Why pointer is giving the same address for all array blocks?

Question

Here is a program, I write it to output all the characters of a string one by one. But I also print the address of individual blocks of the array. The problem is addresses for all blocks are same. Why? Does someone know?

#include<stdio.h>
int main()
{
    char enter[]="Kinsman";
    char *ptr;  
    ptr=enter;
    int i=0;
    while(*ptr!='\0')
    {
        printf("%c%p\n",*ptr,&ptr);
        ptr++;
        for(i=0;i<=100000000;i++);
    }
return 0;
}

@GrijeshChauhan Back on my old ATARI 800XL, waiting one second was accomplished easily with `FOR I=1 TO 500: NEXT I`. But this method - waiting in terms of cycles - has been outdated for about 20 years. — glglgl, Oct 30 '13 at 11:17
@glglgl Yes I read it somewhere in past and it will consume CPU cycles. ~~ But a C compiler can optimize it by replacing `i = 100000000;`.. it is not reliable. Correct? — Grijesh Chauhan, Oct 30 '13 at 11:22
@GrijeshChauhan I used it just to give a pause for each output through loop. — Ashish Tomer, Oct 30 '13 at 11:28
@glglgl From your comments it looks like you both are professionals. — Ashish Tomer, Oct 30 '13 at 11:29
@AshishTomer It is better to use the appropriate function - `sleep()` or `usleep()` on Unix, `Sleep()` on WIndows. — glglgl, Oct 30 '13 at 11:36
@GrijeshChauhan This is right. Most modern compilers will probably optimize it away; forgot about that. — glglgl, Oct 30 '13 at 11:37
@glglgl: Or if you can use C11, [thrd_sleep](http://en.cppreference.com/w/c/thread/thrd_sleep) would be a cross-platform solution. — legends2k, Oct 30 '13 at 12:14

score 4 · Answer 1 · answered Oct 30 '13 at 10:50

4

Because you print the address of the actual pointer.

When you use &ptr you get the address of the actual pointer and not the address if points to. Remove the ampersand (address-of operator &) so you have only ptr.

answered Oct 30 '13 at 10:50

Some programmer dude

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Yes you are right. Thank you! :) Can you tell me how do I clear screen on startup using clrscr()? – Ashish Tomer Oct 30 '13 at 10:55
@AshishTomer By calling the function? – Some programmer dude Oct 30 '13 at 10:56
No i know that actually I added conio.h but GCC gives error: "reverse.c:2:18: fatal error: conio.h: No such file or directory #include ^ compilation terminated." – Ashish Tomer Oct 30 '13 at 11:02
@AshishTomer What is the point of clearing the "screen"? As a user, this would annoy me as I lose track of what happened before. Is this really absolutely necessary? – glglgl Oct 30 '13 at 11:20
@glglgl no! It's not necessary. I just wanted to :P :D – Ashish Tomer Oct 30 '13 at 11:22
@AshishTomer The `` header file and its associated functions are a Windows and specifically VisualStudio (VisualC++) thing (originally from old TurboC if I remember correctly). GCC does not have it. – Some programmer dude Oct 30 '13 at 11:26
@JoachimPileborg okay. thnx – Ashish Tomer Oct 30 '13 at 11:31

score 3 · Accepted Answer · edited May 23 '17 at 10:25

3

You are printing the address of the pointer, not the value of the pointer

Try

printf("%c%p\n",*ptr, static_cast<void*>(ptr));

(https://stackoverflow.com/a/18929285/259)

edited May 23 '17 at 10:25

Community

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answered Oct 30 '13 at 10:51

David Sykes

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I printed the value and address both using %c and %p – Ashish Tomer Oct 30 '13 at 10:53
@AshishTomer Yes but you also added `&`, read answer again. `&ptr` should be just `ptr` in printf – Grijesh Chauhan Oct 30 '13 at 10:54
1

@AshishTomer: You might want to see [this post](http://stackoverflow.com/q/930138/183120) for `clrscr`. – legends2k Oct 30 '13 at 10:59
2

Using `%p` without a `void *` is *undefined behavior*. Unless the argument is a ptr-to-void, a cast is required. – Jens Oct 30 '13 at 11:10
@DavidSykes what are standards? And where can I get them? – Ashish Tomer Oct 31 '13 at 06:54
@AshishTomer If you are referring to the standard c++ library then it should come with your compiler. http://www.cplusplus.com/reference/ – David Sykes Oct 31 '13 at 09:27
@DavidSykes Okay. . . I am using GCC. Thanks! – Ashish Tomer Oct 31 '13 at 13:17

score 1 · Answer 3 · edited Oct 30 '13 at 11:11

1

ptr is a pointer and it is also a variable in stack that has an address. This is fixed, while what it points to is varying by ptr++, thus you've to print the pointed-to value and not the address of the pointer itself.

 printf("%c%p\n",*ptr, (void*)ptr);
 //                   ^  remove & , and add void*

edited Oct 30 '13 at 11:11

Grijesh Chauhan

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answered Oct 30 '13 at 10:53

legends2k

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Can you tell me how do I clear screen?
I added conio.h but GCC gives error: "reverse.c:2:18: fatal error: conio.h: **No such file or directory** #include ^ compilation terminated." – Ashish Tomer Oct 30 '13 at 11:04

Why pointer is giving the same address for all array blocks?

3 Answers3