https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90370
--- Comment #4 from Florian Weimer <fw at gcc dot gnu.org> --- (In reply to Jonathan Wakely from comment #3) > The issue is basically that the C++ Standard Library defines two categories > for error numbers known to the implementation: "generic" and "system", where > the former is for the POSIX errno values, and the latter is for other error > numbers used by the OS for other kinds of errors that aren't covered by a > POSIX errno macro (which might be other non-standard EBLAH macros that the > OS adds to the POSIX ones, or some totally different error numbers used by > the OS, e.g. Windows has its own error numbers unrelated to errno). The > question is which category zero belongs to. This is really confusing for native POSIX systems because the generic/system terminology appears to be swapped for them. This is also reflected in the standard: <http://eel.is/c++draft/syserr.errcat.objects#4> And in the documentation: <https://en.cppreference.com/w/cpp/error/system_category> With this observation in mind, I reach this conclusion: For a native POSIX system, no translation of errno values is needed. That suggests to me that all int values (the ev argument) correspond to a POSIX errno value, and therefore all error conditions crated by default_error_condition should be of the system_category category. > So although zero isn't a POSIX errno value, POSIX does "reserve" the value > zero and give it special meaning (it's the value that won't get set to > report an error). No posix-ish OS is going to give it a different meaning > that clashes with that. I'm not sure if this matters. A non-POSIX systems can use 0 for a real error code that needs to be mapped to an actual C/POSIX E* value (such ERANGE).
