[Solved] Does ccall really convert arguments passed by pointer?

Question

This turned out to be either a bug in the core library or a very misguided documentation, depending on the perspective (issue #29850). The behavior of the function unsafe_convert changed from version 0.4 to 0.5, in a way that makes it more flexible than what is currently suggested.

According to this commit, unsafe_convert changed from this:

unsafe_convert(::Type{Ptr{Void}}, a::Array) = ccall(:jl_array_ptr, Ptr{Void}, (Any,), a)

To this:

unsafe_convert{S,T}(::Type{Ptr{S}}, a::AbstractArray{T}) = convert(Ptr{S}, unsafe_convert(Ptr{T}, a))

For arrays, this relaxed implementation will enable a transformation from an array of T to a pointer of another type S. In practice, unsafe_convert(cconvert(array)) will reinterpret-cast the array’s base pointer, as in the C++ nomenclature. We are left with a dangerously reinterpreted array across the FFI boundary.

The key takeaway is that one needs to take extra care when passing arrays to C functions, as the element type of an array in a C-call function parameter is not statically enforced. Use type signatures and/or explicit conversions where applicable.

Accepted Answer

This turned out to be either a bug in the core library or a very misguided documentation, depending on the perspective (issue #29850). The behavior of the function unsafe_convert changed from version 0.4 to 0.5, in a way that makes it more flexible than what is currently suggested.

According to this commit, unsafe_convert changed from this:

unsafe_convert(::Type{Ptr{Void}}, a::Array) = ccall(:jl_array_ptr, Ptr{Void}, (Any,), a)

To this:

unsafe_convert{S,T}(::Type{Ptr{S}}, a::AbstractArray{T}) = convert(Ptr{S}, unsafe_convert(Ptr{T}, a))

For arrays, this relaxed implementation will enable a transformation from an array of T to a pointer of another type S. In practice, unsafe_convert(cconvert(array)) will reinterpret-cast the array’s base pointer, as in the C++ nomenclature. We are left with a dangerously reinterpreted array across the FFI boundary.

The key takeaway is that one needs to take extra care when passing arrays to C functions, as the element type of an array in a C-call function parameter is not statically enforced. Use type signatures and/or explicit conversions where applicable.