Re: Malloc/Free - freeing memory allocated by malloc

From: Richard Bos (rlb_at_hoekstra-uitgeverij.nl)
Date: 10/22/04 

Date: Fri, 22 Oct 2004 07:45:14 GMT

richard@cogsci.ed.ac.uk (Richard Tobin) wrote:

> In article <41767a4c.179165393@news.individual.net>,
> Richard Bos <rlb@hoekstra-uitgeverij.nl> wrote:
>
> >> but dynamic alloc memory is not dealloc by OS at application teminated. why?
> >
> >Says who? On most OSes it is, and I'd call any modern OS on which it
> >isn't broken.
>
> A sensible, practical view.
>
> >and dereferencing a
> >null pointer causes just as undefined behaviour as dereferencing an
> >invalid pointer.
>
> A limited, theoretical view.
>
> A modern, general-purpose operating system that doesn't make
            ^^^^^^^^^^^^^^^
That is an extra requirement. Not everybody programs on Unix or MacOS.

> dereferencing a null pointer produce an immediate exception (at least
> by default) has a similar kind of "brokenness" to one which does not
> deallocate memory when a process terminates.

The real problem is not whether or not null pointer writes cause a
crash; the problem is that _relying_ on this behaviour, even on systems
where it does happen, is almost guaranteed to cause you to let down your
guard, and write through a copy of a free()d pointer which you forgot to
set to null when you free()d the original.

Richard

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