question on free() in C language [duplicate]

Question

Possible Duplicate:
How do free and malloc work in C?

How does free know how many bytes of memory to be free'd when called in a program?

Answer 1

This is implementation specific, but when malloc is called, the size of the allocated memory is kept somewhere (usually offset from the pointer itself). When free is called, it will use that stored size.

This is exactly why you should only ever call free on a pointer that was returned by malloc.