On 09/16/2011 09:04 PM, John Clements wrote:
On Sep 16, 2011, at 3:06 PM, John Clements wrote:
I'm trying to backport working FFI code to 5.1.3, and I'm having trouble
referring to elements of a C struct. Specifically, in the presence of this
definition
(define-cstruct _rack-audio-closure
([sound _pointer]
[cur-sample _ulong]
[num-samples _ulong]
[stop-now _bool]
[stop-sema-ptr _pointer]))
using the function (rack-audio-closure-cur-sample) on the result of an FFI call
with signature
(_fun _pointer _ulong _pointer -> _rack-audio-closure-pointer)
produces this error:
rack-audio-closure-cur-sample: expected argument of type<struct:rack-audio-closure>;
given #<cpointer:rack-audio-closure>
in 5.1.3, but it works just fine in the git head.
More research suggests that the problem is more limited; it looks like a problem of
generativity. That is, if I have two modules that declare the "same" cstruct
(same name, same fields, same types) then cpointers to these structures returned by
functions from the first module cannot be accessed in the other module.
in 5.1.3 "same" cstructs defined in two different modules could not
inter-operate.
Each place has it own copy of modules, so
the "same" cstruct defined in the same module but in two different
places wouldn't inter-operate either.
Hence the change.
AFAICT, This is not the case in more recent versions; that is, these cstructs
are not generative. Assuming this is deliberate, I could document it.
This is deliberate. It allows foreign pointers to be shared between places.
The following text was removed from the documentation in the head
version because cstructs are not generative anymore.
Note that tags are compared with @racket[eq?] (or @racket[memq]), which
means
an interface can hide its value from users (e.g., not provide the
@racket[cpointer-tag] accessor), which makes such pointers un-fake-able.
If you would like document the fact that cstructs are generative only up
to version 5.1.3. I think that improve the docs.
Thanks,
Kevin
John
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