problem with structure pointers

Hi,

I am getting Segmentation fault error for code below. Please tell me how to remove it.

Code:

#include<stdio.h>
#include<stdlib.h>


struct MATRIX {
	int rows;
	int cols;
	double **t;
};

struct MATRIX new_matrix(int rows, int cols)
{
	struct MATRIX t;
	int i;
	t.rows=rows;
	t.cols=cols;
	t.t=(double **)malloc(t.rows * sizeof(double *));
	for(i = 0; i < t.rows; i++)
		t.t[i] = (double *) malloc(cols * sizeof(double));
	return t;
}

struct MATRIX *transpose(struct MATRIX *a)
{
	struct MATRIX *b;
	int i, j;
	
	*b = new_matrix(a->cols, a->rows);

	for(i=0; i<b->rows; i++)
		for(j=0; j<b->cols; j++)
			 *(*(b->t+i)+j)=*(*(a->t+j)+i);

	return b;
}

int main(){
	struct MATRIX *x;
	struct MATRIX *t;
	int i, j;
	
	*x = new_matrix(3, 4);
	for(i = 0; i < x->rows; i++)
		for(j = 0; j < x->cols; j++)
			*(*(x->t + i) + j) = 1.2*(i+j);
	
	t = transpose(x);
	
	for(i=0; i < x->rows; i++) {
		printf("| ");
		for(j=0; j < x->cols; j++)
			printf("%.2f ",*(*(x->t+i)+j));
		printf("|\n");
	}
	printf("\n");
	
	for(i=0; i < t->rows; i++) {
		printf("| ");
		for(j=0; j < t->cols; j++)
			printf("%.2f ",*(*(t->t+i)+j));
		printf("|\n");
	}
	printf("\n");
}

You never allocated any memory for x or t, so it's natural you get an access violation when dereferencing them.

You can't do *x = new_matrix(3,4), since you don't own the memory x points at. You want your new_matrix to return a MATRIX*, and do x = new_matrix(3,4). Edit: and new_matrix will have to allocate the memory for the matrix, since t is gone at the end of the function.

And don't forget to free and malloc'd memory.

Perhaps start with these signatures:

Code:

struct MATRIX new_matrix(int rows, int cols)
struct MATRIX transpose(struct MATRIX *a)

And make associated . and -> operator changes.