Initializing static array using int variable from inside a struct

Question

I'm working with a typedef used to represent an image, seen here:

typedef struct {
  int rows;             // Vertical height of image in pixels //
  int cols;             // Horizontal width of image in pixels //
  unsigned char *color; // Array of each RGB component of each pixel //
  int colorSize;        // Total number of RGB components, i.e. rows * cols * 3 //
} Image;

So for example an image with three pixels, one white, one blue and one black, the color array would look like this:

{
  0xff, 0xff, 0xff,
  0x00, 0x00, 0xff,
  0x00, 0x00, 0x00
}

Anyway, I'm passing an instance of Image as a parameter in a function. With this parameter I'm trying to initialize a static array using the colorSize variable as its the only variable I am maintaining to track the size of the color array. However I'm getting an error because the initialization value is not constant. How might I get around this?

char *foobar( Image *image, ... ) 
{
  static unsigned char arr[image->colorSize];

  ...
}

Answer 1

Static arrays cannot be variable-length. Use dynamic allocation instead, with a static pointer.

char *foobar(Image *image, ...) {
    static unsigned char *arr;
    if (!arr) {
        arr = malloc(image->colorSize);
    }
    ...
}