Dereferencing a pointer aliased from another of an incompatible type

Question

If I do this assignment

float x = 16.8;
unsigned int i = *(unsigned int*) &x;

What will be the value of the variable i? What it is the logic of this assignment?

its a integer therefore no decimal points. Also why not test it in a compiler simply? Will lead to UB — amanuel2, Nov 01 '16 at 16:46
You should use a union, apparently, or else the behavior will be undefined. See [here](http://stackoverflow.com/questions/98650/what-is-the-strict-aliasing-rule) for more info. — Random Davis, Nov 01 '16 at 17:02
@amanuel2 Testing undefined behaviour in C is pointless, because results are *undefined*. — user694733, Nov 24 '16 at 12:16

score 1 · Accepted Answer · answered Nov 24 '16 at 10:32

With a code like this, you could have seen the answer in your precise case:

#include <stdio.h>

int main(void)
{    
    float x = 1;
    unsigned int i = *(unsigned int*) &x;
    printf("%d\n", i);

    return 0;
}

This type of assignment is illegal, by casting a pointer to a float into a pointer to an unsigned int, then dereferencing it, you are against the Strict Aliasing Rule.

As it was said in the comments, there is no strict answer. Even if with a rounded x like :

float x = 16;

You will probably not have an expected i = 16.

In most cases, your code will seem correct, you will not be warned or anything, and you code will run. So be careful!

Dereferencing a pointer aliased from another of an incompatible type

1 Answers1