# C extension question **URL:** \ **Category:** Ruby\ **Created:** [September 18, 2006, 7:58pm UTC](https://www.ruby-forum.com/t/c-extension-question/69114 "2006-09-18T19:58:42Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![snacktime](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/7feea3/32.png) [@snacktime](https://www.ruby-forum.com/u/snacktime)\ **Post date:** [September 18, 2006, 7:58pm UTC](https://www.ruby-forum.com/t/c-extension-question/69114/1 "2006-09-18T19:58:42Z")
So I’m making some headway on a kerberos 5 extension for ruby. It’s limited at the moment to authorizing a user, changing a user password, creating principals, and deleting principals, but it’s working. Now I’m trying to refactor it a bit and need some assistance. I’ve never really coded in C before so bear with me… In kerberos when doing an administration function I first need to initialize the system with the credentials of an administrative user like so, with handle being a void pointer initialized to NULL. The following kerberos function will fill handle with a handle for the connection which I can use in subsequent calls to other kadm5 functions. kadm5\_init\_with\_password(auser, apass, KADM5\_ADMIN\_SERVICE, NULL, KADM5\_STRUCT\_VERSION, KADM5\_API\_VERSION\_2, &handle); Now what I’d like is to return that handle to ruby and be able to pass it back into my C extension when it’s needed, such as when needing to call the following kerberos function. kadm5\_create\_principal(handle, &princ, mask, pass); So how would I do this? If I understand void pointers correctly, handle could be any type after the call to kadm5\_init\_with\_password, so it seems I would want to check it’s type after it’s set, then find the right type/structure to convert it to or wrap it in before passing it back to ruby? Not that I know how to do that:) Chris ---
**Author:** ![snacktime](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/7feea3/32.png) [@snacktime](https://www.ruby-forum.com/u/snacktime)\ **Post date:** [September 18, 2006, 8:38pm UTC](https://www.ruby-forum.com/t/c-extension-question/69114/2 "2006-09-18T20:38:47Z")
On 9/18/06, snacktime [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > it back to ruby? Not that I know how to do that:) > > Chris Void\* is almost always the same size as the native integer. (There is legitimate controversy over this point but you can depend on it for all ends and intents.) So when the kerb function returns having filled in your void\*, you can cast it to a native unsigned and then pass it back to Ruby as a VALUE returned from INT2NUM. (I wouldn’t use INT2FIX because you’ll need all of the native resolution.) To pass the value back into C, convert it to an int with NUM2INT and cast to a void\*. I would expect (but can’t say for sure) that this will work on 64-bit platforms (where void\* is eight bytes wide) as long as the Ruby conversion functions are defined on the native integer. ---
**Author:** ![snacktime](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/7feea3/32.png) [@snacktime](https://www.ruby-forum.com/u/snacktime)\ **Post date:** [September 18, 2006, 9:14pm UTC](https://www.ruby-forum.com/t/c-extension-question/69114/3 "2006-09-18T21:14:51Z")
> Better to wrap it as a data type in Ruby, which gives you a ruby VALUE > which references the native data. There’s a simple macro to grab the > original pointer given the VALUE. > > DEC Alpha doesn’t necessarily have sizeof(int) == sizeof(void \*), so > please don’t assume so. I hate fixing code that does that. 😉 > > Aria (A kerberos user, too!) Well I’ll do the grunt work and then when I release it I’ll accept patches from those with more C experience than I have. I actually can’t remember if I ever wrote a C program before, but I’ve been meaning to learn it so I figured this was as good a project as any since we have some use for kerberos bindings. ---
**Author:** ![snacktime](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/7feea3/32.png) [@snacktime](https://www.ruby-forum.com/u/snacktime)\ **Post date:** [September 18, 2006, 8:44pm UTC](https://www.ruby-forum.com/t/c-extension-question/69114/4 "2006-09-18T20:44:48Z")
On Tue, 2006-09-19 at 03:39 +0900, Francis C. wrote: > > KADM5\_STRUCT\_VERSION, KADM5\_API\_VERSION\_2, &handle); > > the right type/structure to convert it to or wrap it in before passing > > it back to ruby? Not that I know how to do that:) > Void\* is almost always the same size as the native integer. (There is > legitimate controversy over this point but you can depend on it for all ends > and intents.) Better to wrap it as a data type in Ruby, which gives you a ruby VALUE which references the native data. There’s a simple macro to grab the original pointer given the VALUE. DEC Alpha doesn’t necessarily have sizeof(int) == sizeof(void \*), so please don’t assume so. I hate fixing code that does that. 😉 Aria (A kerberos user, too!) ---
**Author:** ![snacktime](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/7feea3/32.png) [@snacktime](https://www.ruby-forum.com/u/snacktime)\ **Post date:** [September 18, 2006, 9:32pm UTC](https://www.ruby-forum.com/t/c-extension-question/69114/5 "2006-09-18T21:32:53Z")
On 9/18/06, Aredridel [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > DEC Alpha doesn’t necessarily have sizeof(int) == sizeof(void \*), so > please don’t assume so. I hate fixing code that does that. 😉 It’s been over five years since the last time I programmed an Alpha in C. Do you have Ruby running on yours? Are you running DEC/Unix on them? As I recall, pointers are always a flat 64-bits on Alpha but ints come in several flavors, so you would have to cast the void\* carefully to keep from narrowing it, but it would fit just fine in a Ruby Bignum. Are you suggesting to use StringValue and StringValuePtr, and treat the void\* as a char\*? ---
**Author:** ![snacktime](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/7feea3/32.png) [@snacktime](https://www.ruby-forum.com/u/snacktime)\ **Post date:** [September 18, 2006, 9:49pm UTC](https://www.ruby-forum.com/t/c-extension-question/69114/6 "2006-09-18T21:49:00Z")
One more unrelated question. I’m trying to think of the best way to do the interface, particularly how to access error messages and what to return to the caller. For most functions the caller just needs to know if the function returned true, or if there was an error a way to get the error message. So, I could for example return 0 on success or the error message number on failure. Or I could return true/false, store any error message number in a C global variable, and then have a function which returns the text error message if the original function returned false. The latter seems more intuitive. For example krb = Krb5.new if krb.authenticate\_with\_password(user,pass) # authenticated else p krb.errstr end As opposed to: res = krb.authenticate(user,pass) if res == 0 # authenticated else p krb.errstr(res) end And in the case where a method returns a handle that will be used later, I’m thinking return the handle on success, and on failure return nil and make the error available via krb.errstr? Chris ---
**Author:** ![snacktime](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/7feea3/32.png) [@snacktime](https://www.ruby-forum.com/u/snacktime)\ **Post date:** [September 18, 2006, 9:59pm UTC](https://www.ruby-forum.com/t/c-extension-question/69114/7 "2006-09-18T21:59:00Z")
On 9/18/06, snacktime [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > The latter seems more intuitive. For example > And in the case where a method returns a handle that will be used > later, I’m thinking return the handle on success, and on failure > return nil and make the error available via krb.errstr? I’m guessing because I haven’t seen your code but I’d say it’s much more Ruby-esque to return a boolean value rather than a zero (which Ruby evaluates true). Hiding the error string behind an accessor makes sense if it will be accessed infrequently. You do have to remember to clear it on _every_ call into your extension. Why make your user touch the handle at all? Can you hide it inside your Krb5 objects with instance\_eval, and just supply it yourself when you need to pass it to the kerb library? ---
**Author:** ![snacktime](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/7feea3/32.png) [@snacktime](https://www.ruby-forum.com/u/snacktime)\ **Post date:** [September 18, 2006, 10:47pm UTC](https://www.ruby-forum.com/t/c-extension-question/69114/8 "2006-09-18T22:47:07Z")
> Why make your user touch the handle at all? Can you hide it inside > your Krb5 objects with instance\_eval, and just supply it yourself when > you need to pass it to the kerb library? I’m not picturing this for some reason, can you elaborate? Chris