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4.4.3 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.

     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.