That document does not describe long options. I guess that can be treated as a conforming extension. The C implementation does not seem to have support for opt

That document does not describe long options. I guess that can be treated as a conforming extension. The C implementation does not seem to have support for optional option arguments. The C implementation also does not seem to support "--" as a delimiter for option arguments, although that's only a guideline. So I guess it can be used to implement POSIX compliant interfaces that don't use optional option arguments. edit: supporting "--" as delimiter might not be a guideline: The utilities in the Shell and Utilities volume of POSIX.1-2017 that claim conformance to these guidelines shall conform completely to these guidelines as if these guidelines contained the term "shall" instead of "should". edit2: Seems like I was completely wrong about supporting "--", when tried out it does seem to support it. One weird corner cases is the call `<prog> -- -`, where "-" isn't interpreted as enabling stdin, but treated as a regular positional argument. `echo asd | cat -- -` reads from stdin with GNU cat.