That is only true in some environments, since it's not in fact part of the language specification. See the manual page's [1] "STANDARDS" section, which reads:

That is only true in some environments, since it's not in fact part of the language specification. See the manual page's [1] "STANDARDS" section, which reads: getopt() POSIX.1-2008. getopt_long() getopt_long_only() GNU. The use of '+' and '-' in optstring is a GNU extension. [1]: https://www.man7.org/linux/man-pages/man3/getopt.3.html https://www.man7.org/linux/man-pages/man3/getopt.3.html