Re: varptr to structs returns bad pointer value

[Benoît Minisini <benoit.minisini@xxxxxxxxxxxxxxxx>]

Le 27/02/2025 à 21:07, Brian G a écrit :
> I am not sure if this is a bug or not
>
> When passing structures to external c functions varptr fails to produce 
> the correct pointer...
> to the data of the structure. I think maybe I don't understand the 
> difference, but it would seem they should
> produce the same result.
>
> extern myFunction(data as pointer) use "mylib"
> public dataSet as pointer
> public struct myStruct
>      result as long
>      msg as string
> end struct
> public mydata as mystruct
>
> this works as expected
>
> public sub doit()
>    dataSet = alloc(sizeof(mystruct))
>    mydata = dataSet
>    myFunction(dataSet)  ' this works
> end sub
>
> this fails
>
> public sub doit2()
>    mydata = new myStruct
>    dataSet = varptr(myData)
>    myFunction(dataSet)  ' this fails invalid pointer provided
> end sub

VarPtr() tries to return the memory address of where the value of a 
variable is stored.

So when you write:

dataSet = varptr(myData)

you get the the address of where the value of 'mydata' is stored, not 
the address of the 'new MyStruct' object or the address of its contents.

To get the address of a Gambas object contents, you must use the 
'Object.Data()' static method on the object reference.

dataSet = Object.Data(myData)

Regards,

--
Benoît Minisini.