Hacker News Reader

Comment by begriffs | Hacker News Reader

What does the presence or absence of __STDC_ISO_10646__ indicate exactly? I found this part of the C99 spec obscure.

For instance, the macOS clang environment does not define this symbol. Is their implementation of wchar_t or <wctype.h> lacking some aspect of Unicode support?

View on HN

If that macro is defined, then wchar_t is able to represent every character from the Unicode required character set with the same value as the short code for that character. Which version of Unicode is supported is determined by the date value the macro expands to.

Clang defines that macro for some targets (like the Cloud ABI target), but not others. I'm not certain why the macro is not defined for macOS though (it might be worth a bug report to LLVM, as this could be a simple oversight).

Would the following be a correct way to determine whether there's a problem?

* First call setlocale(LC_CTYPE, "en_US.UTF-8")

* Next feed the UTF-8 string representation of every Unicode codepoint one at a time to mbstowcs() and ensure that the output for each is a wchar_t string of length one

* If all input codepoints numerically match the output wchar_t UTF-32 code units, then the implementation is officially good, and should define __STDC_ISO_10646__?

Should work provided your wchar_t type is at least 21-bits wide.

I think this is correct, assuming that locale is supported by the implementation and wchar_t is wide enough, but I am by no means an expert on character encodings.

Reply on news.ycombinator.com