Get current position of pointer

Is there a function to print out the current position of the pointer of the pointer?
(im not sure how to put it)
I mean like ftell() for a file stream.

Code:

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int main()
{
    char * response = "hello world";
    long size;
    for(int i=0;i<6;i++) {
        size=ftell(response);
        printf("%i\n",size); 
        or
        printf("%i\n", *response);
        *response++;
    }

    return 0;
}

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Actually i should get the value of i but without knowing what i is.

ftell is for file position. You can't get the 'cursor' position, because C doesn't concern itself with such things. You'll really need something like the curses library if you want to write to specific spots on the screen.


Quzah.

Quote:

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Originally Posted by Ducky

Actually i should get the value of i but without knowing what i is.

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That's nonsensical -- the value of i is what it is. You don't have to get it.

Are you trying to get the length of the string in response? Use strlen:

Code:

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int size = strlen(response);

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You can get the offset of one pointer relative to another this way:

Code:

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    char * response = "hello world", *p = &response[6];
    printf("%d", p-response);

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No, no, no thats not i want.

Ok i try to put it this way: im "walking a pointer" *response with *response++ and i want to know
where am i. See? Its simple.

PS i meant it to Quzah thank you though

@MK27

Ok i get it, thanks a lot!

Yes, it's simple.

Code:

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int main()
{
    char * response = "hello world", *p =response;
    int size = strlen(response);
    for(int i=0;i<size;i++) {
        printf("%d index is: %d\t offset of p: %d\n",*p,i,p-response);
        p++;
    }

    return 0;
}

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Actually that is exactly what i wanted to know i just wanted to go the other way to understand it.

Why should we substract 'response' from 'p' ?

I guess thats what they call pointer arithmetic, pretty hard to grasp.

Maybe because in my head a pointer was a string, which is apparently not.

A pointer contains a memory address, by incrementing a pointer pointing to the same address as your string you can movie ahead in the string. By subtracting these pointers you get your current relative position.

Quote:

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By subtracting these pointers you get your current relative position.

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Voila this is what not clear: we incremented 'p'. Did this incremented 'response' too?

1. if yes then they have the same value so 6-6=0
2. if no then 'response' is 0. So whats the point substracting 0 ?

response and p are two different pointers pointing to the same memory location. Modifying one does not affect where the other is pointing.

Think of it as a road sign, where two city names : "Strasbourg" and "Paris" both point to the right. Changing one sign to make it point ahead will not modify the other.

So in this case 'response' remains 0 and we are substracting 0. Am i right?

Quote:

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Originally Posted by Ducky

Maybe because in my head a pointer was a string, which is apparently not.

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To add to what Subsonics et. al. said, a memory address in C contains one byte of information. The are laid out in sequential order, and the compiler guarantees in the case of an array (such as a string) that the array occurs in sequence. So, let's use 2 digit addresses for simplicity:

Code:

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char *str = "hello";
printf("%p", str);

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%p gives the value of a pointer, which is an address. Generally, the exact value is not very important, but let's say in our 2 digit addressing system the value is 42 (they are more or less arbitrary and determined at runtime). So the value at memory location 42 is 'h', because each address is one byte. Now:

43: 'e'
44: 'l'
45: 'l'
46: 'o'
47: '\0' null terminator

So from the previous example, if "response" where 42, p is initially 42 as well. 42-42=0. Then it moves up into higher address. Make sense?

Quote:

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Originally Posted by Ducky

Voila this is what not clear: we incremented 'p'. Did this incremented 'response' too?

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No.

Quote:

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Originally Posted by Ducky

2. if no then 'response' is 0. So whats the point substracting 0 ?

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It's not 0, it contains a memory address, that you can print BTW with the %p format specifier in printf.

Quote:

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So from the previous example, if "response" where 42, p is initially 42 as well. 42-42=0. Then it moves up into higher address. Make sense?

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Awesome, now it makes perfect sense! :)

Thanks again MK27 and everybody else for your inputs, this has been bugging me for a long time. :)

Didn't know if this will help but I made a couple changes to MK27's example to show exactly what the memory addresses look like and how subtracting response from p gets the index number.

Code:

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define FALSE 0
#define TRUE 23

int main(int argc, char **argv)
{
        int i;
    char * response = "hello world", *p =response;
    int size = strlen(response);
    for( i=0;i<size;i++) {
               
        printf("%p %c index is:%d\toffset of p:%d %p  \t",response,*p,i,p-response,p);
        printf("(p-response) = %d\n",p-response);
        p++;
    }
    return 0;
}

/*
0x8048590 h index is:0        offset of p:0 0x8048590          (p-response) = 0
0x8048590 e index is:1        offset of p:1 0x8048591          (p-response) = 1
0x8048590 l index is:2        offset of p:2 0x8048592          (p-response) = 2
0x8048590 l index is:3        offset of p:3 0x8048593          (p-response) = 3
0x8048590 o index is:4        offset of p:4 0x8048594          (p-response) = 4
0x8048590  index is:5        offset of p:5 0x8048595          (p-response) = 5
0x8048590 w index is:6        offset of p:6 0x8048596          (p-response) = 6
0x8048590 o index is:7        offset of p:7 0x8048597          (p-response) = 7
0x8048590 r index is:8        offset of p:8 0x8048598          (p-response) = 8
0x8048590 l index is:9        offset of p:9 0x8048599          (p-response) = 9
0x8048590 d index is:10        offset of p:10 0x804859a          (p-response) = 10
*/

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