Pointer Mock Exam Question

This is a discussion on Pointer Mock Exam Question within the C Programming forums, part of the General Programming Boards category; Hi, Ive been given this code: Code: int i=0, j=0, *k=0, *l=0; k = l; l = &i; j = ...

  1. #1
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    Pointer Mock Exam Question

    Hi,

    Ive been given this code:

    Code:
    int i=0, j=0, *k=0, *l=0;
    k = l;
    l = &i;
    j = *k;
    Ive also been told that this code crashes. The question is, explain the cause of the crash, and correct the program by rearranging the istructions without modifiying them.

    Can someone help me to understand why it is crashing. Ive been told that line 4 is the problem, but I dont really understand why.

  2. #2
    Registered User Sharke's Avatar
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    Why not compile it and see what happens?

  3. #3
    and the hat of copycat stevesmithx's Avatar
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    Ok. here's a hint. Try exchanging instructions 2 and 3 and see if it crashes now.
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    It doesnt crash, but I still cant see the problem. Im assuming it is due to the order in which things are being defined? Can k not be pointed to address 0?

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    I would think that pointer can only hold addresses or NUll which is 0 so...I think that this

    statement

    j = *k;

    would be wrong...

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    Sorry , I was wrong...above

    You get the error here

    Code:
    k = l;
    because the pointers have not been initialized yet...

    then as mentioned switching the two lines...


    Code:
    l = &i;
    k = l;
    Now initialization has happened thus is valid...

  7. #7
    ATH0 quzah's Avatar
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    No, the error is in dereferencing a null pointer. There's nothing wrong with assigning NULL to a pointer.


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    Ive been looking at this again and I still dont understand why the error is occuring and its starting to bug me a bit.

    Is it that since k is set equal to l on line 2, and at that point l is equal to 0 (setting k to 0 too), even when l is set equal to the address of i on the next line down, k is still set to 0 from line 2 and isnt affected by line 3.

    Therefore when on line 4 when j is set equal to the pointer k, k is still being set equal to 0 memory address from line 2 and not infact being pointed to i?

  9. #9
    and the hat of copycat stevesmithx's Avatar
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    Therefore when on line 4 when j is set equal to the pointer k, k is still being set equal to 0 memory address from line 2 and not infact being pointed to i?
    Yes. You observed correctly.
    As quzah noted, the crash is caused because of line 4 where you dereference a null pointer.
    Dereferencing a null pointer, results in undefined behavior which causes a segmentation fault and hence the crash.
    Pointer (computing) - Wikipedia, the free encyclopedia
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  10. #10
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    When the value of k is assigned to j, it contains zero. When you are dereferencing a pointer, you are getting the value at the address it contains. 0 is not valid address(Not in protected mode, at least) and that's your error.
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    In the same way, is this why reversing line two and three fixes the problem, because then l is pointing to location i before equating k to l, therefore giving this:

    j = *k = l = i = 0

    (0 = integer 0 not address 0)

  12. #12
    and the hat of copycat stevesmithx's Avatar
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    Quote Originally Posted by spadez View Post
    In the same way, is this why reversing line two and three fixes the problem, because then l is pointing to location i before equating k to l, therefore giving this:

    j = *k = l = i = 0

    (0 = integer 0 not address 0)
    Yes. I suggest using a debugger and trace out and see this for yourself.
    Not everything that can be counted counts, and not everything that counts can be counted
    - Albert Einstein.


    No programming language is perfect. There is not even a single best language; there are only languages well suited or perhaps poorly suited for particular purposes.
    - Herbert Mayer

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    Thank you very much for the help. Ive tried it in the debugger and it seems to confirm these results.

    Would anyone mind explaining what the term "dereferencing a null pointer" actually means?

  14. #14
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by spadez
    Would anyone mind explaining what the term "dereferencing a null pointer" actually means?
    Suppose you have a pointer named p. *p is one way to dereference the pointer. So, if p is a null pointer, we can say that *p dereferences a null pointer. Another way would be to write say, p->foo, where foo is some member of the type that p is a pointer to.
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    I find this slightly confusing, isnt *p defining p as a pointer. In which case how is *p "dereferencing the pointer". I might need this dumbing down a little :s

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