Re: Can I Trust Pointer Arithmetic In Re-Allocated Memory?

On Fri, 18 Aug 2006 23:27:03 UTC, "Bill Reid"
<hormelfree@xxxxxxxxxxxxxxxx> wrote:


Herbert Rosenau <os2guy@xxxxxxxxxxxxx> wrote in message
news:wmzsGguTDN6N-pn2-ysKU5ddsUS8X@xxxxxxxxxxxxxxxxxxxxxxxxx
On Thu, 17 Aug 2006 23:54:41 UTC, "Bill Reid"
<hormelfree@xxxxxxxxxxxxxxxx> wrote:

I guess I got the last two conditions conflated in my mind; it just
seemed
logical to me that if realloc() failed it would free the previous block.
It SEEMS like it should.

But it does not so because you may need to continue your work with the
old block.

Sure, but right off-hand I can't think of single case where I would
actually want to do anything with "half a loaf". When memory
allocation fails in whole or in part, I just want to get out of there
as quickly as possible, maybe just break out of that particular
module operation, quite often just to exit the whole program.

.... and leaving the database in an undefined state. Maybe you needs a
lot of single steps on the incloplete data to do to unwind any action
you've already done with. So the data is needed by your program. Most
programs are more complex in data handling as you currently aware of.

So the best realloc can do for you to flag the error that it is out of
memory and leave data it arrived unchanged. Sometimes you gets a
chance to continue when you free()s some other data area.
Sometimes you'll split the aready occupied block into one or more
smaller ones, write older, smaller blocks out and reuse them.

There are lots of possibilities to continue even without - or with -
asking the user. Exit is seldom the choice in real programs. Often you
will have a solution for "out of memory" on a higher level.

exit() is good for ingenious programs but the real world is more
complex. So realloc gives you the chance for a real cleanup of any
kind you may need. It is simply a bug to overwrite a data area you
have already allocated with a null pointer. You have to cleanup.

I suspect that is true of the vast majority of programs out there.
So not automatically freeing the block seems to be a case of "the
needs of the few outweighing the needs of the many".

You not written a reals program using realloc. You will then quickly
revise your standpoint.

When you have no need for the old block you have to free() that
yourself, else you should free() it. In any case you needs its
address.

In this imperfect world, I guess so...what a friggin' hassle...

As an orthogonal point, what horrible things happen if you try
to free() a NULL pointer?

Nothing. free(NULL); works like a noop. Nothing occures. That is
guaranteed.

So I've kind of been wasting a little time with these types of
generalized pre-exit memory cleanup routines:

void free_time_series_mem(void) {
unsigned ts_idx;

for(ts_idx=0;ts_idx<TS_MAX;ts_idx++) {
if(time_series[ts_idx]!=NULL) {
free(time_series[ts_idx]);
time_series[ts_idx]=NULL;
}
}

num_series=0;
}

Not only don't I need:
if(time_series[ts_idx]!=NULL)

What the hell is the point of:
time_series[ts_idx]=NULL;

That is needed to save yourself from accessing data you've given back
to the system. The only point where you would savely NOT set the
free()'d pointer to NULL is when the pointer and the data it points to
is only alive only inside the function you calls free() AND one of the
next statements is return, In any other case setting it to NULL will
give you a clean "access of pointer to 0 instead of undefined
behavior.


--
Tschau/Bye
Herbert

Visit http://www.ecomstation.de the home of german eComStation
eComStation 1.2 Deutsch ist da!
.

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