c - Adding const-ness to opaque handle -


if have created c module presents handle user pointer forward declared struct, so:

typedef struct foo_obj *foo_handle;

if declare function prototypes use const qualified parameter thusly:

void foo_work(const foo_handle foohandle);

how const-ness applied? 

const struct foo_obj *foo_handle // struct foo_obj *const foo_handle  // b const struct foo_obj *const foo_handle  // c

or ub?

b. ( there no undefined behavior code presented. )

the function call

void foo_work(const foo_handle foohandle);

is equivalent 

void foo_work(struct foo_obj* const foohandle);

variable foohandle in function becode const pointer non-const struct foo_obj object. not able add const qualifier foohandle make pointer const object.

instead, if want have pointer const object, , keep struct hidden, must make typedef:

typedef const struct foo_obj* foo_consthandle;

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