(guile.info.gz) Type checking
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18.3.3 Type checking
--------------------
Functions that operate on smobs should aggressively check the types of
their arguments, to avoid misinterpreting some other datatype as a smob,
and perhaps causing a segmentation fault. Fortunately, this is pretty
simple to do. The function need only verify that its argument is a
non-immediate, whose first word is the type tag returned by
`scm_make_smob_type'.
For example, here is a simple function that operates on an image
smob, and checks the type of its argument. We also present an expanded
version of the `init_image_type' function, to make `clear_image' and
the image constructor function `make_image' visible to Scheme code.
SCM
clear_image (SCM image_smob)
{
int area;
struct image *image;
SCM_ASSERT (SCM_SMOB_PREDICATE (image_tag, image_smob),
image_smob, SCM_ARG1, "clear-image");
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 (image->update_func != SCM_BOOL_F)
scm_apply (image->update_func, SCM_EOL, SCM_EOL);
return SCM_UNSPECIFIED;
}
void
init_image_type (void)
{
image_tag = scm_make_smob_type ("image", sizeof (struct image));
scm_set_smob_mark (image_tag, mark_image);
scm_set_smob_free (image_tag, free_image);
scm_set_smob_print (image_tag, print_image);
scm_c_define_gsubr ("clear-image", 1, 0, 0, clear_image);
scm_c_define_gsubr ("make-image", 3, 0, 0, make_image);
}
Info Catalog
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