Using Ctypes to decompose struct stored in OCaml string?

Is it possible to use Ctypes to decompose the value of a C struct that is stored in an OCaml string? I’m not familiar in detail with Ctypes but the normal use case is that one calls a C function which returns a pointer to a struct which then is made accessible to OCaml. Here the value is already in an OCaml string and I would like to decompose it.