Hi Linus, On Tue, Jun 7, 2022 at 8:15 PM Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Tue, Jun 7, 2022 at 3:23 AM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote: > > These header files are heavy users of large constants lacking the "U" > > suffix e.g.: > > > > #define NB_ADAPTER_ID__SUBSYSTEM_ID_MASK 0xFFFF0000L > > As Andreas says, this is not undefined behavior. > > A hexadecimal integer constant will always get a type that fits the > actual value. So on a 32-bit architecture, because 0xFFFF0000 doesn't > fit in 'long', it will automatically become 'unsigned long'. > > Now, a C compiler might still warn about such implicit type > conversions, but I'd be a bit surprised if any version of gcc actually > would do that, because this behavior for hex constants is *very* > traditional, and very common. > > It's also true that the type of the constant - but not the value - > will be different on 32-bit and 64-bit architectures (ie on 64-bit, it > will be plain "long" and never extended to "unsigned long", because > the hex value obviously fits just fine). > > I don't see any normal situation where that really matters, since any > normal use will have the same result. > > The case you point to at > > https://lore.kernel.org/r/CAK8P3a0QrihBR_2FQ7uZ5w2JmLjv7czfrrarCMmJOhvNdJ3p9g@xxxxxxxxxxxxxx > > is very different, because the constant "1" is always just a plain > signed "int". So when you do "(1 << 31)", that is now a signed integer > with the top bit set, and so it will have an actual negative value, > and that can cause various problems (when right-shifted, or when > compared to other values). > > But hexadecimal constants can be signed types, but they never have > negative values. Thank you, I stand corrected. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
