checking if pointer is NULL allowed?

[atv]

Hi,
Just to check, if i set a pointer explicitly to NULL, i'm not allowed
to dereference it? Why is that, it's not
like it's pointing to any garbage right? Why else set it to NULL. I can
remember some instances where
i did just that and printf reported something like : (null).

I'm asking because i read somewhere that it is not valid. I thought it
simply wasn't valid to dereference
a pointer who is _not_ set to point to anything.

But i can check if it is NULL or not right ? ( i hope ) :-)

Rgds,
-alef

[tedu]

On Feb 2, 12:10 pm, Eric Sosman <[e-mail]:

tedu wrote On 02/02/07 13:28,:

On Jan 25, 4:03 pm, [e-mail] (Richard Tobin) wrote:

>In article <[e-mail] .com>,

>tedu <[e-mail]:

>>>>It is common practice in many shops I've worked at to have a function or
macro used in debugging routines that can take a pointer and return the
pointer or "null", depending on if the pointer is valid to dereference
or not.

>>>Maybe, but that's not possible in standard C.

>>why not?

>You can tell whether it's null (which I suspect is what was meant) but
you can't check that it's "valid to dereference" - it might be a
free()ed pointer for example.

[little slow noticing the followup.] but one can keep a list of freed
pointers (or conversely, a list of all allocated pointers).

In isgood(p), what if p points to a perfectly valid
object that wasn't obtained from malloc()?
Or, what if p points "into" rather than "at" a block
of allocated memory?

good points. i was focusing too heavily on "can this go to free?"
perhaps. i generally try to avoid too many 'pointers into objects'
running loose, as they frequently end up in unexpected places. so in
that sense, i would count them as bad.

Or, what if p points to a block of memory that has
been freed and then reallocated?

it goes back in the list. true, it won't find stale pointers in that
case.

[tedu]

On Feb 2, 11:16 am, "Harald van Dijk" <[e-mail]:

tedu wrote:

[little slow noticing the followup.] but one can keep a list of freed
pointers (or conversely, a list of all allocated pointers).

In standard C, you're not even allowed to compare a freed pointer for
equality to another pointer.

right. i haven't had too many problems with [violating] that rule,
but it's a rule nonetheless.