Working except for the thread part. LUL, C (gcc)

Language: Layout:

#include <stdio.h>
#include <math.h>
#include <sys/mman.h>
#include <pthread.h>
#include <malloc.h>
#include <stdlib.h>

int a[18][16]; //first array 
int b[16][18]; //second array
int c[18][18]; // The result

//%how to pseudo this code?

struct thread_info {
int shape; // The difference is the stakes. 
int colour; // Out of place
};

//%Completes the arthimetic for one element of a product of matrix multiplication. 
void *compute_C_ij(void *arg)
{ 
int i, j, k, sum; //col, row;
 
//i = ; //is passed by the pthread call. 
//j = ; //distinguishes which cij being solved.

//%information necessary to be passed is row of a and col of b. and which C value it is. 
sum = 0;    
        for (k = 0; k < 16; k++) {
          sum = sum + a[i][k]*b[k][j];
        }
 
        c[i][j] = sum;
return(c); //%or the pointer for C. 
}

int main(int argc, char *argv[])
{
	
int i,j,k;

for(k=0; k<324; k++){
   pthread_t thread[k]; //Thread variables
   thread_info info[k]; //structs to be passed
  }

double a[18][16]; //arrays
double b[16][18]; // arrays

//%initializing

for(i=0; i<18; i++){
	for(j=0; j<16; j++){
		a[i][j] = i+j;
	}
}
 
for(i=0; i<16; i++){
	for(j=0; j<18; j++){
		b[i][j] = i+(2*j);
	}
}

//%this is from load.c Want to recycle the lines below to run the threads to solve for the matrix concurrently. 
	
	for (i = 0; i < 18; i++) {
		
            for (j = 0; j < 18; j++) {
                thread_info.shape = i; 
                thread_info.colour = j;
                if (pthread_create(&thread[i], NULL, &compute_C_ij, (void *)&info[i+j]) != 0) {
			
                printf("Error creating thread %d.%d\n", i, j);
                }    
            }
	}

pthread_exit(NULL);

printf("The Result of matrix multiplication a * b is: \n");
for(i=0; i<18; i++){
	for(j=0; j<18; j++){
		printf("%d\t", c[i][j]);
	}
	printf("\n");
}

printf("The Diagonal of matrix multiplication a * b is: \n");
    for(i=0; i<18; i++){
		printf("%d", c[i][i]);
	printf("\n");
}

}

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Compilation time: 0.12 sec, absolute service time: 0,21 sec