# Ref vs. Ptr and parametric types

**URL:** <https://discourse.julialang.org/t/ref-vs-ptr-and-parametric-types/13569>\
**Category:** General Usage\
**Created:** [August 16, 2018, 1:53pm UTC](https://discourse.julialang.org/t/ref-vs-ptr-and-parametric-types/13569 "2018-08-16T13:53:04Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![ssfrr](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ssfrr/32/3736_2.png) [@ssfrr](https://discourse.julialang.org/u/ssfrr)\
**Post date:** [August 16, 2018, 1:53pm UTC](https://discourse.julialang.org/t/ref-vs-ptr-and-parametric-types/13569/1 "2018-08-16T13:53:04Z")

</div>

I have a C function I’m calling with the signature:

```nohighlight
SNDFILE* sf_open_virtual	(SF_VIRTUAL_IO *sfvirtual, int mode, SF_INFO *sfinfo, void *user_data) ;

```

(from [libsndfile](http://www.mega-nerd.com/libsndfile/api.html#open_virtual))

the `user_data` pointer is passed as an argument to some callbacks that are defined in the `SF_VIRTUAL_IO` struct. I’m calling `sf_open_virtual` and passing in a pointer to a value that could have varying type, so I have my wrapper defined as:

```julia
function sf_open(io::T, mode, sfinfo) where T <: IO
    virtio = SF_VIRTUAL_IO(T)
    filePtr = ccall((:sf_open_virtual, libsndfile), Ptr{Cvoid},
                    (Ref{SF_VIRTUAL_IO}, Int32, Ref{SF_INFO}, Ref{T}),
                    virtio, mode, sfinfo, io)
    if filePtr == C_NULL
        error("LibSndFile.jl error while opening stream: ", sf_strerror(C_NULL))
    end

    filePtr
end

```

I was under the impression that generally when passing pointers to Julia-owned data you should define the argument type as a `Ref`, and also that when passing the value to `ccall` you can just pass the value and Julia will handle making the `Ref`. As I understand it this makes sure that the value remains GC-rooted for the duration of the `ccall`. This seems to work fine in 0.7 and 1.0, but in 0.6 I get the error:

```julia
error compiling sf_open: ccall: argument type Ref should have an element type, not Ref{T}

```

I can fix it by making the argument type `Ptr{T}` and passing in `pointer_from_objref(io)`, and in this case it shouldn’t matter (clearly `io` needs to stay GC-rooted anyways because it’ll be referenced later from the callback), but I’m wondering why this doesn’t work as expected.