No-copy type annotation

Question

In C, if a structure contains a futex or for whatever reason doesn't make sense to copy or move to a new address, is there any way (type annotation or something) to restrict/warn users from accidentally making copies of those objects?

Answer 1

No, there are not. Looks you picked the wrong language, try Rust, Go, Java, Python etc. Though I guess some (or all) of them doesn't support this feature too but it's the right direction.

As the comment points out, if it is not desirable to let user copy the structure, just always use pointers.

If it would be problem giving the structure intels to the user, it is possible to substitute the structure members with same-size nonsenses in the header file for the user, and reserve the real header file yourself, like how WinAPI does. However, this would make everything complicated since the type size, structure alignment etc. are different sometimes in different architectures.

Answer 2

You do could do something similar with opaque type and private encapsulation.

something.h

typedef struct something something; // forward declaration, incomplete/opaque type

something* something_create  (void);
void       something_dostuff (something* obj);

something.c

#include "something.h"

struct something { ... }; // actual definition, only visible privately in this file

something* something_create  (void)
{
  something* result = malloc( sizeof *result );
  ...
  return result;
}

void something_dostuff (something* obj)
{
  /* do stuff on the object */
}

caller.c

#include "something.h"

something* st = something_create();
something_dostuff(st);

Now of course this doesn't stop the caller from doing dirty hacks on purpose such as wild memcpy on the pointed-at data. But is stops a sane application programmer from doing things by mistake, which ought to be the aim of this.