How to extract

[Nate Griswold]

I had a question. In embedded racket, I am passing a _cpointer value back to c code by way of racket_apply's return value.

Looking over https://docs.racket-lang.org/inside/cs-values_types.html , there appears to be a group of functions associated with extracting values from ptrs. I do not see one for a pointer ptr there.

Is there a way to get at a returned _cpointer value from c code?

Thanks

Nate

[Nate Griswold]

A bit more context:

I am using a communications library (zeromq) and i need to share a c context object between c and racket for in-process communication.

I can either create it in c or create it in racket. The library i'm using happens to create it in racket.

The only way i know of solving this problem is to create an init foreign function in racket and pass in the _cpointer data and grab the c pointer when my c function is called.

Is there any other way to solve this problem?

Nate

[Matthew Flatt]

I see that there's not a good way right now, but here's a workaround
that uses information about the current layout:

A cpointer value is implemented as a Chez Scheme record with either 1
field or 2 fields. There are 2 fields only when the cpointer has an
offset as a result of `ptr-add`, so you can probably ignore that.

To extract the first field, assume that a record has the same layout as
a vector, so use `Svector_ref(p, 0)` to extra the field from the
cpointer `p`.

Then you can use `Sunsigned_value()` to convert that field value to a
pointer-sized integer, then case.

I might have some part of that wrong, but it should be close... Of
course, there should be better support for record-field access and
cpointer extraction, so I'll add to the API.

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[Nate Griswold]

Awesome, thanks!

I actually just implemented it with the roundabout callback system, i think i might try out your suggestion though.

Thanks for working on racket!

Nate