Pointers in college

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I am studying pointers and this is code used as an example that is in the material given by the faculty. I have been replicating this code in my machine and the output is different from the one presented in the material. Code: 

#include <stdio.h>

int main()
{
    int x=5, y=7, teste=21, aux=17;
    int *px, *py;

    px = &y;
    py = &x;

    printf("py - px = %d\n",py-px);
    printf("px = %u, *px=%d, &px = %d\n",px, *px, &px);
    printf("py = %u, *py=%d, &py = %d\n",py, *py, &py);
    px++;
    printf("px = %u, px=%d, &px = %d\n",px, *px, &px);

    py = px+3;
    printf("py = %u, *py=%d, &py = %d\n",py, *py, &py);
    printf("py - px = %d\n",py - px);

    return 0;
}

The following is the output of the program on my machine: 

py - px = -1<br>
px = 3782591132, *px=7, &px = -512376152
py = 3782591128, *py=5, &py = -512376144
px = 3782591136, *px=21, &px = -512376152
py = 3782591148, *py=32767, &py = -512376144
py - px = 3

I also use a IDE Online to study and the exit of the program is the same as this one in college material .

Output in the Online IDE: 

py - px = 1                                                                                                                                                               
px = 2271354488, *px=7, &px = -2023612824                                                                                                                                 
py = 2271354492, *py=5, &py = -2023612832                                                                                                                                 
px = 2271354492, *px=5, &px = -2023612824                                                                                                                                  
py = 2271354504, *py=593585370, &py = -2023612832                                                                                                                         
py - px = 3

According to the material, the px pointer points to the variable x and after the increment it should point to the variable y.

However on my machine, it is pointing to the test variable. And in the subtraction line between the px and py pointers the result would be 1 instead of -1.

I do not know what might be wrong.

I'm using Ubuntu 16.4.5 with gcc 5.4 and gcc 7.1 both with the same output. And IDE Online is RED HAT 7.1.1-3 with gcc 7.1.1.

    
asked by anonymous 29.10.2017 / 00:08

1 answer

1

In the case presented the program will actually display different outputs on each machine, and sometimes every execution, because the value stored in a pointer is a memory address. The problem is that because it is not an array, it may be that x and y are not contiguous positions in memory so incrementing x will not necessarily stop at y. (Remember that in pointer arithmetic, sum and subtraction move backward in memory positions)

To illustrate what I'm saying, see the change I made to your code: 

#include <stdio.h>

int main(){

    int array[] = {5, 7, 21, 17, 50};
    int *px, *py;

    px = array;
    py = px+1;

    printf("py - px = %d\n", py - px);
    printf("px = %u, *px=%d, &px = %d\n",px, *px, &px);
    printf("py = %u, *py=%d, &py = %d\n",py, *py, &py);
    px++;
    printf("px = %u, px=%d, &px = %d\n",px, *px, &px);

    py = px+3;
    printf("py = %u, *py=%d, &py = %d\n",py, *py, &py);
    printf("py - px = %d\n",py - px);

    return 0;
}

Note: I had to put one more position in the array because py + 3 accessed a variable outside the array.

The output shown is as follows: 

py - px = 1 // 1 espaço de diferença
px = 2865090736, *px=5, &px = -1429876568
py = 2865090740, *py=7, &py = -1429876576
px = 2865090740, px=7, &px = -1429876568
py = 2865090752, *py=50, &py = -1429876576
py - px = 3

Now notice the positions in memory: 

 x ->  0. 2865090736 // 2865090736 + 4*0
 y ->  1. 2865090740 // 2865090736 + 4*1
       2. 2865090744 // 2865090736 + 4*2
       3. 2865090748 // 2865090736 + 4*3
y+3->  4. 2865090752 // 2865090736 + 4*4

4 bytes is the space that an int occupies.

    
29.10.2017 / 18:24
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