Re: Deallocating an object twice

[Peter Chapin <PChapin@vtc.vsc.edu>]

On 2014-07-27 07:30, anon@att.net wrote:

> For most OS, deallocation of memory happens only if the program 
> exits and returns it's allocated resources back to the OS. And 
> for speed the underlying C library routine "free" does not truly
> deallocate or mark the memory as unused.

C's free() marks the memory as unused in the sense that a future call to
malloc() might use memory that was previously freed. If that were not
the case it wouldn't be possible, even in principle, to write a C
program that didn't leak memory.

What you're talking about is happening at a lower level. It may be the
case that once a block of memory is requested from the OS, the C library
never releases it. As you say, the OS reclaims the memory when the
process terminates. However, the C library will still distinguish
between used and unused memory within the allocation it has.

A program that loops forever repeatedly allocating and freeing memory
for a single integer, say, could run infinitely without ever exhausting
storage or requesting additional memory from the OS. It will simply
reuse the space it already has.

Peter