C: long pointer pointing to address with an integer

Question

Why does the first printf() output 1 and the second one 8589934593? EDIT: Why does the second one output exactly 8589934593 and not some other number?

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

int main(int argc, char *argv[]){
    int *intPtr = NULL;
    long *longPtr = NULL;

    int array[5] = {0,1,2,3,4};
    intPtr = &array[1];
    longPtr = &array[1];
    printf("%d\n", *intPtr);
    printf("%ld\n", *longPtr);
}

Possible duplicate of [What is the strict aliasing rule?](https://stackoverflow.com/questions/98650/what-is-the-strict-aliasing-rule) — machine_1, Mar 25 '18 at 19:33
Learn what **undefined behaviour** is. You definitively will meet it many times in the future. — too honest for this site, Mar 26 '18 at 00:29

score 2 · Accepted Answer · answered Mar 25 '18 at 19:22

2

Because it's undefined behavior. Pointing a long* to the address of an int and acting like it's a long violates the strict aliasing rule, giving you undefined behavior.

answered Mar 25 '18 at 19:22

Hatted Rooster

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But why does it output 8589934593 and not some other number? – AndrejCoding Mar 25 '18 at 19:23
1

@AndrejCoding Because it's undefined, it can print anything or blow up your computer. – Hatted Rooster Mar 25 '18 at 19:24
So for some compilers, it could output e.g. 123 or whatever? It just outputs 8589934593 for my particular compiler because the compiler defined it somewhere? – AndrejCoding Mar 25 '18 at 19:26
1

@AndrejCoding it's because that's just what happens to be in memory at that address on your computer at the time you run it. If you run it again later you may get a different result, or if you change something that causes the compiler to use a different address, etc. – nemequ Mar 25 '18 at 19:26
1

print in hex `>>> hex(8589934593 ) '0x200000001'`. Now you're sure the compiler read the data with a shift or something. – Jean-François Fabre Mar 25 '18 at 19:27
@AndrejCoding Because it happened to be in memory. – Hatted Rooster Mar 25 '18 at 19:27
Thanks, guys! I will mark this answer as correct in 8min! – AndrejCoding Mar 25 '18 at 19:27
@Jean-FrançoisFabre Fancy seeing you here – Hatted Rooster Mar 25 '18 at 19:27

C: long pointer pointing to address with an integer

1 Answers1