Pointer to specific memory address

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  1. #1
    Resident nerd elnerdo's Avatar
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    Pointer to specific memory address

    Is it possible to have a pointer point to a SPECIFIC memory address? I want to be able to edit one certain address, and I'm not sure how to go about doing it.

    I would've assumed that something similar to

    Code:
    int* ptr;
    ptr = 0x83fc22;
    but it turns out that this is untrue.

    (Background info: I'm using devc++, I am a novice to the concept of pointers, but I am not a novice to programming. I generally program in .NET (vb.net and c#) and Java. )

    Do you know how or if this can be done?

    Also, another pointer question, while I'm at it: How can I get something's memory address into an int?

    As an example, let's say that I want to do something like this
    Code:
    int x;
    int y;
    y = &x; //Obviously, this won't work.  But, I want y to equal
    //something like '0x2fecd0'
    y += 5;
    //so that y can NOW equal 0x2fecd5
    cout << "Five more than the memory address of x is  "<< y;
    Last edited by elnerdo; 05-16-2006 at 08:08 PM.
    nerds unite!

    I'm using windows XP.
    I'm using dev-C++ by bloodshed.

  2. #2
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    To make this work you can use a cast
    Code:
    int* ptr;
    ptr = (void *)0x83fc22;
    But this isn't really a good thing to do. You don't know what's at that addres or even if it lies within your address space.

    For your second question you can cast to an int

  3. #3
    and the hat of wrongness Salem's Avatar
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    > Is it possible to have a pointer point to a SPECIFIC memory address?
    Given your current OS/Compiler, no.

    Programs operate in virtual memory, so the only memory you have is given to you by the OS. You can't just invent addresses and hope that something useful will happen.

    If you're writing device drivers, then obviously you need access to physical resources at some point, but that's pretty heavy stuff.
    Likewise, programs like debuggers can inspect and modify the program and data space of the program being debugged, but this is also in "voodoo" territory for most people.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    Resident nerd elnerdo's Avatar
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    Thanks, you two. Quanatum, your solution did the trick. I guess I just wasn't thinking.

    Salem, I'll keep that in mind.
    nerds unite!

    I'm using windows XP.
    I'm using dev-C++ by bloodshed.

  5. #5
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    Quote Originally Posted by elnerdo
    Thanks, you two. Quanatum, your solution did the trick. .
    That solution only allowed it to compile -- doesn't mean it will work at runtime

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    Just throwin' this in, although you say you've already solved your problem. Back in the day when I did graphics programming, I would use this to get a pointer to the 320x200 (8-bit color) vga screen, which is at 0A0000 in 32-bit protected mode:

    Code:
    unsigned char *vga=(unsigned char *)0x0A0000;

  7. #7
    char main() RoshanX's Avatar
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    answer to your second question, since a pointer is an integer, it is possible to do it like this

    Code:
     y=(int)&x;
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    Just Lurking Dave_Sinkula's Avatar
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    Quote Originally Posted by RoshanX
    since a pointer is an integer, it is possible to do it like this
    Bad habit.
    http://c-faq.com/ptrs/int2ptr.html
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    Quote Originally Posted by RoshanX
    answer to your second question, since a pointer is an integer, it is possible to do it like this

    Code:
     y=(int)&x;
    won't work when sizeof(int) != sizeof(int *). So your code snippet is non-standard and probably produce unpredictable results with many compilers.

  10. #10
    and the hat of wrongness Salem's Avatar
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    > since a pointer is an integer,
    Oops - back to school for you.
    The standard doesn't even guarantee that all pointers have the same size, never mind being the same size as an int.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  11. #11
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    it is possible and dangerous that a pointer points a SPECIFIC memory address
    because may be this address is address of your graphic card or sound card etc... so this fact can spoil your pc (%1 chance )

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