How does free know how much to free?

Question

In C programming, you can pass any kind of pointer you like as an argument to free, how does it know the size of the allocated memory to free? Whenever I pass a pointer to s

Answer 1

The original technique was to allocate a slightly larger block and store the size at the beginning, then give the application the rest of the blog. The extra space holds a size and possibly links to thread the free blocks together for reuse.

There are certain issues with those tricks, however, such as poor cache and memory management behavior. Using memory right in the block tends to page things in unnecessarily and it also creates dirty pages which complicate sharing and copy-on-write.

So a more advanced technique is to keep a separate directory. Exotic approaches have also been developed where areas of memory use the same power-of-two sizes.

In general, the answer is: a separate data structure is allocated to keep state.