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

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Message-ID: <20250426104520.GD17549@1wt.eu>
Date: Sat, 26 Apr 2025 12:45:20 +0200
From: Willy Tarreau <w@....eu>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
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 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;

> 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:

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

Willy