Doubt regarding pointer

This is a discussion on Doubt regarding pointer within the C Programming forums, part of the General Programming Boards category; In the code below Code: int main() { char *p; p = 12; *p = 20; } I want a ...

  1. #1
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    Doubt regarding pointer

    In the code below

    Code:
    int main()
    {
         char *p;
         p = 12;
         *p = 20;
    }
    I want a clarification regarding pointer p that in the second stmt, p is getting an address 12 and in 3rd stmt we are assigning the value 20 at the address 12...right?

    If not please give the justifiction in detail....

  2. #2
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    Seems right, I think..

  3. #3
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    Yep. Addresses are typically given in hex, so the location is actually 0x0000000C.
    It is too clear and so it is hard to see.
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  4. #4
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    Quote Originally Posted by karthik537 View Post
    In the code below

    Code:
    int main()
    {
         char *p;
         p = 12;
         *p = 20;
    }
    I want a clarification regarding pointer p that in the second stmt, p is getting an address 12 and in 3rd stmt we are assigning the value 20 at the address 12...right?

    If not please give the justifiction in detail....

    Correct.

    The posted code, however, will give warnings in most compilers, as the compiler thinks you are doing something "strange" if you assign an integer constant to a pointer - a cast will remove the warning.

    It will also, almost certainly, cause something to go wrong in your system - either it will change memory that you shouldn't change, or it will crash, because address 12 is not at all available to you.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  5. #5
    DESTINY BEN10's Avatar
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    Addresses are typically given in hex
    can u please explain

  6. #6
    spurious conceit MK27's Avatar
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    Quote Originally Posted by matsp View Post
    It will also, almost certainly, cause something to go wrong in your system - either it will change memory that you shouldn't change, or it will crash, because address 12 is not at all available to you.
    It's my computer, I can do what I want with it!!

    can u please explain
    BEN10: you do not have to (and usually should never try to) assign pointer addresses unless it's an address that you know has already been assigned to a pointer.
    Code:
    char *ptr, *p2;
    p2=ptr;  //okay
    ptr=12; //not okay
    If you want to see the address assigned by the compiler (or I guess actually the linker?):
    Code:
    printf("%p",ptr);
    Notice, as matsp sayeth, this is the hexadecimal address of a location in the physical memory of the computer.
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    cpwiki -- our wiki on sourceforge

  7. #7
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    Quote Originally Posted by MK27 View Post
    Notice, as matsp sayeth, this is the hexadecimal address of a location in the physical memory of the computer.
    Pedantically, it's the virtual address - if you do not deal with kernel code, you never see the actual physical location for anything [assuming we're talking 32-bit OS's on x86, that is].

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  8. #8
    C++ Witch laserlight's Avatar
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    Strictly speaking, the %p format specifier is only used to print a pointer to void, so it should be:
    Code:
    printf("%p", (void*)ptr);
    Also, it is not guaranteed that the representation of the address will be in hexadecimal, though it is typically the case.
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