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Date: Thu, 27 Aug 2020 11:27:59 +0200
From: Jens Gustedt <>
Subject: Re: direct coding of asctime_r


on Mon, 24 Aug 2020 12:14:00 -0400 you (Rich Felker <>)

> I'm not *strongly* opposed to this, but my reasoning is fairly much in
> line with the POSIX side, that these interfaces are legacy/deprecated,
> and in general musl practice is to choose maximum simplicity over
> size/performance optimality for deprecated/legacy or junk interfaces.
> In particular, asctime[_r] formats dates in a legacy US format,
> whereas modern applications should be using either ISO date format or
> a locale-specific format.

But which is also a format used by the language itself (or refered to)
by `__TIME__` and similar macros.

> Note that ISO C specifies asctime in terms of a particular printf
> format string, meaning the results are well-defined for any values
> that don't overflow the specified buffer, even if they are somewhat
> nonsensical.

I don't think so. The general rules for valid arguments to C library
functions always apply, so according to 7.1.4 calls to the functions
with values that are outside the specified ranges for the type have

In the <time.h> header the only exception from this rule seems to be
`mktime`, which makes such exceptions explicit and says how the
argument is normalized if it is not in the ranges as specified.

The sample code that I posted does range checks with simple means that
never results in unbounded UB and always returns a string that is null
terminated. I would think that this is reasonable behavior.


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