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You can set errno to 0 and see if it changes.

I'm not sure there is any guarantee that a function that completes successfully hasn't changed the value of errno anyway.

Yeah, I didn't mean to imply that you can ignore the return value.

The syscall is allowed to set it even in case of success.

This is sort of right, but the behaviour is a bit subtle. errno should never be set to zero by any POSIX-compliant syscall or library call. However it may be set to something (which is non-zero) on a successful call. Ref: https://pubs.opengroup.org/onlinepubs/9699919799/functions/e...

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