a simple question about pointers

This is a discussion on a simple question about pointers within the C Programming forums, part of the General Programming Boards category; Hi, I am a freshman computer engineering student and I've just started programming in C, it may seem a bit ...

  1. #1
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    Post a simple question about pointers

    Hi, I am a freshman computer engineering student and I've just started programming in C, it may seem a bit simple but I couldn't find the answer of my question. Here is the code I write:
    Code:
    #include <stdio.h>
     int main()
     {
             int i=1, j, *ip;
             ip = &i;
             printf("%d\n", ip);
            return 0;
     }
    When I compile and run this code it gives me warning and prints random numbers like :
    -1079398400 or -1077595728 or -1080068272
    and when i change the code tho this :
    Code:
    #include <stdio.h>
     int main()
     {
             int i=1, j, *ip;
             ip = &i;
             printf("%p\n", ip);
            return 0;
     }
    it prints these (that I think they are adresses ) : 0xbfa20780 or 0xbfc0e170 or 0xbfaf6850

    Can you tell me why do I get these negative numbers and why do they change each time i run the code, and also what is the difference between these two codes?

    Thanks a lot
    bahada

  2. #2
    Kernel hacker
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    The negative numbers are because, techncially anything with the highest bit set which means for 32-bit numbers above decimally 2 billion and a bit is a negative number in computers. If you do not want "large" numbers to appear negative, then you should use %u instead of %d - %u stands for "unsigned", which means "do not treat the highest bit as sign" - then you get a number in the 3 billion range.

    The reason it changes is because Linux has a scheme to randomly move the stack around to prevent people from using known locations on the stack for stack-smashing attacks. Here is a Wiki article on why this is a meaningfull:
    Address space layout randomization - Wikipedia, the free encyclopedia

    --
    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.

  3. #3
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    Yeah I understand now, thanks! Can I also learn why do these two codes behave differently when i change the line
    printf("%d\n", ip);
    to the line:
    printf("%p\n", ip);

    thanks.

    bahada

  4. #4
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    They do not behave differently in any other way than the actual representation of the value in the output. If you treat a memory address as a signed decimal number, and the value is above 2^31, then it will show as a negative number (and 0xbfffxxxx is about 1 billion/1GB into the negative region). If you use %x, it will look similar to your %p format - the difference here is that %p knows about the size of a pointer, and %x will show "an integer size" value (unless you specify %lx or %llx for long and long long values respectively - and one of these three options is MOST LIKELY right for a pointer, but the trick is to know which - %p knows that - it would even know architectures that have strange pointers and represent that in a sensible manner).

    [And for the pedants, yes, we should cast the pointer to void *].

    --
    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
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    I understand now, thank you very much...

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