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by simcop2387
2806 days ago
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You can't use memcpy here, you have to modify the original memory to mark it as freed. You also can't allocate a new buffer to do the copy into because you are the allocator. The C standard specifically states that a cast (T ) to and from (void ) is validly defined behavior and that you must get the original pointer back. It's also valid to go from (T ) to (U ) and back again if and only if T and U have the same alignment requirements. (void ) is required to have no alignment requirements because you can't directly de-reference it since the result would have type (void). __attribute__((__packed__)) is a completely different topic about how the layout of the struct gets decided and what padding might be used. What's going on here is that malloc() gives the caller a (void ) that points to one position past a (struct header_t ) and then free is casting it back from (void ) to (struct header_t ) and then going back one element in the set to get the original header with the meta-data about the allocation, this is perfectly fine because the pointer is never anything other than (void ) or (struct header_t ) as far as the language is concerned. The caller might turn the element after the (struct header_t ) into something else but the original (struct header_t *) is always the same. |
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You can use the stack in this case.