linux-kernel - Re: [PATCH 11/15] tools/nolibc: add difftime()

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Message-ID: <20250428123809-70bb912e-51e0-4b4b-9870-a02e41a079fe@linutronix.de>
Date: Mon, 28 Apr 2025 12:41:19 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Willy Tarreau <w@....eu>
Cc: Thomas Weißschuh <linux@...ssschuh.net>, 
	Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 11/15] tools/nolibc: add difftime()

On Sat, Apr 26, 2025 at 12:45:20PM +0200, Willy Tarreau wrote:
> On Wed, Apr 23, 2025 at 05:01:41PM +0200, Thomas Weißschuh wrote:
> > This is used in various selftests and will be handy when integrating
> > those with nolibc.
> > 
> > Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
> > ---
> >  tools/include/nolibc/time.h                  | 7 +++++++
> >  tools/testing/selftests/nolibc/nolibc-test.c | 1 +
> >  2 files changed, 8 insertions(+)
> > 
> > diff --git a/tools/include/nolibc/time.h b/tools/include/nolibc/time.h
> > index 28a1549adb14e2087fa8fbdb7e9c35e1c3f22c2a..760133c574ece97165e3bba5616a387deaf07aff 100644
> > --- a/tools/include/nolibc/time.h
> > +++ b/tools/include/nolibc/time.h
> > @@ -105,6 +105,13 @@ int clock_settime(clockid_t clockid, struct timespec *tp)
> >  }
> >  
> >  
> > +static __inline__
> > +double difftime(time_t time1, time_t time2)
> > +{
> > +	return time1 - time2;
> > +}
> 
> Thanks for making me discover difftime(). I've never heard about it, and
> seeing it return a double looks totally weird, but that's how it is indeed.
> 
> In case time_t would be unsigned, this would return a large unsigned
> value. I think it could be more robust to explicitly cast the two
> inputs to long:
> 
> 	return (long)time1 - (long)time2;

time_t is explicitly defined as "signed long" in nolibc's std.h.
If we change it at some time in the future we will have to audit any usage site
of time_t in any case.

> > diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
> > index aed71de4b4f3dd1f183c7fc25e5a5cee466600ed..fd8bab42e75157967658690005bc9142360fc135 100644
> > --- a/tools/testing/selftests/nolibc/nolibc-test.c
> > +++ b/tools/testing/selftests/nolibc/nolibc-test.c
> > @@ -1423,6 +1423,7 @@ int run_stdlib(int min, int max)
> >  		CASE_TEST(toupper_noop);            EXPECT_EQ(1, toupper('A'), 'A'); break;
> >  		CASE_TEST(abs);                     EXPECT_EQ(1, abs(-10), 10); break;
> >  		CASE_TEST(abs_noop);                EXPECT_EQ(1, abs(10), 10); break;
> > +		CASE_TEST(difftime);                EXPECT_EQ(1, difftime(200., 100.), 100.); break;
> 
> Then here maybe test it in reverse to make sure the types don't cause trouble:

Ack.

> 		CASE_TEST(difftime);                EXPECT_EQ(1, difftime(100., 200.), -100.); break;