On Thu, 6 Oct 2016, Martin Sebor wrote: > > The problem is that extracting via an integer of the opposite signedness > > is *not* defined unless the argument has a value representable in both > > types - both as a matter of ISO C rules on variadic functions, and as a > > practical matter of what various ABIs say about how sub-word arguments are > > extended when passed as function arguments. > > No, sign mismatch is not the problem we're discussing. GCC doesn't > warn for %lc when wchar_t and wint_t differ in signedness except > with -Wformat-signedness. The cause of the warning is that wchar_t > is a distinct type from wint_t such as long from int (irrespective > of their sign), even if the two have the same size, and the checker > doesn't distinguish the typedefs from the underlying types.
For most configurations of GCC, wint_t is unsigned int (the default from defaults.h). Only a few specific (OS, CPU) pairs make a different choice. > I did say, however, that the warning might have some merit even > in this case if there also were a target to which the code could > be ported where wchar_t and wint_t had different sizes (i.e., as > a portability warning). But for such a portability warning to > be useful it would need to be issued by all GCC compilers, not > just when compiling for the target with that property. (But > since neither of us knows(*) of such a target this is a moot > point.) As I pointed out, wint_t and wchar_t are ABI-incompatible when passed to functions on powerpc64-linux-gnu, because of the different signedness, even though they have the same size. Thus, diagnosing the wrong choice of type is useful as a portability warning. Unfortunately, it's hard to diagnose it on architectures where wchar_t and wint_t are the same type or too similar, but by diagnosing where they are sufficiently different at the C level (but possibly not at the ABI level), code can be fixed so it doesn't break in cases where the difference matters. > The text of the warning itself indicates that even suggesting how > to fix it isn't trivial. GCC warns for just for the %lc directive > (not for the sign mismatch) and suggests to fix it by replacing > the %lc directive with %ld. That's clearly wrong. Printing It's true the hint is wrong, but the warning is right. -- Joseph S. Myers jos...@codesourcery.com
