Am Di., 27. Okt. 2020 um 02:18 Uhr schrieb Wolfgang Corcoran-Mathe <xxxxxx@sigwinch.xyz>: > > On 2020-10-26 18:18 -0400, John Cowan wrote: > > On Mon, Oct 26, 2020 at 2:07 PM Wolfgang Corcoran-Mathe <xxxxxx@sigwinch.xyz> > > wrote: > > > > Would it be sufficient to provide C versions of > > > nan-quiet?, nan-negative?, etc., operating on doubles? > > > > > > > That's exactly what I had in mind. Create a union type in C that can be > > either a double or a uint64, put in the double, get out the uint64, and > > bit-twiddle it, probably 10-15 lines of code. > > This would be undefined behavior in C89, but, after a lot of Is it undefined in C89? I think it is just implementation-defined behavior. > searching, it does seem that type-punning through unions is valid in > C99 (and C11, allegedly; I don't have a copy of that standard). > Footnote 82 to section 6.5.2.3 (present in more recent drafts of the C99 > standard) clarifies this (somewhat): > > If the member used to access the contents of a union object is not > > the same as the member last used to store a value in the object, the > > appropriate part of the object representation of the value is > > reinterpreted as an object representation in the new type as > > described in 6.2.6 (a process sometimes called "type punning"). > > This might be a trap representation. "Reinterpreted as an object ..." is again implementation-defined, isn't it? Is there a way to find out how an IEEE double is mapped to an array of chars?
