Functions that operate on foreign objects should check that the passed
SCM value indeed is of the correct type before accessing its
data. They can do this with scm_assert_foreign_object_type.
For example, here is a simple function that operates on an image object, and checks the type of its argument.
SCM
clear_image (SCM image_obj)
{
int area;
struct image *image;
scm_assert_foreign_object_type (image_type, image_obj);
image = scm_foreign_object_ref (image_obj, 0);
area = image->width * image->height;
memset (image->pixels, 0, area);
/* Invoke the image's update function. */
if (scm_is_true (image->update_func))
scm_call_0 (image->update_func);
return SCM_UNSPECIFIED;
}