Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH] lib: string: reduce unnecessary loop in strncpy
To:     Liu Xiang <liuxiang_1999@126.com>, linux-kernel@vger.kernel.org
References: <1572444859-3687-1-git-send-email-liuxiang_1999@126.com>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <c6452618-0161-eefc-0730-b10eac823228@rasmusvillemoes.dk>
Date:   Wed, 30 Oct 2019 15:59:58 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.9.0
MIME-Version: 1.0
In-Reply-To: <1572444859-3687-1-git-send-email-liuxiang_1999@126.com>
Content-Type: text/plain; charset=windows-1252
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 30/10/2019 15.14, Liu Xiang wrote:
> Now in strncpy, even src[0] is 0, loop will execute count times until
> count is 0. It is better to exit the loop immediately when *src is 0.

Please read "man strncpy".

There's a reason the loop is written in that somewhat convoluted way:
The behavior of strncpy is mandated by the C standard, and if the src
string is shorter than the destination buffer, the rest must be
0-filled. So if we hit a nul byte before running out of count, we keep
copying that nul byte to the rest of the destination.

Rasmus