A quick test I created to interact with libgit2.

git2types.jl

type GitAtomic
  val::Clong
  function GitAtomic()
    new(zero(Clong))
  end
end
 
type GitRefcount
  refcount_val::Clong
  owner::Ptr{Void}
  function GitRefcount()
    new(zero(Clong), C_NULL)
  end
end
 
type GitVectorCmp
  a::Ptr{Void}
  b::Ptr{Void}
  function GitVectorCmp()
    new(C_NULL, C_NULL)
  end
end
 
type GitVector
  _alloc_size::Csize_t
  _cmp_a::Ptr{Void}
  _cmp_b::Ptr{Void}
  contents::Ptr{Ptr{Void}}
  length::Csize_t
  sorted::Cint
  function GitVector()
    new(zero(Csize_t), C_NULL, C_NULL, C_NULL, zero(Csize_t), zero(Cint))
  end
end
 
type GitConfig
  rc_recfcount_val::Clong
  rc_owned::Ptr{Void}
  files_alloc_size::Csize_t
  files_cmp_a::Ptr{Void}
  files_cmp_b::Ptr{Void}
  files_contents::Ptr{Ptr{Void}}
  files_length::Csize_t
  files_sorted::Cint
  function GitConfig()
    new(zero(Clong), C_NULL, zero(Csize_t), C_NULL, C_NULL, C_NULL, zero(Csize_t), zero(Cint))
  end
end

repl_output.txt

julia> include("git2types.jl")

julia> config = GitConfig()
GitConfig(0,Ptr{Void} @0x0000000000000000,0x0000000000000000,Ptr{Void} @0x0000000000000000,Ptr{Void} @0x0000000000000000,Ptr{Ptr{None}} @0x0000000000000000,0x0000000000000000,0)

julia> err = ccall((:git_config_new, :libgit2), Cint, (Ptr{GitConfig},), &config)
0