Applied XOR operator with an uninitialized variable in C

Question

I'm a little bit confused. As far as I know, if you declare uninitialized variable in C, so its value is indeterminate.

If you don’t initialize an variable that’s defined inside a function, the variable value remain undefined.That means the element takes on whatever value previously resided at that location in memory.

If I applied ^(XOR) operator to uninitialized integer variable itself. Like,

#include <stdio.h>

int main()
{
        int a;
        printf("%d\n", a^a);
}

it's clear that a^a should be zero because the result is zero only when we have two zeroes or two ones. So, I have a question : Is it undefined behaviour?

see also [(Why) is using an uninitialized variable undefined behavior in C?](http://stackoverflow.com/q/11962457/995714) — phuclv, Mar 16 '17 at 06:19
its undefined behavior only when `a==a` ... oh wait, that's UB also. — technosaurus, Mar 16 '17 at 06:20
[Value of int i = i ^ i ; Is it always zero or undefined behavior?](http://stackoverflow.com/q/17485814/995714) — phuclv, Mar 16 '17 at 07:15

score 1 · Accepted Answer · answered Mar 16 '17 at 06:10

1

Yes, this is undefined behaviour.

Attempt to use a variable with indeterminate value which

can have trap representation
not have it's address taken

will cause undefined behavior.

answered Mar 16 '17 at 06:10

Sourav Ghosh

133,132
16
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261

Shrey · Answer 2 · 2017-03-16T06:39:07.877

1

Yes it's undefined behavior. Just declaring and not initializing the variable, stuff the variable with a garbage value .

edited Mar 16 '17 at 06:39

answered Mar 16 '17 at 06:14

Shrey

98
1
12

`Performing xor on a value with itself gives 0 . It's undefined behavour.` isn't that contradictory? If you have a defined value, how is it UB? – Sourav Ghosh Mar 16 '17 at 06:25
@SouravGhosh , Sorry I guess my answer is a bit confusing . They are two distinct statements . – Shrey Mar 16 '17 at 06:36
That was the point for the comment, please refine the answer and thanks for your efforts, :) – Sourav Ghosh Mar 16 '17 at 06:36

Applied XOR operator with an uninitialized variable in C

2 Answers2