in modern C, errno is a macro that calls a function

[rumbledethumps]

When stdio was added, cc65's errno behavior was inherited. Errors are sent through __mappederrno by the standard library. But why?

Modern C uses a macro that calls a function for errno. If we did this, all the error checking in the standard library can be skipped and apps that don't look at errno won't link in all the code and translation tables.

Also, __oserror should be a platform defined macro too if this is changed. Some platforms, like the RP6502, would then get this feature for 0 code and 0 data.

[mysterymath]

Modern C uses a macro that calls a function for errno. If we did this, all the error checking in the standard library can be skipped and apps that don't look at errno won't link in all the code and translation tables.

To my knowledge the primary reason that the C standard allows errno to be a macro is that it was historically the only plausible way to meet the requirement that errno be thread-local. On modern toolchains, thread_local achieves this, and in any case, it's not really a concern for us, as the SDK is (currently) decidedly single-threaded.

When stdio was added, cc65's errno behavior was inherited. Errors are sent through __mappederrno by the standard library. But why?

This is mostly because for many of our platforms, I/O is a direct port of the routines from cc65. This was largely done to achieve semantic compatibility with cc65 and to get them working quickly.

But, this should really be a platform-by-platform decision. The C stdio routines should bottom out in a generic API; e.g. read and write. The specifics of this were taken from cc65, again mostly for compatibility, but this interface also shouldn't directly include __oserror or __mappederrno.

Taking a quick search through the SDK, the only reference to __oserror or __mappederrno in common/c is remove and rename. But these two routines are themselves to be considered primitives in the generic API for stdio; it's expected that targets would replace them wholesale.