July 2006 - cffi-devel - mailman3.common-lisp.net

[cffi-devel] Enums, structs, and field sizes
by Dan Knapp 15 Jul '06

15 Jul '06

I just uncovered this issue with cffi-net. This is more of a design question than a bug; I know at least one way that I could work around it, but it's distressingly inelegant. To get the cffi-net low-level primitives up and running, we feed cffi-grovel the names of some preprocessor constants which it turns into an enum called address-family, with values such as :af-inet -> 2. This enum is used as a parameter to a bunch of different function calls, and it's also a member of the struct sockaddr-in. We tell the groveller that the "family" slot should be of the type address-family, so instead of figuring out the size of it and putting in :uint8, :uint16, or whatever, it declares the slot to be of type address-family. The problem is that the slot of course has a correct size. In this case it happens to be :uint8. In the absence of other guidance, cffi assumes that an enum type should be a regular integer, which seems to wind up as :sint32. So when it comes time to access the slot, too much memory is used and it overwrites adjacent slots. Since cffi does provide the option to give a base type for the enum, we could specify that type. If we do that by hand, though, it defeats the purpose of having cffi-grovel; it would introduce a portability issue and would have to be manually maintained for new architectures with surprising new slot sizes. We could make cffi-grovel smart enough to notice that the enum is used for a slot, and take some appropriate action. It could set the base type of the enum, but this would cause trouble with the functions that use it as a parameter type and expect it to be int-sized. It could create a new enum, address-family-uint8, that duplicates everything in address-family but is a different size. This has the benefit of working, so if we can't come up with anything better it's probably what we'll do. We also can't reasonably hook in any special-case code for when this field is accessed, because cffi only dispatches on type when it's deciding how to access a field, and if we create a new type we're back to one of the above scenarios. It occured to me that the reason this is a problem at all is that enums are considered to be types. Perhaps they really need to be something outside the type system, that can be overlayed on a type as needed? Anyway, since there's no particular hurry, we thought we'd post to the list and ask for suggestions. It would be comforting to find a clean way to do this. -- Dan Knapp

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[cffi-devel] Mutable Pointers
by Stuart Sierra 14 Jul '06

14 Jul '06

Hello, I'm working on a CFFI interface to the Perl API (I thought it would be fun - ha!) and I need to be able to do C-style pointer arithmetic. That is, I have a foreign pointer declared with DEFCVAR and I need to alter the address stored there. I couldn't see a way to do this with CFFI, so I created a new foreign type called :ADDRESS that would treat the foreign pointer as an integer value that I could modify. Here's my code: (defvar *pointer-size* (foreign-type-size :pointer) "The size of a pointer on the current platform, in bytes.") (ecase *pointer-size* (1 (defctype :address :uint8)) ; unlikely (2 (defctype :address :uint16)) ; possible (4 (defctype :address :uint32)) ; most common (8 (defctype :address :uint64))) ; possible (defmacro address-incf (address &optional (n 1)) "Increment ADDRESS by N pointers, like ++i on a C pointer." `(incf ,address (* ,n *pointer-size*))) (defmacro address-decf (address &optional (n 1)) "Decrement ADDRESS by N pointers, like --i on a C pointer." `(decf ,address (* ,n *pointer-size*))) (defmacro address-ref (address type) "Dereference ADDRESS to get the TYPE it points to, like MEM-REF." `(mem-ref (make-pointer ,address) ,type)) This does work for my purposes, but is there a better way using CFFI internals? Thanks, -Stuart

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Re: [cffi-devel] trying to understand foreign types and translations
by rif 06 Jul '06

06 Jul '06

> The two arguments to TRANSLATE-TO-FOREIGN are the Lisp object to be > converted, and the name of the foreign type to convert to. First, > declare a foreign typedef for :POINTER (since you will ultimately be > returning a pointer from the type translator) that you use to hang > your type translator on: > > (defctype fnv-double :pointer) > > Then you can specialize TRANSLATE-TO-FOREIGN on both the Lisp value > and the foreign type (using an EQL specializer): > > (defmethod translate-to-foreign ((value fnv-double) (name (eql 'fnv-double))) > (fnv-foreign-pointer value)) > > It might be confusing to call both the Lisp and foreign types > FNV-DOUBLE, but it is legal because foreign type names are a separate > namespace from Lisp types (viva Lisp-N!). It is important to remember > the distinction between the two though. > > > and then declaring something like > > > > (defcfun ("foo" :%foo) :double > > (x fnv-double)) > > should just work! > > (This is untested and I haven't really been doing any CFFI hacking > lately, so I apologize if I missed something obvious here.) > > James I don't quite understand. fnv-double is NOT a pointer. It's a CL struct that holds two arguments, one of which is the foreign pointer. I see I was misusing translate-to-foreign, so maybe I don't need a defctype at all, just an appropriate translate-to-foreign? rif

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[cffi-devel] trying to understand foreign types and translations
by rif 06 Jul '06

06 Jul '06

I've been poking at this for over an hour, but I'm just not sure how to get this to work. I have a foreign object, points to a vector of (C) double-floats: (defstruct (fnv-double (:constructor make-fnv-double-internal)) (foreign-pointer nil :read-only t) (size 0 :type fixnum :read-only t)) I can create and manipulate it. I need to know how to define (with defcfun) an interface to a C function which expects a vector of double-floats. My attempts have been along the lines of: (defmethod translate-to-foreign (val (f fnv-double)) (fnv-foreign-pointer f)) and then declaring something like (defcfun ("foo" :%foo) :double (x fnv-double)) but neither this nor any of the variants I've tried seems to work at all. I think I need to either defctype or define-foreign-type first, but I haven't gotten that to work either. Can anyone suggest how to do this? Cheers, rif

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[cffi-devel] Chase Online. Banking Account registration information
by Chase＠common-lisp.net 03 Jul '06

03 Jul '06

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