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C programming : How does free know how much to free?
When we are allocating some memory from the heap using mallolc or calloc or even realloc, we are giving the pointer and the size of the memory which we want to allocate. But while freeing that allocated memory, we are providing only pointer (base address of the allocated memory). So my Question is how the free() function knows how much memory it has to free. means the size of the memory.
One correction here - malloc() wants only the size, and returns the pointer.
As for free(), c-runtime library maintains the list of all blocks that malloc() returned, and when you free(), it searches that list for your pointer, then releases the memory and updates the list.
This list is called a heap.
You cannot free part of the memory which was allocated in a single call.
Each call to alloc() sets aside a block of memory. Memory that was alloc'ed as a block in one call must also be freed as a block in one call. Since there is no way to free part of this memory, there is no need to specify the amount of memory to be freed in the call to free().
The system keeps track of how much memory was allocated for each pointer. When you free the pointer, the system already knows exactly how much memory was allocated, and thus, how much to free.
There are several techniques. One is to store information at addresses before the base address. Another is to maintain a record of allocated addresses and sizes separately.
See the memory management reference for lots of information on memory management and the discussion of free lists etc.
It depends on the specific malloc/free implementation that you are using. Typically there might be some kind of linked list of allocated blocks and another linked list of free blocks. Each block will typically have a header which contains the size of the block and maybe pointers to the next/prev blocks, but there are various other possible implementations which are commonly found.