how free() method know where to stop deallocated memory

[YyYo]

Hi All...

I have a question on free() method.
How free() method know until what address to stop freeing memory ?
Let says I allocated 4 bytes of memory using void: *p = malloc(4)
If I will free(p), the allocated memory will be free again.
My question is how free() method know to stop deallocating memory until 4 bytes?
is there some kind of NULL so free() method stop when occur it?

Best Regards
YyYo

[Bazzy]

malloc may store somewhere the information of how much memory was allocated
in "The C programming language" there's an example implementation of malloc and free that explains this

[jsmith]

Typical memory management allocators will allocate slightly more memory than you request, then store
information about the allocation (such as the size) in the first few bytes and return you a pointer just past
that data. When you free the pointer, it looks in the few bytes before the pointer to get the size of the buffer.

[kbw]

... which is why buffer overruns are catastrophic.