foreign function returning pointer to struct

Ayup! I'm trying to use FLI to call a C function that returns a pointer to a struct. But when I try to access struct fields in the returned pointer all the fields are set to null.

In section 4.2.1 of the FLI manual it says that C and Lisp does not share memory. Does that mean that it is impossible to use pointers returned by foreign functions?


Kind regards

André Lynum


_______________________________________________
Lisp Hug - the mailing list for LispWorks users
lisp-hug@lispworks.com
http://www.lispworks.com/support/lisp-hug.html

Re: foreign function returning pointer to struct

Following up to myself, a test library returning a pointer to an int appear to work. So I guess the problem lies elsewhere.


-andré


On Jun 11, 2012, at 1:31 PM, André Lynum wrote:

> 
> Ayup! I'm trying to use FLI to call a C function that returns a pointer to a struct. But when I try to access struct fields in the returned pointer all the fields are set to null.
> 
> In section 4.2.1 of the FLI manual it says that C and Lisp does not share memory. Does that mean that it is impossible to use pointers returned by foreign functions?
> 
> 
> Kind regards
> 
> André Lynum
> 
> 
> _______________________________________________
> Lisp Hug - the mailing list for LispWorks users
> lisp-hug@lispworks.com
> http://www.lispworks.com/support/lisp-hug.html
> 
> 


_______________________________________________
Lisp Hug - the mailing list for LispWorks users
lisp-hug@lispworks.com
http://www.lispworks.com/support/lisp-hug.html

Re: beginner

Hi,

If you are asking about a LispWorks library for drawing graphs (in the
mathematical sense, aka charts), I do not know any!

But If you mean, a set of core functions (or primitives, like draw a
line, draw a circle) that you can use to accomplish your task, then
yes,

In the Listener type:

(example-file "capi/graphics/")

This should give you the path to the directory where LW graphics examples live.

Also search on LispWorks site documentation for Graphics-Port or for short GP.

HiH

Ala'a

to get the path to the examples that ships with

On Mon, Jun 11, 2012 at 5:37 PM, vincent.douce.2012
<vincent.douce.2012@gmail.com> wrote:
>
> hi everyone
> i just installed lispworks on my mac os X
> i dont know if it is the right place for my question
> i a a beginner in lisp
> i just know still nothing
> what i would like to find a real motivation is just being able to draw something like the graph of r=theta
> after that, i can search for myself, being guided by my aim which is graphical rendering
> i am wondering wether with lispworks i need something else to produce graphics
> for example if i define some functions in my first lisp programs, could i draw their graph just with lispworks or do i need a sort of interface ?
> Vincent
>
> _______________________________________________
> Lisp Hug - the mailing list for LispWorks users
> lisp-hug@lispworks.com
> http://www.lispworks.com/support/lisp-hug.html
>

_______________________________________________
Lisp Hug - the mailing list for LispWorks users
lisp-hug@lispworks.com
http://www.lispworks.com/support/lisp-hug.html

Re: beginner

Unable to parse email body. Email id is 11628

RE: foreign function returning pointer to struct

For dereferencing a struct you need to use the :copy-foreign-object keyword.. As the doc for FLI:DEREFERENCE says, 'An error is signaled if value is an aggregate type and copy-foreign-object is not set accordingly.' Setting it to T or NIL should work (default is ERROR).

Chris

Subject: Re: foreign function returning pointer to struct
From: and-ly@online.no
Date: Mon, 11 Jun 2012 19:39:51 +0200
CC: lisp-hug@lispworks.com
To: massung@gmail.com

Thanks for the reply.

It's not possible to dereference such a pointer

CL-USER 91 > (fli:dereference *ptr*)

Error: Attempted to dereference a pointer to aggregate type (:STRUCT MY-STRUCT).

CL-USER 93 > *ptr*

#<Pointer to type (:STRUCT MY-STRUCT) = #x00148400>

Reading the docs further it appears that I should omit the :pointer in the :result type

(fli:define-foreign-function (a-test "a_test" :source)

()

:result-type my-struct

:module "/Users/stinky/Work/ML/libtest.so")

And call an allocated foreign object for the :result-pointer key

CL-USER 94 > (setf *x* (fli:allocate-foreign-object :type 'my-struct))

#<Pointer to type (:STRUCT MY-STRUCT) = #x003A3B20>

CL-USER 95 > (a-test :result-pointer *x*)

#<Pointer to type (:STRUCT MY-STRUCT) = #x003A3B20>

But this results in completely rubbish struct field values

CL-USER 101 > (fli:foreign-slot-value *x* 'a)

1581056

CL-USER 102 > (fli:foreign-slot-value *x* 'b)

I haven't found any examples of this in the docs or elsewhere so hopefully someone who knows how this works can help me out :)

Kind regards

André Lynum

On Jun 11, 2012, at 5:36 PM, Jeffrey Massung wrote:

On Jun 11, 2012, at 7:59 AM, André Lynum wrote:

When calling my function and accessing the struct fields I get garbage in the second field

CL-USER 21 > (setf *ptr* (a-test))
#<Pointer to type (:STRUCT MY-STRUCT) = #x0015C7D0>

CL-USER 22 > (fli:foreign-slot-value *ptr* 'b)
1383793

CL-USER 23 > (fli:foreign-slot-value *ptr* 'a)
13

Andre,

I don't have time at the moment to put together my own test, but the above behavior looks correct. LispWorks doesn't have the C equivalent of the -> operator, only the . operator. Accessing slow 'a only appears to work because it's at byte offset 0 from the pointer address. You'll need to dereference the pointer first (to get a my-struct) and then access the slots in it.

(let ((s (dereference *ptr*)))
(values
(foreign-slot-value s 'b)
(foreign-slot-value s 'a)))
=> 14 13

Again, the above isn't tested, but I'm pretty confident. ;-)

- jeff