Thread: Pointer to a matrix - Segmentation fault

  1. #1
    Registered User
    Join Date
    Apr 2009
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    2

    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]);
        }
    }

  2. #2
    Registered User
    Join Date
    Jun 2005
    Posts
    6,815
    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.

  3. #3
    Hurry Slowly vart's Avatar
    Join Date
    Oct 2006
    Location
    Rishon LeZion, Israel
    Posts
    6,794
    you allocating sizeof(int) *SIZE*SIZE and assigning to the pointer of type
    struct Matrix*

    sizeof (struct Matrix) could be different from size of its members

    also - it is good idea - to check the pointer before using it. some functions could fail, you know...
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

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