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Date: Fri, 5 Sep 2014 22:39:45 +0400 (MSK)
From: Alexander Monakov <>
Subject: Re: New static analysis results

On Fri, 5 Sep 2014, Rich Felker wrote:
> > > ctime.c:5
> > >     localtime(t) may return NULL, but that will cause UB in asctime
> > 
> > Yes, I need to look into what ctime should do in this case though...
> Found it:
> The ctime function
>   2 The ctime function converts the calendar time pointed to by timer
>   to local time in the form of a string. It is equivalent to
>            asctime(localtime(timer))
> The standard basically specifies the implementation, so it's clearly
> UB if localtime(t) would return a null pointer. Looks like no action
> is needed here; the most-desirable-behavior (crash) for UB happens
> automatically anyway.

I suspect what happened is, at some point localtime was not specified to
return NULL and set errno, and at that time it made perfect sense to specify
asctime as you quoted, and then at some later point localtime specification
was expanded with error cases, but asctime specification was not adjusted.
Is that possible?

It doesn't look very nice for a libc to invoke UB where it could easily
propagate error to the caller, but "that's exactly what the standard requires"
can't be argued with I guess.


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