realloc return value

[subramanian]

I have taken the following prototype from K & R.

void *realloc(void *p, size_t size);

Suppose p was earlier allocated by malloc. Suppose I am calling realloc
with larger size value.
If realloc is successful, will the return pointer be the same as p or
will it be different.
K & R 2nd edition says "realloc returns a pointer to the new space".

Why I am asking is that if they are different, the older pointer value
of p should be freed.

Thanks

[jacob navia]

subramanian a écrit :

I have taken the following prototype from K & R.

void *realloc(void *p, size_t size);

Suppose p was earlier allocated by malloc. Suppose I am calling realloc
with larger size value.
If realloc is successful, will the return pointer be the same as p or
will it be different.
K & R 2nd edition says "realloc returns a pointer to the new space".

Why I am asking is that if they are different, the older pointer value
of p should be freed.

Thanks

You do not have to free the old value. realloc will do it for you

[Richard Heathfield]

subramanian said:

I have taken the following prototype from K & R.

void *realloc(void *p, size_t size);

Suppose p was earlier allocated by malloc. Suppose I am calling realloc
with larger size value.
If realloc is successful, will the return pointer be the same as p or
will it be different.

It *may* be different. That depends on whether realloc relocated the block
(and also on whether realloc succeeded). If it did relocate the block, p's
value is now indeterminate and should not be used. And if it failed,
realloc will return NULL. See below.

K & R 2nd edition says "realloc returns a pointer to the new space".

Why I am asking is that if they are different, the older pointer value
of p should be freed.

No, realloc will take care of that for you if it is necessary.

Note that realloc may *fail*, in which case it will return NULL. So don't do
this:

p = realloc(p, new_size); /* BAD DOG! NO BISCUIT! */

Instead, do this:

tmp = realloc(p, new_size);
if(tmp != NULL)
{
p = tmp;
tmp = NULL;
}
else
{
the reallocation failed, but at least p still points to the
old, untouched memory block, so you haven't *lost* any memory
}

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.

[mark_bluemel]

subramanian wrote:

I have taken the following prototype from K & R.

void *realloc(void *p, size_t size);

Suppose p was earlier allocated by malloc. Suppose I am calling realloc
with larger size value.
If realloc is successful, will the return pointer be the same as p or
will it be different.

We don't know, and we don't need to care. If it's not null, the
reallocation worked (and that could conceivably be by growing the
allocated memory in place, AFAIK), the old data has been (if necessary)
copied into the new location. If the old base address and the new base
address are different, the previously allocated space will have been
freed.

K & R 2nd edition says "realloc returns a pointer to the new space".

Why I am asking is that if they are different, the older pointer value
of p should be freed.

But not by you. That's realloc()'s job.

[mark_bluemel]

subramanian wrote:

I have taken the following prototype from K & R.

void *realloc(void *p, size_t size);

Suppose p was earlier allocated by malloc. Suppose I am calling realloc
with larger size value.
If realloc is successful, will the return pointer be the same as p or
will it be different.

I've just checked with a copy of the draft C89 standard and it says
that the return pointer could be either...

[indro]

>Da: jacob navia

>Data: Mer 20 Dic 2006 13:20

....

>Da: Richard Heathfield
Data: Mer 20 Dic 2006 13:29

...
really strange:
jacob posted his message a couple of hours ago, and richard hadn't
"attaked" (harrassed ?) him ...
really strange
ah: got it ! they're disputating in the good ol' "size of pointers"
thread !
i would say: "they're not multithreaded" ... but the standard doesn't
know about threads
;-)

[Richard Heathfield]

indro said:

>>Da: jacob navia
Data: Mer 20 Dic 2006 13:20

...

>>Da: Richard Heathfield
Data: Mer 20 Dic 2006 13:29

..
really strange:
jacob posted his message a couple of hours ago, and richard hadn't
"attaked" (harrassed ?) him ...

If he didn't screw up, why should I reply? What do you want me to say -
"Well done Mr Navia, you finally got something right"? Would not that, too,
be construed as an "attack" by those who think (wrongly) that I've got it
in for him?

And if he did screw up and I didn't reply, well, so what? I'm under no
obligation to correct /all/ the screw-ups posted here.

really strange
ah: got it ! they're disputating in the good ol' "size of pointers"
thread !

What dispute? The Standard is clear on the matter.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.

[Richard Tobin]

In article <11************ **********@a3g2 000cwd.googlegr oups.com>,
<ma**********@p obox.comwrote:

>Suppose p was earlier allocated by malloc. Suppose I am calling realloc
with larger size value.
If realloc is successful, will the return pointer be the same as p or
will it be different.

>We don't know, and we don't need to care.

Of course we have to care: we have to change any variables that
pointed to the old data so they point to the new data. In effect,
you have to assume that it *is* different.

-- Richard
--
"Considerat ion shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.

[mark_bluemel]

Richard Tobin wrote:

In article <11************ **********@a3g2 000cwd.googlegr oups.com>,
<ma**********@p obox.comwrote:

Suppose p was earlier allocated by malloc. Suppose I am calling realloc
with larger size value.
If realloc is successful, will the return pointer be the same as p or
will it be different.

We don't know, and we don't need to care.

Of course we have to care: we have to change any variables that
pointed to the old data so they point to the new data. In effect,
you have to assume that it *is* different.

Fair comment. (This group helps me learn...)

[Default User]

ma**********@po box.com wrote:

>
subramanian wrote:

I have taken the following prototype from K & R.

void *realloc(void *p, size_t size);

Suppose p was earlier allocated by malloc. Suppose I am calling
realloc with larger size value.
If realloc is successful, will the return pointer be the same as p
or will it be different.

I've just checked with a copy of the draft C89 standard and it says
that the return pointer could be either...

Of course. If you think about things in practical fashion (i.e.
implementation details) if it's possible to expand the existing memory
region in place by adjusting a number in a table or something, then it
makes sense to do so. If not, then it pretty much has to be moved.


Brian