malloc and free

[Ed]

Hi,

When free() is used to free the memory allocated by malloc(), how does
it know the size of the memory to be freed? I guess I'm asking where
does the info stored? Thanks

Ed

[Dave Vandervies]

Re: malloc and free

In article <c959d82c.04101 30818.4980c9f6@ posting.google. com>,
Ed <kurtoo@netscap e.net> wrote:[color=blue]
>Hi,
>
>When free() is used to free the memory allocated by malloc(), how does
>it know the size of the memory to be freed?[/color]

Compiler magic.
(Well, in this case, library magic. But implementation magic in any
case.)
[color=blue]
> I guess I'm asking where
>does the info stored?[/color]

Somewhere you can't get at it. Or possibly it gets reconstructed from
other information and doesn't need to be stored. Or possibly free()
just needs to request that the hardware mark the segment as unallocated
and doesn't need the size at all.

If you need to know the size of a chunk of memory you got from malloc,
you'll need to store the size you asked it for somewhere.


dave

--
Dave Vandervies dj3vande@csclub .uwaterloo.ca

[I]t's less of an ego-boost to be right than it is to be *righter*.
--Micah Cowan in comp.lang.c

[Nils O. Selåsdal]

Re: malloc and free

Ed wrote:[color=blue]
> Hi,
>
> When free() is used to free the memory allocated by malloc(), how does
> it know the size of the memory to be freed? I guess I'm asking where
> does the info stored? Thanks[/color]
Implementation dependant.
It could store it in memory just before the memory given back by malloc,
it could use the address as an index into an array/tree/whatnot, and
many other more or less clever things.

[Default User]

Re: malloc and free

Ed wrote:
[color=blue]
> Hi,
>
> When free() is used to free the memory allocated by malloc(), how does
> it know the size of the memory to be freed?[/color]

Magic.
[color=blue]
> I guess I'm asking where does the info stored?[/color]

Someplace secret. None of your business, seriously. How a particular
implementation does it is highly platform-specific and of little or no
use to the programmer.




Brian Rodenborn

[Dan Pop]

Re: malloc and free

In <I5JACD.IJE@new s.boeing.com> "Default User" <first.last@boe ing.com.invalid > writes:
[color=blue]
>Ed wrote:
>[color=green]
>> When free() is used to free the memory allocated by malloc(), how does
>> it know the size of the memory to be freed?[/color]
>
>Magic.[/color]

It derives it, one way or another, from the address of the memory block.
See chapter 8 from K&R2 for one possible answer.
[color=blue][color=green]
>> I guess I'm asking where does the info stored?[/color]
>
>Someplace secret. None of your business, seriously. How a particular
>implementati on does it is highly platform-specific and of little or no
>use to the programmer.[/color]

In an ideal world, where programmers didn't make any mistakes, maybe.
In the real world, this kind of information helps understanding the
(mis)behaviour of a buggy program, e.g. why a free() call generates a
segfault or why some implementations complain about a corrupted malloc
arena.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Dan.Pop@ifh.de
Currently looking for a job in the European Union

[Method Man]

Re: malloc and free

> > I guess I'm asking where does the info stored?[color=blue]
>
> Someplace secret. None of your business, seriously.[/color]

That may be true.
[color=blue]
> How a particular
> implementation does it is highly platform-specific and of little or no
> use to the programmer.
>[/color]

It can be educational to know how a particual implementation of malloc is
done. Perhaps the OP wishes to write his own memory manager and is gathering
information. In any case, there are definitely purposes and uses for seeking
such knowledge.

[E. Robert Tisdale]

Re: malloc and free

Ed wrote:
[color=blue]
> When free() is used to free the memory allocated by malloc(),
> how does it know the size of the memory to be freed?[/color]

Obviously, malloc(size_t) stores the size somewhere
where free(void*) can find it.
[color=blue]
> I guess I'm asking where does the info stored?[/color]

Typically, information about the amount of storage allocated
is stored immediately before the allocated storage.
Try this:
[color=blue]
> cat main.c[/color]
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char* argv[]) {
if (1 < argc) {
const
size_t n = atoi(argv[1]);
size_t* p = (size_t*)malloc (n*sizeof(size_ t));
fprintf(stdout, "p[-2] = %u\n", p[-2]);
fprintf(stdout, "p[-1] = %u\n", p[-1]);
}
return EXIT_SUCCESS;
}
[color=blue]
> gcc -Wall -std=c99 -pedantic -o main main.c
> ./main 1[/color]
p[-2] = 0
p[-1] = 17

On my computer, malloc allocates storage
in "double word" aligned increments.
It always allocates one double word to store the size
and at least one other double word for storage.

[Nils O. Selåsdal]

Re: malloc and free

E. Robert Tisdale wrote:[color=blue]
> Ed wrote:
>[color=green]
>> When free() is used to free the memory allocated by malloc(),
>> how does it know the size of the memory to be freed?[/color]
>
>
> Obviously, malloc(size_t) stores the size somewhere
> where free(void*) can find it.
>[color=green]
>> I guess I'm asking where does the info stored?[/color]
>
>
> Typically, information about the amount of storage allocated
> is stored immediately before the allocated storage.
> Try this:[/color]
....
[color=blue]
> On my computer, malloc allocates storage
> in "double word" aligned increments.
> It always allocates one double word to store the size
> and at least one other double word for storage.[/color]
fyi;
bash-2.05b$ ./main 1
p[-2] = 4294967295
p[-1] = 4294967295

bash-2.05b$ ./main 5
p[-2] = 0
p[-1] = 0

:-)

[E. Robert Tisdale]

Re: malloc and free

Nils O. Selåsdal wrote:
[color=blue]
> E. Robert Tisdale wrote:
>[color=green]
>> Ed wrote:
>>[color=darkred]
>>> When free() is used to free the memory allocated by malloc(),
>>> how does it know the size of the memory to be freed?[/color]
>>
>> Obviously, malloc(size_t) stores the size somewhere
>> where free(void*) can find it.
>>[color=darkred]
>>> I guess I'm asking where does the info stored?[/color]
>>
>> Typically, information about the amount of storage allocated
>> is stored immediately before the allocated storage.
>> Try this:[/color]
>
> ...
>[color=green]
>> On my computer, malloc allocates storage
>> in "double word" aligned increments.
>> It always allocates one double word to store the size
>> and at least one other double word for storage.[/color]
>
> fyi;
> bash-2.05b$ ./main 1
> p[-2] = 4294967295
> p[-1] = 4294967295
>
> bash-2.05b$ ./main 5
> p[-2] = 0
> p[-1] = 0
>
> :-)[/color]

That' not a lot of information.
Would you care to elaborate a little?

[Default User]

Re: malloc and free

Dan Pop wrote:
[color=blue]
> In <I5JACD.IJE@new s.boeing.com> "Default User"
> <first.last@boe ing.com.invalid > writes:[/color]
[color=blue][color=green]
> > Someplace secret. None of your business, seriously. How a particular
> > implementation does it is highly platform-specific and of little or
> > no use to the programmer.[/color]
>
> In an ideal world, where programmers didn't make any mistakes, maybe.
> In the real world, this kind of information helps understanding the
> (mis)behaviour of a buggy program, e.g. why a free() call generates a
> segfault or why some implementations complain about a corrupted malloc
> arena.[/color]

Normally this question is a prelude to accessing this information for
the wrong purposes, like figuring out the size of dynamically-allocated
arrays or such.




Brian Rodenborn

[Nils O. Selåsdal]

Re: malloc and free

[color=blue][color=green]
>> fyi;
>> bash-2.05b$ ./main 1
>> p[-2] = 4294967295
>> p[-1] = 4294967295
>>
>> bash-2.05b$ ./main 5
>> p[-2] = 0
>> p[-1] = 0
>>
>> :-)[/color]
>
>
> That' not a lot of information.
> Would you care to elaborate a little?[/color]
Just seems that my computer behaves diffrently than yours ;)
(Running NetBSD on x86 btw)

[Flash Gordon]

Re: malloc and free

On Wed, 13 Oct 2004 12:24:23 -0700
"E. Robert Tisdale" <E.Robert.Tisda le@jpl.nasa.gov > wrote:
[color=blue]
> Nils O. Selåsdal wrote:
> [color=green]
> > E. Robert Tisdale wrote:
> > [color=darkred]
> >> Ed wrote:
> >>
> >>> When free() is used to free the memory allocated by malloc(),
> >>> how does it know the size of the memory to be freed?
> >>
> >> Obviously, malloc(size_t) stores the size somewhere
> >> where free(void*) can find it.
> >>
> >>> I guess I'm asking where does the info stored?
> >>
> >> Typically, information about the amount of storage allocated
> >> is stored immediately before the allocated storage.
> >> Try this:[/color]
> >
> > ...
> > [color=darkred]
> >> On my computer, malloc allocates storage
> >> in "double word" aligned increments.
> >> It always allocates one double word to store the size
> >> and at least one other double word for storage.[/color]
> >
> > fyi;
> > bash-2.05b$ ./main 1
> > p[-2] = 4294967295
> > p[-1] = 4294967295
> >
> > bash-2.05b$ ./main 5
> > p[-2] = 0
> > p[-1] = 0
> >
> > :-)[/color]
>
> That' not a lot of information.
> Would you care to elaborate a little?[/color]

It shows that your suggesting of what is typically done does not apply
to Nils system, thus demonstrating that you are talking non-portable
rubbish again. I would have said that was completely obvious to someone
with two brain cells to run together.
--
Flash Gordon
Sometimes I think shooting would be far too good for some people.
Although my email address says spam, it is real and I read it.

[E. Robert Tisdale]

Re: malloc and free

Nils O. Selåsdal wrote:[color=blue]
>[color=green][color=darkred]
>>> fyi;
>>> bash-2.05b$ ./main 1
>>> p[-2] = 4294967295
>>> p[-1] = 4294967295
>>>
>>> bash-2.05b$ ./main 5
>>> p[-2] = 0
>>> p[-1] = 0
>>>
>>> :-)[/color]
>>
>>
>>
>> That' not a lot of information.
>> Would you care to elaborate a little?[/color]
>
> Just seems that my computer behaves diffrently than yours ;)
> (Running NetBSD on x86 btw)[/color]

Which compiler?

[Nils O. Selåsdal]

Re: malloc and free

E. Robert Tisdale wrote:[color=blue]
> Nils O. Selåsdal wrote:
>[color=green]
>>[color=darkred]
>>>> fyi;
>>>> bash-2.05b$ ./main 1
>>>> p[-2] = 4294967295
>>>> p[-1] = 4294967295
>>>>
>>>> bash-2.05b$ ./main 5
>>>> p[-2] = 0
>>>> p[-1] = 0
>>>>
>>>> :-)
>>>
>>>
>>>
>>>
>>> That' not a lot of information.
>>> Would you care to elaborate a little?[/color]
>>
>>
>> Just seems that my computer behaves diffrently than yours ;)
>> (Running NetBSD on x86 btw)[/color]
>
>
> Which compiler?[/color]

bash-2.05b$ gcc -v
Using built-in specs.
Configured with:
/home/nick/work/netbsd/src/tools/gcc/../../gnu/dist/gcc/configure
--enable-long-long --disable-multilib --enable-threads --disable-symvers
--build=i386-unknown-netbsdelf --host=i386--netbsdelf
--target=i386--netbsdelf
Thread model: posix
gcc version 3.3.3 (NetBSD nb3 20040520)

[E. Robert Tisdale]

Re: malloc and free

Nils O. Selåsdal wrote:[color=blue]
> E. Robert Tisdale wrote:
>[color=green]
>> Nils O. Selåsdal wrote:
>>[color=darkred]
>>>
>>>>> fyi;
>>>>> bash-2.05b$ ./main 1
>>>>> p[-2] = 4294967295
>>>>> p[-1] = 4294967295
>>>>>
>>>>> bash-2.05b$ ./main 5
>>>>> p[-2] = 0
>>>>> p[-1] = 0
>>>>>
>>>>> :-)
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> That' not a lot of information.
>>>> Would you care to elaborate a little?
>>>
>>>
>>>
>>> Just seems that my computer behaves diffrently than yours ;)
>>> (Running NetBSD on x86 btw)[/color]
>>
>>
>>
>> Which compiler?[/color]
>
>
> bash-2.05b$ gcc -v
> Using built-in specs.
> Configured with:
> /home/nick/work/netbsd/src/tools/gcc/../../gnu/dist/gcc/configure
> --enable-long-long --disable-multilib --enable-threads --disable-symvers
> --build=i386-unknown-netbsdelf --host=i386--netbsdelf
> --target=i386--netbsdelf
> Thread model: posix
> gcc version 3.3.3 (NetBSD nb3 20040520)[/color]
[color=blue]
> gcc -v[/color]
Reading specs from /usr/local/lib/gcc/i686-pc-linux-gnu/3.4.1/specs
Configured with: ../gcc-3.4.1.src/configure
Thread model: posix
gcc version 3.4.1