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5.5.1 Defining Foreign Object Types

To create a new foreign object type from C, call scm_make_foreign_object_type. It returns a value of type SCM which identifies the new type.

Here is how one might declare a new type representing eight-bit gray-scale images:

#include <libguile.h>

struct image {
  int width, height;
  char *pixels;

  /* The name of this image */
  SCM name;

  /* A function to call when this image is
     modified, e.g., to update the screen,
     or SCM_BOOL_F if no action necessary */
  SCM update_func;

static SCM image_type image_type;

init_image_type (void)
  SCM name, slots;
  scm_t_struct_finalize finalizer;

  name = scm_from_utf8_symbol ("image");
  slots = scm_list_1 (scm_from_utf8_symbol ("data"));
  finalizer = NULL;

  image_type =
    scm_make_foreign_object_type (name, slots, finalizer);

The result is an initialized image_type value that identifies the new foreign object type. The next section describes how to create foreign objects and how to access their slots.