How to get the address of next block in memory management?

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I read the code of memory management, and I can't understand the meaning of the following function, could you please tell how does the function work?

// Get the reference of next block's address in current block.
static void*& GetNextBlock(void* p_block)
{
      return *(reinterpret_cast<void**>(p_block));
}

According to the comment upon the code, the function returns the address of p_block's next block, but I think the function returns the address of p_block.

pointers memory c++
1 Answers

The p_block variable is an address of some piece of memory. You cast that to a void** with reinterpret_cast<void**>(p_block), meaning it's now considered the address (A) of an address (B) in memory. Then you dereference that (with *something) to get the address B.

This scheme is often used in memory arenas where a memory block (usually a control segment before the memory addresses returned from malloc) contains the address of the next block (whether allocated or not). For example:

             +----------+    +----------+
firstFree -> | nextFree | -> | nextFree | -> nullptr
             +----------+    +----------+
             | user bit |    | user bit |
             +----------+    +----------+
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