Thread: Pointer to Pointer + Pointer casting; Need Explanation Please.

Hi all,

Have a little question about pointer casting (the meaning of it);

Here is my code (working on Eclipse):

Code:

#include<stdio.h>

int main(void){

int x = 5;
int *myPtr1;
int **myPtr2;

myPtr1 = &x;

printf("The addr of x variable is: %p\n", &x);
printf("The addr myPtr1 refers to, is: %p\n", myPtr1);
printf("The value myPtr1 refer to: %d\n", *myPtr1);
printf("The addr of myPtr1 variable is: %p\n\n", &myPtr1);

*myPtr2 =(int *) myPtr1;

printf("The 1-st addr myPtr2 refers to: %p\n", myPtr2);
printf("The 2-nd addr myPtr2 refers to:%p\n", *myPtr2);
printf("The value myPtr2 refers to: %d\n", **myPtr2);
printf("The addr of myPtr2 variable is: %p\n", &myPtr2);

return 0;
}

The output example is:

Code:

The addr of x variable is: 0xbfd96624
The addr myPtr1 refers to, is: 0xbfd96624
The value myPtr1 refer to: 5
The addr of myPtr1 variable is: 0xbfd96628

The 1-st addr myPtr2 refers to: 0xb7759000
The 2-nd addr myPtr2 refers to:0xbfd96624
The value myPtr2 refers to: 5
The addr of myPtr2 variable is: 0xbfd9662c

Still, I can't truly understand what does this line mean:

Code:

*myPtr2 =(int *) myPtr1;

By this, I don't sure if the text descriptions I've provided in"printf" functions are correct.


The "roots" of this question come from my investigation of "strod" function source code. There I've found something similar and just tried to understand what they have done there.

Furthermore:

In case this line is commented:

Code:

printf("The addr of myPtr2 variable is: %p\n", &myPtr2);

Then Eclipse underlines this line:

Code:

*myPtr2 =(int *) myPtr1;

and saying:

Code:

myPtr2 is used uninitialized in this function

But in case the above "printf" is used, all works good.


Any help/explanation/clarification will be highly appreciated.

Thanks in advance.

Originally Posted by laserlight

It means: cast myPtr1 to be a pointer to int, then assign the result to what myPtr2 points to.

Actually, myPtr1 is already a pointer to int, so this would suffice:

Code:

*myPtr2 = myPtr1;

My apologies, but this is still not fully clear to me. What happens to myPtr2 when I use casting (reading Deitel's book all the day though ^_^) is it kinda "downgraded" to "int *" insted "int **"?


I've checked your code change and this is the result I get:

Code:

The addr of x variable is: 0xbfad3da4
The addr myPtr1 refers to, is: 0xbfad3da4
The value myPtr1 refer to: 5
The addr of myPtr1 variable is: 0xbfad3da8

The 1-st addr myPtr2 refers to: 0xb76e9000
The 2-nd addr myPtr2 refers to:0xbfad3da4
The value myPtr2 refers to: 5
The addr of myPtr2 variable is: 0xbfad3dac

And still I get a warning message when the last "printf" is commented.


The other situation is when:

Code:

myPtr2 = &myPtr1;

In this case I get fully consistent address picture:

Code:

The addr of x variable is: 0xbfee18e4
The addr myPtr1 refers to, is: 0xbfee18e4
The value myPtr1 refer to: 5
The addr of myPtr1 variable is: 0xbfee18e8

The 1-st addr myPtr2 refers to: 0xbfee18e8
The 2-nd addr myPtr2 refers to:0xbfee18e4
The value myPtr2 refers to: 5
The addr of myPtr2 variable is: 0xbfee18ec

Thanks for your explanation laserlight.