a simple question about pointers

bahada · 04-16-2009, 04:29 AM

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

matsp · 04-16-2009, 04:38 AM

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

bahada · 04-16-2009, 04:58 AM

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

matsp · 04-16-2009, 05:10 AM

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

bahada · 04-16-2009, 05:15 AM

I understand now, thank you very much...

04-16-2009, 04:29 AM	#1
bahada Registered User Join Date: Apr 2009 Posts: 10	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
bahada is offline

04-16-2009, 04:38 AM	#2
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	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.
matsp is offline

04-16-2009, 04:58 AM	#3
bahada Registered User Join Date: Apr 2009 Posts: 10	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
bahada is offline

04-16-2009, 05:10 AM	#4
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	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.
matsp is offline

04-16-2009, 05:15 AM	#5
bahada Registered User Join Date: Apr 2009 Posts: 10	I understand now, thank you very much...
bahada is offline

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