problem with structure pointers

Hi,

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

Code:

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#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");
}

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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:

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

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And make associated . and -> operator changes.