malloc

malloc

[Earl R. Lapus]

hi,
   I tried doing this:
       ucp = (unsigned char *) malloc(0);
   it was successful and returned a valid address. I was kinda hoping
   it would do something weird...When I looked at the man page It said 
   something like.. "If your application needs to generate empty objects, 
   you may use malloc(0) for this purpose..."

   So my questions are:
       What do you mean when you say EMPTY OBJECT?
       Does it ocuppy any memory storage?

=========================
 ...what i want and what i need
 IS and WILL always be FREE...
                    -brandon boyd-
=========================

Re: malloc

[Glynn Clements]

Earl R. Lapus wrote:

>    I tried doing this:
>        ucp = (unsigned char *) malloc(0);
>    it was successful and returned a valid address. I was kinda hoping
>    it would do something weird...When I looked at the man page It said 
>    something like.. "If your application needs to generate empty objects, 
>    you may use malloc(0) for this purpose..."
> 
>    So my questions are:
>        What do you mean when you say EMPTY OBJECT?

A block of memory which is zero bytes long. You can't store anything
in the block, but the returned pointer is guaranteed to be unique, in
that it differs from any other valid pointer.

>        Does it ocuppy any memory storage?

Every block of memory returned from malloc() has a header, which
occupies memory.

-- 
Glynn Clements <glynn.clements@virgin.net>

Re: malloc

[Jason P. Winters]

 
> hi,
>    I tried doing this:
>        ucp = (unsigned char *) malloc(0);

Most malloc() calls have been "fixed" to return at least one byte of
useable memory when passed a 0, as too many people were doing that by
accident and crashing.  (It might be more, like sizeof(int), but I'm
not sure).

   After all, why would you call malloc to get nothing? So it assumes
you want *something* and returns it.

Ciao!

Jason
-- 
|UUCP  jason@txt.com                        Who wills, Can.
|VOICE (408) 243-3425                         Who tries, Does.
|LOCAL Hey, Jason!                              Who loves, Lives.          o_.
|Disclaimer:  Not me! I didn't do *THAT!*                                 <|
|Local Disclaimer:  I'm not here!                   A. McCaffrey           4

Re: malloc

[Mehran Rezaei]

> hi,
>    I tried doing this:
>        ucp = (unsigned char *) malloc(0);
>    it was successful and returned a valid address. I was kinda hoping
>    it would do something weird...When I looked at the man page It said
>    something like.. "If your application needs to generate empty objects,
>    you may use malloc(0) for this purpose..."
>
>    So my questions are:
>        What do you mean when you say EMPTY OBJECT?
>        Does it ocuppy any memory storage?
Hi there,
Of course it depends on the implementation. However, since Doug Lea'
allocator is the base and everybody else follows his setup here I am writing
about his allocator:
malloc(size_t n)
if "n" is zero, malloc returns a minimum-sized chunk (the minimum size is 16
bytes on most 32 bit machines, and 24 or 32 bytes on 64 bit machines).
From 16 bytes or 32 bytes returned, you do not see the header (2*address
size of machine - 64 bits in case of 32 bit machine). It leaves you 8 bytes
to use.
I believe gnu uses Doug Lea' allocator.
So long,

Mehran

malloc

[ratheesh k]

Is the below statement right or wrong ?

malloc tries to allocate continous memmory space in virtual address
space.  not in physical address space.

Re: malloc

[Santosh Sivaraj]

On 12/20/2010 10:09 AM, ratheesh k wrote:
> Is the below statement right or wrong ?
>
> malloc tries to allocate continous memmory space in virtual address
> space.  not in physical address space.
It is right, you could have googled it.

Re: malloc

[Uriel Corfa]

On Mon, Dec 20, 2010 at 5:39 AM, ratheesh k <ratheesh.ksz@gmail.com> wrote:
> Is the below statement right or wrong ?
>
> malloc tries to allocate continous memmory space in virtual address
> space.  not in physical address space.

Hi,

This is true, but in theory, malloc doesn't have a notion of a virtual
or physical address space. Malloc is defined as : it tries to allocate
a contiguous memory chunk of at least the required size in a
system-defined pool. The question of setting up virtual address
spaces, of lazy allocation of physical pages, etc. is left up to the
underlying OS. In other terms : it's mmap(2) and brk(2) that take care
of this. Malloc relies on them for this kind of questions.

Regards,

-- 
Uriel Corfa
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