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. Linus
