Thread: Pointer to a matrix - Segmentation fault

The following code works, but when I change my size larger than 10 I get segmentation faults. I'm pretty sure it has to do with the pointer somehow I just can't figure it out!

I need to make my matrix larger, by setting the matrix[SIZE][SIZE] to size 10000.
#define SIZE 100

It only works whenamatrix[SIZE][SIZE] to size 100.
#define SIZE 10

Any help will be greatly appreciated! Thanks!

Endomlic

Code:

//I had to change the size to 10 from 100 because
//of segmentation faults
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/shm.h>
#include <sys/ipc.h>
#include <sys/types.h>
#define SIZE 10                                            <---------------------------HERE
struct matrix
{
    int C[SIZE][SIZE];
}*c_ptr;
int A[SIZE][SIZE], B[SIZE][SIZE];
void randomize(int dim, int);
void compute_element(int, int);
void print();
int main(int argc, char * argv[])
{
    int shmid, status;
    int i,j,l,m,z, children = 1;
    pid_t pid;
    shmid = shmget((key_t)1234, sizeof(int) * SIZE * SIZE, 0666 | IPC_CREAT);
    c_ptr = shmat(shmid, 0, 0); 
    // place random values in A and B
    randomize(SIZE,10000);

	//Spawns chlidren based on user parameter
	//default is 1
	if(!(argc < 2))
	{
		children = atoi(argv[1]);
	}
	printf("%d process(es) spawned. To spawn more processes type number after program name.\n", children);

	pid = fork();
	if(pid == 0){
		for( l = 0; l < (SIZE/2); l++ ){
			for( m = 0; m < (SIZE/2); m++ )
			{		
				for(z = 0; z < children; z++){
					pid = fork();
					if(pid == 0){
						(void) shmat(shmid, 0, 0); 
						compute_element(l,m);
						//exit child
						_exit(status);				
					}
				}
			}
		}
		//exit child
		_exit(status);
	}

	//the other half the parent will handle
	for( i = (SIZE/2); i < SIZE; i++ ){
        for( j = (SIZE/2); j < SIZE; j++ )
        {
			(void) shmat(shmid, 0, 0); 
            compute_element(i,j);            
        }
	}

	//wait for child to exit before printing
	wait(&status);
    print();
	
	return 0;
}
void randomize(int dim, int big)
{
    int i,j;
    for( i = 0; i < dim; i++ )
        for( j = 0; j < dim; j++ )
		{
            A[i][j] = random() % big;
            B[i][j] = random() % big;
		}
}
void compute_element(int iNum, int jNum)
{   
    int k;
	{
		c_ptr->C[iNum][jNum] = 0;
		for( k = 0; k < SIZE; k++ )
			if( (k % 2) == 0 )
				c_ptr->C[iNum][jNum] += A[iNum][k] * A[k][jNum];
			else 
				c_ptr->C[iNum][jNum] -= A[iNum][k] * A[k][jNum];
	}
}
void print()
{
	int i;
    for(i = 0; i < SIZE; i++)
    {
        int row = random() % SIZE;
        int col = random() % SIZE;
        printf("C[%d][%d] = %d\n", row, col, c_ptr->C[row][col]);
    }
}

Nothing particularly obvious in the code, on a quick look. It's possible, however, it's running into system-imposed limits (eg quotas). Check the return values from shmget() and shmat() to see if an error occurred.