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  • You are mistaken, XSI is (if ever) made mandatory in newer versions of the standard. Also note that according to the author of the POSIX validation test suite, XSI compliance is mandatory unless you like to validate an embedded UNIX platform only. Commented Jun 7, 2018 at 12:29
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    @schily: Citation? Commented Jun 7, 2018 at 17:38
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    The current documentation for unistd.h reads: "The <unistd.h> header shall define the following symbolic constant only if the implementation supports the XSI option; see XSI Conformance. If defined, its value shall be suitable for use in #if preprocessing directives. _XOPEN_VERSION" Commented Jun 7, 2018 at 17:39
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    Thanks. unix.stackexchange.com/questions/467010/… Commented Sep 5, 2018 at 12:49
  • BTW: for a UNIX branding (which is more than POSIX compliance), you need to implement all XSI enhancements. With dash, you cannot get XSI compliance since dash does not implement multibyte character support and with the usual echo on Linux there is another problem, since echo on Linux does not implement support for escape sequences. Commented Feb 11, 2020 at 13:08