constant variable

constant variable

[Nasir Simbolon]

Hi All, 
I just curious. I have the following codes

main()
{
 	const int i=9;          //declare constant  integer variable and initialize
       	int* j=NULL;   		     //declare a pointer to integer
	 j = (int*) &i;         //j point to address of memory i 
 	*j = 3;		     //modify value  of  j point to 

	//Now print the result
	 printf("value of i = %d\n",i);
  	 printf("value of j = %d\n",*j);
	 printf("Address of i = %x\n",&i);
 	 printf("Address of j = %x\n",j);
}
when I compile using gcc version 3.3.2 (Debian) and run, I have the result :

value of i = 9
value of *j = 3
Address of i = bffff8a4
Address of j = bffff8a4

Now, I notice  that j point to address of i BUT they do not  have the same 
value.

Anybody have an explanation?


Regards,

Nasir 


-- 
"timor Domini principium scientia (Prov 1:7)"

RE: constant variable

[Sandro Dangui]

If you look at the generated assembly you will notice that the compiler uses
(in the call printf("value of i = %d\n",i);) the value 9 instead of the
contents of i. I mean, the assembly pushes directly the value 9 to the
stack... It assumes that if you declared a constant it will not be changed.

[]'s

Sandro.


-----Original Message-----
From: linux-c-programming-owner@vger.kernel.org
[mailto:linux-c-programming-owner@vger.kernel.org] On Behalf Of Nasir
Simbolon
Sent: Thursday, March 11, 2004 8:19 AM
To: linux-c-programming@vger.kernel.org
Subject: constant variable


Hi All, 
I just curious. I have the following codes

main()
{
 	const int i=9;          //declare constant  integer variable and
initialize
       	int* j=NULL;   		     //declare a pointer to integer
	 j = (int*) &i;         //j point to address of memory i 
 	*j = 3;		     //modify value  of  j point to 

	//Now print the result
	 printf("value of i = %d\n",i);
  	 printf("value of j = %d\n",*j);
	 printf("Address of i = %x\n",&i);
 	 printf("Address of j = %x\n",j);
}
when I compile using gcc version 3.3.2 (Debian) and run, I have the result :

value of i = 9
value of *j = 3
Address of i = bffff8a4
Address of j = bffff8a4

Now, I notice  that j point to address of i BUT they do not  have the same 
value.

Anybody have an explanation?


Regards,

Nasir 


-- 
"timor Domini principium scientia (Prov 1:7)"

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Re: constant variable

[Steven Smith]

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>  	const int i=9;
>      	int* j=NULL;
> 	 j = (int*) &i;
>  	*j = 3;

> when I compile using gcc version 3.3.2 (Debian) and run, I have the result :
> 
> value of i = 9
> value of *j = 3
> 
> Now, I notice  that j point to address of i BUT they do not  have the same 
> value.

Probably because of compiler optimisations.  If you've specified that
something is const, then the compiler is perfectly at liberty to
assume that it is.  This means that it can assume the initial value of
i still applies, and so, rather than going to memory for i, it can
just substitute the constant 9 in.  This is likely to be much quicker
at run time.

I can only reproduce this behaviour if I have compiler optimisations
turned on, which adds some more weight to the explanation.

Steven.

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