intrenal process of malloc and free?

Hi,
What i want to know is that malloc returns a pointer, then how come that pointer knows the size of allocated memory? and so with free it dellocates that amount of memory only.
where is the total size of memory allocated stored and how that pointer(returned by malloc) takes care of that.

May 9 '07 #1

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DeMan

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1GB

I'm not ceratin, but I think (basically) a simple implementation is a linked list.

To begin with you have 1 node (to the beginning of free memory)
Each node can be in two states, free or used.
when you allocate memory, you add a single node as many memory locations further as you need, and mark the start node used.
You now have 2 Nodes, one marking the start of allocated memory, one marking the start of free memory.
Repeating this process for several different mallocs gives you x nodes to the start of different mallocs, and 1 to the beginning of free memory.

When you free memory, you do one of two things:
- if the Node marking the end of the memory you want to free is itself free, you remove it, and mark the start node as free (linking it to the end of the free node)
- if the Node marking the end of the memory you want to free is used, you mark the current node as free

When you want to find free memory, you iterate through the linked list looking for a Node that is marked free and has enough memory before the next Node

May 9 '07 #2

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