Re: This is undefined, but is it legal?

=?UTF-8?B?TWFyY2VsIE3DvGxsZXI=?= <>
Sat, 26 Jul 2008 23:51:42 +0200
Pete Becker wrote:

On 2008-07-26 10:31:04 -0400, "Andrew Koenig" <> said:

Not true. It's not true in theory for int, and definitely not true in
practice for double -- because IEEE floating-point, which most modern
computers use, has a notion of "signaling not-a-number" values that
cause a
run-time error condition if accessed.

My reading of IEEE-754 is that acessing a signaling NaN causes an
invalid operation exception by default, and the result of that exception
is just to return a quite NaN. Unless the program has installed a trap
handler, this is completely innocuous.

In theory.
In practice a had a bug in some debug output a few days ago that
accidentally read a double from a union that was initialized as a
pointer. In many cases nothing strange happened. But under some
conditions the whole operating system immediately froze, even when
running in the debugger. I don't know what exception the random bits
caused, but obviously it was strange enough that nobody tested it before.


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