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In at least some locales tolower segfaults under that condition, in glibc at least.

(before anyone says that ‘in reality it’s probably fine’)

I would love to know an example of that.

https://drewdevault.com/2020/09/25/A-story-of-two-libcs.html

That quotes a glibc comment "we also support negative `signed char' values for broken old programs" which means the OP code is in fact going to behave OK with glibc, providing that "support for broken old programs" is never removed.

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