How to initialize structure in main function

Question

I am trying to write simple pgm file reading C program. I had to create simple structure:

typedef struct pgmImage {
   int R; //rows
   int C; //collumns
   int G; //grey scale
   int **pix;  // array of pixels
}Image;

Now i have to initialize empty Image structure.I need one to set all variables based on *.pgm file in other function. All the time i am getting "unable to read memory" and 'unitialized local variable x used'. I have tried to simply do :

Image *x=0;

but program crashes when read function try to set R,C,G values.

Answer 1

If you want a pointer to Image you have to initialize like this.

Image *x = NULL;

Accessing the image (x) camps like this :

x-> C = 0;
x-> ...
x->pix = NULL;

But first you need to allocate memory to your image.

x = (Image*) malloc(sizeof(Image));

Answer 2

Image x = {0}

it s a static memory allocation of image element x

or

Image *x = calloc(1,(sizeof(Image));

it s a dynamic memory allocation of image and x is a pointer to the allocated memory

the calloc will allocate emory and initiate all memory to 0, so the pointer in the structure will be initiated systematically to NULL

Answer 3

If you want to use a pointer, you have to allocate enough memory to hold a structure. However, you are trying to dereference a null pointer: this leads to undefined behavior.

#include <stdlib.h>
Image *x = malloc(sizeof *x);

And then you can initialize the members. NB: Use calloc here is non-portable, because a null-pointer is not guaranteed to have a memory representation all-bits-zero.

Anyway, it seems that you don't need to use a pointer here.

#include <stddef.h>
Image x = { 0, 0, 0, NULL };