Variable 2D array in a C struct

Question

This is probably a question with a simple answer but I am not finding anything similar to it with a proper solution. I am trying to create a struct in C that has two variables and then a two dimensional array with dimensions equal to that of the two variable parameters used to create the struct .

struct image{
  int width;
  int hight;
  int pixles[width][height];
};

Now I knew this wouldn't work before I even compiled it but I'm not sure how to go about making this work.

Answer 1

You cannot do that directly as you were said in comments. There are two common idioms to simulate it (assuming VLA are supported):

1. you only store in the struct a pointer to a (dynamically allocated) array, then you cast it to a pointer to a 2D VLA array:

    typedef struct _Image { int width; int height; unsigned char * data; } Image; int main() { Image image = {5, 4}; image.data = malloc(image.width * image.height); unsigned char (*data)[image.width] = (void *) image.data; // you can then use data[i][j]; 

2. If you dynamically allocate the structs, you can use an array of 0 size as its last element (and again cast it to a VLA pointer):

    typedef struct _Image { int width; int height; unsigned char data[0]; } Image; int main() { Image *image = malloc(sizeof(Image) + 5 * 4); // static + dynamic parts image->width = 5; image->height = 4; unsigned char (*data)[image->width] = (void *) &image->data; // you can then safely use data[i][j] 

If VLA are not supported by your C implementation, you must revert to the old idiom of simulating a 2D array through a 1D pointer: data[i + j*image.width]