undefined | Better HN

0 pointsGhotiFish12y ago0 comments

lets say __ is 0x00101101

then it's ~__+__

so

   0x00101101
  +0x11010010

You can see how that always works.

0 comments

4 comments · 1 top-level

CJefferson12y ago· 3 in thread

Unfortunately not. When you have not written to a C variable, it doesn't have a random value, reading it is undefined. Different reads can produce different values.

babuskov12y ago

Not really. When you declare a variable the system assigns it an address on stack. When you try to read it, it reads the value currently stored at that address. Subsequent reads all read from the same address.

Unless you have something else writing to this address, the value will not change. Since the program's stack is only available to that program, it will stay the same.

twoodfin12y ago

Yes, really. I don't think you understand the leeway that C/C++ give implementations when they encounter undefined behavior. Your mental model of how the code "should" be compiled can go out the window when an optimizer designed to work properly only on well-defined code goes to work:

http://blog.llvm.org/2011/05/what-every-c-programmer-should-...

1 more reply

foxhill12y ago

as you said, reading it is just undefined. the compiler has to do something, and it's improbable that a read before write, followed by another read will result in two different values.

if you could produce a piece of code that exhibits the behaviour you describe, i'd very much like to see it.

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