I have a C function that expects an opaque void* pointer to hold some user
data. This is specifically about a small problem with Sundials.jl, but it's
also unclear to me generally.
How do I safely pass a Julia value to the function? What are the
conventions? The value will later be passed to Julia callbacks, so it needs
to remain valid forever. I don't mind leaking it, if necessary.
The function CVodeUserData in Sundials.jl expects either
(Ptr{None}, Ptr{None})
or
(Ptr{None}, Any)
(the 2nd argument is the Julia value, the first is library's internal
structure).
Which of these is correct? The original code for CVodeUserDataB seems to
use Ptr{None} (why not Ptr{Void} or Any?), but I keep getting an error
saying that my Julia has no method matching convert(::Type{Ptr{Union()}},
::MyType).
Is Any the correct type for a Julia value to be passed to C? Will the value
be moved or deallocated?
