Relative pointers in memory mapped file using C

Question

Is it possible to use a structure with a pointer to another structure inside a memory mapped file instead of storing the offset in some integral type and calculate the pointer?

e.g. given following struct:

typedef struct _myStruct_t {
  int number;
  struct _myStruct_t *next;
} myStruct_t;
myStruct_t* first = (myStruct_t*)mapViewHandle;
myStruct_t* next = first->next;

instead of this:

typedef struct _myStruct_t {
  int number;
  int next;
} myStruct_t;
myStruct_t* first = (myStruct_t*)mappedFileHandle;
myStruct_t* next = (myStruct_t*)(mappedFileHandle+first->next);

I read about '__based' keyword, but this is Microsoft specific and therefore Windows-bound.

Looking for something working with GCC compiler.

Answer 1

I'm pretty sure there's nothing akin to the __based pointer from Visual Studio in GCC. The only time I'd seen anything like that built-in was on some pretty odd hardware. The Visual Studio extension provides an address translation layer around all operations involving the pointer.

So it sounds like you're into roll-your-own territory; although I'm willing to be told otherwise.

The last time I was dealing with something like this it was on the palm platform, where, unless you locked down memory, there was the possibility of it being moved around. You got memory handles from allocations and you had to MemHandleLock before you used it, and MemPtrUnlock it after you were finished using it so the block could be moved around by the OS (which seemed to happen on ARM based palm devices).

If you're insistent on storing pointer-esque values in a memory mapped structure the first recommendation would be to store the value in an intptr_t, which is an int size that can contain a pointer value. While your offsets are unlikely to exceed 4GB, it pays to stay safe.

That said, this is probably easy to implement in C++ using a template class, it's just that marking the question as C makes things a lot messier.