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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 21 Dec 2018 12:01:48 -0800
Message-ID: <CAHk-=wgSvRhj6KNRdn9gbK9RJVAswdtKSNjAxdf9O9vn04T5XQ@mail.gmail.com>
Subject: Re: [for-next][PATCH 23/24] string.h: Add strncmp_prefix() helper macro
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Joe Perches <joe@perches.com>,
        Tom Zanussi <zanussi@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On Fri, Dec 21, 2018 at 11:40 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> OK, what about if we just use strlen() and say that this macro is not
> safe for parameters with side effects.

I think gcc follows simple assignments just fine, and does the
optimized strlen() for them too.

So why not just do

   #define have_prefix(str,prefix) ({ \
        const char *__pfx = prefix; \
        size_t __pfxlen = strlen(__pfx); \
        strncmp(str, __pfx, __pfxlen) ? 0 : __pfxlen); })

and be done with it safely?

The above is ENTIRELY untested.

      Linus