Saturday, June 27, 2009

return mem cells to freelist, then fill your pointer's 32 bits with 0

Suppose a 32-bit pointer p2 contains address 0x1234,and 22==sizeof(Derived)

******* q[base p2 = new Derived()] implements --
- grab 22 bytes from freelist
- initialize by ctor
- return 0x1234 i.e. starting address of the 22 bytes
--> from now on, "system" won't give those 22 bytes to other malloc requests, since those are ALLOCATED to us.

****** q(delete p2) performs
- call dtor, possibly virtual
- determine size of the (polymorphic) pointee object, possibly via vtbl. Suppose it's 22.
(The compile-time declared type of p2 doesn't matter. If we did a Base p2 = new Derived(), then deletion will reclaim all the mem cells of Derived.)
- mark that many bytes starting from Address 0x1234
- return these bytes to freelist
--> Any time from now on, those 22 bytes can be allocated to anyone. Note p2 continues to be seated at 0x1234. The deletion performs no state change in the 32-bit object p2. If you read/write/delete on p2, you die.

Rule -- after you call q(delete p2), always fill the 32-bit object p2 with zeros, by calling q(p2 = 0)

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