Re: Allocate a variable in a known physical location

[isuru herath <isuru81 at yahoo dot com>]

Hi Ian,

Thanks a lot for your quick response and I am sorry for not explaining the 
problem correctly. 

I have a separate piece of memory for which I have given physical address 
range 0x10001000 to 0x10101000. I want to allocate variables in this 
address range. To achieve this I create a structure with variables I need 
to allocate there. For example if I need to allocate i and j in the above 
address range, I define a structure like following.

struct my
{
     int i;
     int j;
};

and then allocate memory for the structure using mmap like below.(bear with 
me if syntax are wrong).

struct my *p = mmap(........);

when ever I need to access i, j in my code I access them via pointer p like 
following.

p->i or p->j

All what I need is to allocate i and j in the above address range. Due to 
lack of my  knowledge in compiler and gcc this is how I did it. The 
drawback of this is that to access i, it has to access p first. This 
introduces an unnecessary access to my statistics. Therefore if I could 
allocate i and j without using the above method I thought my problem will 
be solved. 

As you mentioned in your reply can I use section attribute to achieve this or do you have any other suggestion. 

Any help/advice is greatly appreciated.

regards,
Isuru

--- On Mon, 1/31/11, Ian Lance Taylor <iant@google.com> wrote:

> From: Ian Lance Taylor <iant@google.com>
> Subject: Re: Allocate a variable in a known physical location
> To: "isuru herath" <isuru81@yahoo.com>
> Cc: gcc-help@gcc.gnu.org
> Date: Monday, January 31, 2011, 11:21 AM
> isuru herath <isuru81@yahoo.com>
> writes:
> 
> > I need to allocate a variable in a known physical
> location, let's say I need 
> > to allocate void *p in location 0x10001000.? I
> was using mmap to to do this, 
> > but in that manner I can only allocate p[0],
> p[1]...p[n] in that physical 
> > address range. Therefore when I access p[i], accesses
> to p results in 
> > outside {0x10001000,? 0x10001000+offset} and p[i]
> results as an access in 
> > the range I am interested in.
> 
> I don't understand the last sentence there.
> 
> > I was wondering is there a was for me to force 
> > to allocate variable p in that address range or I am
> looking for something 
> > totally unrealistic. Because of the nature of my
> research I can use any 
> > optimization(-O2, O3).
> 
> If you don't want to use mmap, the simplest way to put a
> variable at a
> specific location is to put it in a specific section using
> __attribute__
> ((section ("..."))) and then put that section at a specific
> address
> using a linker script.
> 
> Ian
>