4 ms·

When you hear talk of "conservative" Garbage Collectors, what the "conservative" refers to is that anything on the stack that looks like a pointer is treated as

When you hear talk of "conservative" Garbage Collectors, what the "conservative" refers to is that anything on the stack that looks like a pointer is treated as if it is a pointer. After all, it doesn't matter what you cast a pointer to, it's all just 1s and 0s on the stack...

dllthomas 13y ago
Of course, you can make something look like not a pointer and then make it a pointer again if you actually change the zeros and ones.
- _delirium 13y ago
  Most languages don't give you any guarantees if you do that, though. In C, for example, pointer arithmetic on a void*, or on a pointer cast to an integer, is not guaranteed to produce sensible results. The standard only provides guarantees for pointer arithmetic if it's performed on a non-void pointer that points within an array, and the results remain within the bounds of the same array (arithmetic that produces a pointer pointing outside the array of the base pointer is undefined, with the exception that pointing to one element past the end of the array is defined).
  - dllthomas 13y ago
    In ordinary C, without a garbage collector, I'd be baffled to find an implementation where one could not: 1) Cast a pointer to an integer of sufficient size. 2) Xor that integer with another value. 3) Xor the result with the same value. 4) Cast back to a pointer. 5) Dereference. It's sufficiently corner-case that I am not certain what the standard says about it. In any event, pointers matter when coding in assembly, where language standards aren't even relevant.
    - aidenn0 13y ago
      That is actually allowed in the standard; it specifically says that casting a pointer to an integral type of sufficient size is a reversible operation (though it I'm not sure if this is not well defined: (int i[2]; (int )((int)i)+sizeof(int)
      - dllthomas 13y ago
        Could you have a standards-conforming implementation where pointers were big-endian and other integral values little-endian? That would break your example code. I don't know of any architecture that would motivate that, of course...
- fhars 13y ago
  You can serialize it to a hex string, put it in a json request that you send off to some cloud service, and then an hour later parse it back out of the json reply, with nothing left in RAM that looks like that pointer for the time inbetween.

dllthomas 13y ago

Of course, you can make something look like not a pointer and then make it a pointer again if you actually change the zeros and ones.

_delirium 13y ago
Most languages don't give you any guarantees if you do that, though. In C, for example, pointer arithmetic on a void*, or on a pointer cast to an integer, is not guaranteed to produce sensible results. The standard only provides guarantees for pointer arithmetic if it's performed on a non-void pointer that points within an array, and the results remain within the bounds of the same array (arithmetic that produces a pointer pointing outside the array of the base pointer is undefined, with the exception that pointing to one element past the end of the array is defined).
- dllthomas 13y ago
  In ordinary C, without a garbage collector, I'd be baffled to find an implementation where one could not: 1) Cast a pointer to an integer of sufficient size. 2) Xor that integer with another value. 3) Xor the result with the same value. 4) Cast back to a pointer. 5) Dereference. It's sufficiently corner-case that I am not certain what the standard says about it. In any event, pointers matter when coding in assembly, where language standards aren't even relevant.
  - aidenn0 13y ago
    That is actually allowed in the standard; it specifically says that casting a pointer to an integral type of sufficient size is a reversible operation (though it I'm not sure if this is not well defined: (int i[2]; (int )((int)i)+sizeof(int)
    - dllthomas 13y ago
      Could you have a standards-conforming implementation where pointers were big-endian and other integral values little-endian? That would break your example code. I don't know of any architecture that would motivate that, of course...
fhars 13y ago
You can serialize it to a hex string, put it in a json request that you send off to some cloud service, and then an hour later parse it back out of the json reply, with nothing left in RAM that looks like that pointer for the time inbetween.