I read about memory, stack, etc but I didn't understand something!

This is a discussion on I read about memory, stack, etc but I didn't understand something! within the C++ Programming forums, part of the General Programming Boards category; This is the memory model Code: ---------------------- Code segment "Execution code" ----------------------- Data segment "Global variables and constants" ----------------------------------- Heap ...

  1. #1
    System Novice siavoshkc's Avatar
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    I read about memory, stack, etc but I didn't understand something!

    This is the memory model
    Code:
    ----------------------
    Code segment
    "Execution code"
    
    -----------------------
    Data segment
    "Global variables and constants"
    
    -----------------------------------
    Heap
    "Dynamically allocated memory"
       .
       .
       .
    
    ---------------------------------
    Stack
    "Holds function arguments and local variables and constants"
     
                      ^
    --------------------------------
    Right?

    Now my question is when heap is between data segment and stack, how there is no limitation in using new? We can allocate memory as much as we want (With an unlimited swap file :-P).
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  2. #2
    and the hat of wrongness Salem's Avatar
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    They're 4 separate areas of memory, there is no "between" as such. Not all systems lay out memory in the same order.

    > how there is no limitation in using new?
    The OS will not allocate overlapping memory, so at some point asking the OS for more memory will fail, and at that point calling new will throw an exception.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  3. #3
    The larch
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    Are you sure you can allocate infinite memory? There are still limits somewhere? new can fail too.

    Then, are you sure this is how memory must always be laid out?
    With a little test program, if I allocate a small array of ints with new I get the following addresses:
    global int: 0x43f000
    local int: 0x22ff74
    new int: 0x3d3850

    However, if I allocate a large amount of ints (1 million), I get
    global int: 0x43f000
    local int: 0x22ff74
    new int: 0x510020
    Dynamically allocated array is not between the globals and stack.

    After all, why worry about it?

  4. #4
    System Novice siavoshkc's Avatar
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    The OS will not allocate overlapping memory, so at some point asking the OS for more memory will fail, and at that point calling new will throw an exception.
    But it seems it never overlaps. It makes swap file bigger and bigger until it gets to its defined limitation.
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  5. #5
    pwns nooblars
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    That is because memory is randomly accessable... the execution code is given a pointer where to find the variable when it wants to read/write to it. There is no defined order to things, something to keep in mind is that programs grow and shrink in the amount of memory they use... and they have to share the envirorment with other programs who the OS can decide that it wants to give a slice to, both just would write to the next avaiable slot of memory (afaik).

    Then again I could have interpreted things wrong when I read up on this. (I think I read it in one of my CS books.)

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