[tz] strftime and nonnormalized dates/times

Clive D.W. Feather clive at davros.org
Tue Jul 3 08:04:04 UTC 2018

Robert Elz said:
>   | But if strftime doesn't restrict the ranges of the various struct
>   | tm fields to their "normal" values, it makes me wonder how much
>   | latitude a push-the-boundaries programmer actually has when
>   | calling strftime.  Are the values as unlimited as they are when
>   | calling, say, mktime?
> What POSIX says is:
> 	If any of the specified values are outside the normal range,
> 	the characters stored are unspecified.
> which means that if you provide a value for tm_hour outside the 0..23
> range, what happens is anyone's guess (and you cannot complain.)

Not quite. There's an important difference between "unspecified" and

In this case, the resulting string could be anything but it must still be a
string and the call can't break anything else.

If it had been "undefined", then the call - or even the *existence* of the
call in your code - can cause the program to behave in any way the
implementation likes, including - for example - wiping or corrupting the
entire filesystem.

Clive D.W. Feather          | If you lie to the compiler,
Email: clive at davros.org     | it will get its revenge.
Web: http://www.davros.org  |   - Henry Spencer
Mobile: +44 7973 377646

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