Incrementing variables using pointers

I'm very new to dealing with pointers, and my C knowledge is fairly small. I'm trying to understand pointers. I wrote the following code to print a list of variables (a to f) like so:

开发者_运维问答0
1
2
3
4
5

I wrote the following code to do this:

#include <stdio.h>
int main(){
  int a,b,c,d,e,f;
  int *p;
  int i;
  a = b = c = d = f = 0;
  p = &a;
  for (i = 0; i < 5; i++){
    *p += i;
    printf("%d\n", *p);
    p++;
  }
  return 0;
}

The idea was it works through the variables and increments each by an ever-increasing number (i). I am assuming that as you initialize the variables at the same time, they'd be placed next to each other in memory. However, I get the following output:

0
1
2
3
-1218283607

If I change the for loop to only go from 0 to 3 (i < 4), it works fine, printer 0 1 2 and 3. But when I wish to print the variable f as well, it doesn't seem to set it.

As I said, I'm very new to pointers so I've probably overlooked something silly, but I've been looking through my code over and over, trying to work it out.

Thanks in advance.

There is no guarantee that a, b, c, d, e and f will be adjacent in memory. If you want that sort of guarantee you need to use an array.

#include <stdio.h>
int main() {
    int a[6];
    int *p;
    int i;
    a[0] = a[1] = a[2] = a[3] = a[4] = a[5] = 0;
    p = &a[0];
    for (i = 0; i < 6; i++){
        *p += i;
        p++;
    }
    for(i = 0; i < 6; i++) {
        printf("%d\n", a[i]);
    }
    return 0;
}

Here int a[6] is declaring an array named a that can hold six integers. These six integers can obtained via a[0], a[1], a[2], a[3], a[4] and a[5]. You are guaranteed that a[0], a[1], a[2], a[3], a[4] and a[5] are layed out contiguously in memory. Thus the line

p = &a[0];

sets p to the address of the first element. Each increment of this pointer moves us forward one position in the array.

The second for loop shows that first for loops correctly sets a[i] to i for i in {0, 1, 2, 3, 4, 5}. If you run this program you will see

0
1
2
3 
4
5

on the console.

You forgot to initialize e. But yes, use a packed array.

It isn't safe to assume that stack variables are arranged in memory in any particular order.

You need to use an array, a struct or possibly a union to gurantee the ordering of your ints.

union {
   int ary[6];
   struct {
      int a;
      int b;
      int c;
      int d;
      int e;
      int f;
      } s;
   } u = {0};

  p = &u.s.a;
  for (i = 0; i < 5; i++){
    *p += i;
    printf("%d\n", *p);
    p++;
  }