][0] = 0.0
return 0;
}
int main() {
Matrix* m;
//read size and set it to int size (miissing here, does work)
double MA[][], MB[][], MC[][];
int i, j;
pthread_t threads[size];
MA = (double **) malloc(size * sizeof(double *));
MB = (double **) malloc(size * sizeof(double *));
MC = (double **) malloc(size * sizeof(double *));
for(int i=0;i<size;++i){
MA[i] = (double *) malloc(size * sizeof(double));
MB[i] = (double *) malloc(size * sizeof(double));
MC[i] = (double *) malloc(size * sizeof(double));
}
for (i = 0; i < size; i++) {
m = malloc(sizeof(Matrix*));
m->MA = MA;
m->MB = MB;
m->MC = MC;
m->size = size;
m->col = i;
pthread_create(&threads[i], NULL, multiply, m);
}
for (i = 0; i < size; i++) {
pthread_join(threads[i], NULL);
}
return 0;
}
But you must TAKE CARE that the thread can access to the data concurrently and so you should use some locks if different threads can use and change same values.