direct memory address to a pointer

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hi i have a question about pointers,
can we give address directly to a pointer?
like this code:

int *p2;
p2=0x22fefc;

Posted 17-Jul-11 22:09pm

Reza Oruji

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Comments

Mohibur Rashid 18-Jul-11 10:25am

If you want to share data between process then this is not the way at all.
Try ICP, client/server solutions

3 solutions

Solution 3

Yes. But, there is a very good chance of a complete failure when you try to use it - it is unlikely that a random address will be within your programs memory space, and the OS will spot the access attempt and kill your app.

Posted 17-Jul-11 22:33pm

OriginalGriff

Comments

Sergey Alexandrovich Kryukov 18-Jul-11 4:45am

You answer is correct for most modern systems, but in fact this is platform-dependent. My 4. Please see my answer.
--SA

hakz.code 18-Jul-11 8:25am

This is what I too think,by the way I dont find a point in accessing direct memory unless we are programming on smaller platforms like 8085 microprocessor.

Sergey Alexandrovich Kryukov 18-Jul-11 10:32am

That's correct, direct memory is typical for smaller platforms.
--SA

Solution 1

yes, that is very much a valid way of assigning an address to a pointer in C.
When writing to the address you'd have to ensure that that your memory is set up
accordingly though... e.g. though a linker directive file.

I would rather do it as follows though:

#define SOME_ADDRESS (0x22fefc)
int *p2 = (int*)SOME_ADDRESS;

Below is an example of a linker directive file for a power pc processor:

MSIL

-sec
{
    .PPC.EMB.sdata0 0xffff8000  ABS :
    .PPC.EMB.sbss0          CLEAR ABS :

    .text 0x10000 :
    .syscall :
    .secinfo :
    .rodata :
    .sdata2 :
    .fixaddr :
    .fixtype :
    .ROM.data ROM(.data) :
    .ROM.sdata ROM(.sdata) :
    .sdabase align(8) :
    .sdata :
    .sbss :
    .data :
    .bss :
    .heap align(16) pad(0x100000) :
    .stack align(16) pad(0x80000) :

    SOME_ADDRESS 0x0022fefc MIN_SIZE(0x4) :
}

Posted 17-Jul-11 22:26pm

R. Erasmus

Updated 18-Jul-11 21:18pm

v6

Comments

R. Erasmus 18-Jul-11 4:34am

Not true, this will work in C, I don't know about C++.

R. Erasmus 18-Jul-11 4:42am

For the noob that voted me a 1... Think its time to maybe look into another field of expertise as this one is clearly not working for you.

hakz.code 18-Jul-11 8:18am

Hi your code flagged a compiler error for me,I think the typecast should be to int* not to int.

R. Erasmus 19-Jul-11 3:19am

Indeed. Well spotted, thx. ;-)

hakz.code 19-Jul-11 6:11am

welcome :-)

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Sergey Alexandrovich Kryukov · Accepted Answer · 2011-07-17T22:44:00

Solution 4

You can do it:

C++

int * p2 = reinterpret_cast<int*>(0x22fefc);

//C-style cast:
int * p1 = (int*)0x22fefc;

//easy to check-up: p1==p2, but de-referencing them could be a problem :-)

The result depends on the platform. On most modern platforms is would looks like some random address; and you will very likely get General Protection Fault exception. In some platforms it may work if the address makes sense. For example, with MS-DOS you could access video card this way or some other physical memory buffer. (However, in real-mode of Intel CPU the address would be the segment:offset pair, so this code won't work anyway.)

—SA

Posted 17-Jul-11 22:44pm

Sergey Alexandrovich Kryukov

Updated 18-Jul-11 4:35am

v2

Comments

Аslam Iqbal 18-Jul-11 5:28am

good ans. my 5

Sergey Alexandrovich Kryukov 18-Jul-11 10:36am

Thank you, Aslam.
--SA

Espen Harlinn 18-Jul-11 8:33am

Nice and simple, my 5

Sergey Alexandrovich Kryukov 18-Jul-11 10:36am

Thank you, Espen.
Updated with C-style cast.
--SA

Espen Harlinn 18-Jul-11 10:42am

Have you taken a look at this article:
http://www.codeproject.com/KB/cross-platform/BenchmarkCppVsDotNet.aspx
I think you'll like it :)

Sergey Alexandrovich Kryukov 18-Jul-11 11:32am

Yes, pretty interesting info; thank you for sharing.
--SA