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[QUOTE="Eric Sosman, post: 5088243"]
Aside: This won't always do what you want.  The compiler
has a lot of freedom in laying out bit-fields, and different
compilers will do it differently.

The short (and uninformative) answer is: When the argument
and the conversion specification disagree, the behavior is
undefined.

Most likely, the "%x" specifier looks at half of the `double'
argument,[*] finds a whole lot of zero bits, interprets them as
the `unsigned int' it was expecting, and prints their value.

Even that much isn't guaranteed, though.  Some machines pass
floating-point arguments in a different way than others (perhaps
in dedicated F-P registers), so the "%x" would look for its
argument in one place while the `1.0' inhabits another.  GIGO.

[*] Yes, the type of `1.0' is `double', not `float'.  Even
if you were to write `1.0f' it wouldn't help: the "default
argument promotions" would convert the `1.0f' to `double' while
marshalling the arguments for printf.  There is no way to pass
a `float' -- or `short', or `char' -- argument to printf.
[/QUOTE]

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