Hello Elliot, On 01/18/2015 02:54 AM, enh wrote: > http://man7.org/linux/man-pages/man2/utimensat.2.html > > If both tv_nsec fields are specified as UTIME_OMIT, then no file > ownership or permission checks are performed, and the file timestamps > are not modified, but other error conditions may still be detected. > > fs/utimes.c > > /* Nothing to do, we must not even check the path. */ > if (tstimes[0].tv_nsec == UTIME_OMIT && > tstimes[1].tv_nsec == UTIME_OMIT) > return 0; > > if you're intending to match POSIX, the current text is correct. they > say "If both tv_nsec fields are set to UTIME_OMIT, no ownership or > permissions check shall be performed for the file, but other error > conditions may still be detected (including [EACCES] errors related to > the path prefix)." > (http://pubs.opengroup.org/onlinepubs/9699919799/functions/futimens.html) > > but as the snippet above shows, the kernel doesn't, and so will > silently return success even for a non-existent file if both times are > UTIME_OMIT. That's an interesting detail. I'm not sure if it was intended, but I agree with what you seem to be saying: it doesn't violate the standard. > > i'm not sure why the kernel says "must", but even if they fix that, it Here's my hypothesis. The implementer of the system call was a German speaker. The German verb "müssen" can be translated as "must" or "to need to". But, while the verb seems like English "must", it's also a "false friend", as language teachers sometimes say. "Wir müessen nicht" might seem like it it means "We must not" but rather, it means "We need not", which clearly is different. I suspect that the author of the code and comments made an English language misstep in the comment. (It's not every day that I get to apply my mediocre German to man-pages work!) > would be useful for the man page to mention this subtlety. something > like "(Note that at present the kernel performs no checks in this > case, not even that the file exists, and will always return > success.)". (strictly that's not true because the EFAULT check for the > struct timespec* will have happened, but it seems less clear to try to > include that fact.) I'm reluctant to note that point too prominently, since it's obviously not the kind of thing we want to encourage user-space programmers to do, but at the bottom of NOTES, I added: If both tv_nsec fields are specified as UTIME_OMIT, then the Linux implementation of utimensat() succeeds even if the file referred to by dirfd and pathname does not exist. Okay? Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
