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Used by sysfatal(2):

Also mentioned in sysfatal(2)'s manual:

Also used in the beautiful (for those who like being creative with the preprocessor) options parser https://9fans.github.io/plan9port/man/man3/arg.html

Thank you, that bit was very interesting. I like the idea for single threaded applications of having argv0 be a global.

Doesn't have to be single-threaded even. Main writes the pointer at the start, and no one else is going to change it, just read from it.