If I have created a C module that presents a handle to the user with a pointer to a forward declared struct, like so:
typedef struct FOO_Obj *FOO_Handle;
If I then declare function prototypes that use it as a const qualified parameter thusly:
void FOO_work(const FOO_Handle fooHandle);
How is the const-ness applied?
const struct FOO_Obj *FOO_Handle // A
struct FOO_Obj *const FOO_Handle // B
const struct FOO_Obj *const FOO_Handle // C
Or is it UB?
B. ( There is no undefined behavior with the code you presented. )
The function call
void FOO_work(const FOO_Handle fooHandle);
is equivalent to
void FOO_work(struct FOO_Obj* const fooHandle);
Variable fooHandle in the function will becode a const pointer to a non-const struct FOO_Obj object. You will not be able to add the const qualifier to fooHandle to make it a pointer to a const object.
Instead, if you want to have a pointer to a const object, and keep the struct hidden, you must make another typedef:
typedef const struct FOO_Obj* FOO_ConstHandle;