Some libraries define types like this:

typedef struct TF_Tensor TF_Tensor;

Where the thing being typedef'ed is undefined (this is possible since the library only uses opaque pointers to the undefined thing).

In these cases, jextract doesn't emit any layout for TF_Tensor, which is understandable, but then leaves the user no option than to use a very general layout such as C_POINTER to workaround the issue, which makes the code harder to follow.