How is dynamically allocated memory kept track in C

we allocate memory dynamically in C using malloc() and we receive a pointer to a location in the heap.
now we use free() to deallocate the memory, passing the same pointer value as its argumnet.

the Question now is how does free() know how much to deallocate.. considering the fact that we can always resize the memory block allocated by malloc().

is there anything related to Hash Tables here?

Answer

The original technique was to allocate a slightly larger block and store the size at the beginning, a part the application didn't see. 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 also create 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.