Warning: This is the manual of the legacy Guile 2.0 series. You may want to read the manual of the current stable series instead.

5.5.3 Type checking

Functions that operate on smobs should check that the passed SCM value indeed is a suitable smob before accessing its data. They can do this with scm_assert_smob_type.

For example, here is a simple function that operates on an image smob, and checks the type of its argument.

SCM
clear_image (SCM image_smob)
{
  int area;
  struct image *image;

  scm_assert_smob_type (image_tag, image_smob);

  image = (struct image *) SCM_SMOB_DATA (image_smob);
  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);

  scm_remember_upto_here_1 (image_smob);

  return SCM_UNSPECIFIED;
}

See Remembering During Operations for an explanation of the call to scm_remember_upto_here_1.