Proper way to ccall mutable types

General Usage
1

Suppose I have a type

mutable struct MyCType
  a::Cint
  b::Ptr{Cdouble}
end

corresponding exactly to a struct in C. Then I call a C library which has functions like

void modify_struct(myctype *a)

which mutates a. I want to call these functions from Julia and need to know which ::T to specify it as in the @ccall.

The manual says:

T* (where T represents an appropriately defined type) … Ref{T}` (T may be safely mutated only if T is an isbits type)|

Which, because MyCType is not an isbits type and I want to use the C code to mutate it, makes it sounds like I cannot use

typeof(a) == MyCType # true
@ccall modify_struct(a::Ref{MyCType})::Cvoid

However this seems to work completely fine. What am I missing?