Does standard C guarantee same memory content after realloc()?

[Jonas]

Hi,

I'm 99 % sure that Standard C guarantees to do a memory move inside
realloc() in case the new, returned memory block (address) is different than
the original one. Can any C expert confirm this to me, please?

Thanks,
Jonas

PS. I using C90, not C99--if it makes a difference.

[Andreas Kahari]

In article <rJ************ *******@newsb.t elia.net>, Jonas wrote:

Hi,

I'm 99 % sure that Standard C guarantees to do a memory move inside
realloc() in case the new, returned memory block (address) is different than
the original one. Can any C expert confirm this to me, please?

I haven't got the C90 standard in my hand, but SUSv3 (which is a
ISO C superset) says about realloc() that "The contents of the
object shall remain unchanged up to the lesser of the new and
old sizes".

--
Andreas Kähäri

[cody]

> I'm 99 % sure that Standard C guarantees to do a memory move inside

realloc() in case the new, returned memory block (address) is different than the original one. Can any C expert confirm this to me, please?

The memory location will only change if at the current position there is no
room for a continous block with the requested size.

--
cody

[Freeware, Games and Humor]
www.deutronium.de.vu || www.deutronium.tk

[Irrwahn Grausewitz]

Andreas Kahari <ak*******@free shell.org> wrote:

In article <rJ************ *******@newsb.t elia.net>, Jonas wrote:

Hi,

I'm 99 % sure that Standard C guarantees to do a memory move inside
realloc() in case the new, returned memory block (address) is different than
the original one. Can any C expert confirm this to me, please?

I haven't got the C90 standard in my hand, but SUSv3 (which is a
ISO C superset) says about realloc() that "The contents of the
object shall remain unchanged up to the lesser of the new and
old sizes".

ISO/IEC 9899:1999 states:

7.20.3.4 The realloc function
[...]
2 The realloc function deallocates the old object pointed to by ptr
and returns a pointer to a new object that has the size specified
by size. The contents of the new object shall be the same as that
of the old object prior to deallocation, up to the lesser of the
new and old sizes. Any bytes in the new object beyond the size of
the old object have indeterminate values.
A draft copy of ANSI C89 I found reads:

4.10.3.4 The realloc function
[...]
The realloc function changes the size of the object pointed to by
ptr to the size specified by size . The contents of the object shall
be unchanged up to the lesser of the new and old sizes. If the new
size is larger, the value of the newly allocated portion of the object
is indeterminate.

Regards
--
Irrwahn
(ir*******@free net.de)

[Irrwahn Grausewitz]

"cody" <do************ *********@gmx.d e> wrote:

I'm 99 % sure that Standard C guarantees to do a memory move inside
realloc() in case the new, returned memory block (address) is different

than

the original one. Can any C expert confirm this to me, please?

The memory location will only change if at the current position there is no
room for a continous block with the requested size.

Maybe true. Maybe not. (At least) C99 does not require this behaviour,
though many implementations will behave like this. However, how does
your statement address the OPs question?

To OP: see my other reply.

Regards
--
Irrwahn
(ir*******@free net.de)

[Dan Pop]

In <bm************ @ID-176797.news.uni-berlin.de> "cody" <do************ *********@gmx.d e> writes:

I'm 99 % sure that Standard C guarantees to do a memory move inside
realloc() in case the new, returned memory block (address) is differentthan

the original one. Can any C expert confirm this to me, please?

You can be safely 100% sure of that. Under both C standards.
The memory location will only change if at the current position there is no
room for a continous block with the requested size.

1. This statement is false. realloc() is free to change the block's
address even if the old size is equal to the new size. Under NO
circumstance is realloc() constrained to return the old address.

2. This has absolutely NOTHING to do with the OP's question! He was
talking about the *contents* of the reallocated memory block, not
about the circumstances when the memory block will have its address
changed.

So, could you, please, stop posting bullshit to this newsgroup?

The actual C89 specification is:

4.10.3.4 The realloc function

Synopsis

#include <stdlib.h>
void *realloc(void *ptr, size_t size);

Description

The realloc function changes the size of the object pointed to by
ptr to the size specified by size . The contents of the object shall
be unchanged up to the lesser of the new and old sizes. If the new
size is larger, the value of the newly allocated portion of the object
is indeterminate. If ptr is a null pointer, the realloc function
behaves like the malloc function for the specified size. Otherwise,
if ptr does not match a pointer earlier returned by the calloc ,
malloc , or realloc function, or if the space has been deallocated by
a call to the free or realloc function, the behavior is undefined. If
the space cannot be allocated, the object pointed to by ptr is
unchanged. If size is zero and ptr is not a null pointer, the object
it points to is freed.

Returns

The realloc function returns either a null pointer or a pointer to
the possibly moved allocated space.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[cody]

> >> I'm 99 % sure that Standard C guarantees to do a memory move inside

realloc() in case the new, returned memory block (address) is different than the original one. Can any C expert confirm this to me, please?
You can be safely 100% sure of that. Under both C standards.

The memory location will only change if at the current position there is

noroom for a continous block with the requested size.

1. This statement is false. realloc() is free to change the block's
address even if the old size is equal to the new size. Under NO
circumstance is realloc() constrained to return the old address.

2. This has absolutely NOTHING to do with the OP's question! He was
talking about the *contents* of the reallocated memory block, not
about the circumstances when the memory block will have its address
changed.

So, could you, please, stop posting bullshit to this newsgroup?

Instead of giving stupid and rude comments you should read the original
question.
The OP clearly asked wheather the returned memory address will/might/can be
different than the original one.

You are talking bullshit here. Why in the world should realloc cause the
content of the memory block to be changed???

--
cody

[Freeware, Games and Humor]
www.deutronium.de.vu || www.deutronium.tk

[Irrwahn Grausewitz]

PLEASE do not drop references when quoting!

[quoting restored]

"cody" <do************ *********@gmx.d e> wrote:

Dan Pop wrote:

cody wrote:

> Jonas wrote:
>> I'm 99 % sure that Standard C guarantees to do a memory move inside
>> realloc() in case the new, returned memory block (address) is differentthan >> the original one. Can any C expert confirm this to me, please?
You can be safely 100% sure of that. Under both C standards.

>The memory location will only change if at the current position there isno >room for a continous block with the requested size.

1. This statement is false. realloc() is free to change the block's
address even if the old size is equal to the new size. Under NO
circumstance is realloc() constrained to return the old address.

2. This has absolutely NOTHING to do with the OP's question! He was
talking about the *contents* of the reallocated memory block, not
about the circumstances when the memory block will have its address
changed.

So, could you, please, stop posting bullshit to this newsgroup?

Instead of giving stupid and rude comments you should read the original
question.

I never encountered a stupid comment given by Dan Pop. If you dislike
his, um, direct way of commenting, or consider it rude, feel free to
ignore his posts. But be aware you definitely miss something when doing
so.
The OP clearly asked wheather the returned memory address will/might/can be
different than the original one.
Oh yes? Let's review the subject line of this thread:

"Does standard C guarantee same memory content after realloc()?"

Now, which part of "same memory content" didn't you understand?
You are talking bullshit here.
Hmwah. Hmmwaha. Hmwahaaaha-ha-haaa. Well, I know for sure that /if/
somebody is posting bovine excrement here at all, it is you.
Why in the world should realloc cause the
content of the memory block to be changed???

I do not know, but the question was: is it required by the standard
for realloc to retain the contents of the memory (except that truncation
occurs when the new size is less than the previous size)?
[ Answer: definetely yes, in C89/C90 and C99 ]

Now stop trolling and go read a book on C.

Irrwahn
--
ERROR 103: Dead mouse in hard drive.

[Default User]

cody wrote:

Instead of giving stupid and rude comments you should read the original
question.
Stop spewing a bunch of incorrect information then.
The OP clearly asked wheather the returned memory address will/might/can be
different than the original one.

I think you better reread.

"I'm 99 % sure that Standard C guarantees to do a memory move inside
realloc() in case the new, returned memory block (address) is different
than
the original one. Can any C expert confirm this to me, please?"
The OP already acknowledges that the address may change, he wanted to
know if the data went with it.


Brian Rodenborn

[Mark Gordon]

On Fri, 10 Oct 2003 21:35:31 +0200
"cody" <do************ *********@gmx.d e> wrote:

Please don't trim out the attribution lines for bits of the message
still quoted.

> I'm 99 % sure that Standard C guarantees to do a memory move ^^^^^^^^^^^> inside realloc() in case the new, returned memory block
> (address) is different than
> the original one. Can any C expert confirm this to me, please?
You can be safely 100% sure of that. Under both C standards.

The memory location will only change if at the current position
there is noroom for a continous block with the requested size.

1. This statement is false. realloc() is free to change the block's
address even if the old size is equal to the new size. Under NO
circumstance is realloc() constrained to return the old address.

2. This has absolutely NOTHING to do with the OP's question! He was
talking about the *contents* of the reallocated memory block, not
about the circumstances when the memory block will have its
address changed.

So, could you, please, stop posting bullshit to this newsgroup?

Instead of giving stupid and rude comments you should read the
original question.

Dan did both read and answer the OPs question.
The OP clearly asked wheather the returned memory address
will/might/can be different than the original one.
No, the OP asked if it does a memory move *when* the return address is
different. I.e. whether he runs the risk of loosing his data.
You are talking bullshit here. Why in the world should realloc cause
the content of the memory block to be changed???

It is allowed to therefor one should allow for the fact it could happen.
As to why it might happen in a real system, it could do it to
consolidate free space.
--
Mark Gordon
Paid to be a Geek & a Senior Software Developer
Although my email address says spamtrap, it is real and I read it.