linux-kernel - Re: [PATCH v3] string.h: Add str_has

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Date:   Sat, 22 Dec 2018 11:58:45 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joe Perches <joe@...ches.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>
Subject: Re: [PATCH v3] string.h: Add str_has_prefix() helper

* Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 21 Dec 2018 16:32:58 -0800
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > On Fri, Dec 21, 2018, 16:06 Steven Rostedt <rostedt@...dmis.org wrote:
> > 
> > > On Fri, 21 Dec 2018 18:13:16
> > >
> > > And I'll make a separate patch that adds:
> > >
> > > static __always_inline bool
> > > str_has_prefix_len(const char *str, const char *prefix, unsigned int *len)
> > 
> > 
> > Why would this ever be a good idea? What's the advantage over returning the
> > length?
> 
> Style?
> 
> I was just thinking that some people (like Joe) think it's in bad taste
> to have:
> 
> 	if ((len = str_has_prefix(str, "const"))) {
> 
> and it might look better to have:
> 
> 	if (str_has_prefix_len(str, "const", &len)) {
> 
> Honestly, I'm good with either and don't really have a preference.

The first one is infinitely more readable and less ambiguous than a 
random series of arguments with unknown semantics for 'len': does 'len' 
have to be pre-initialized or does it always get set by the function, is 
the 'len' return always the same as the str_has_prefix_len() return value 
or is it a separate error code, etc.

I have no idea in what universe it's preferrable to pass it as an 
argument to a function.

We only punt return parameters to arguments when we are *forced* to, 
because there's too many of them, or there's some separate error and 
value path that cannot be encoded via any of the well-known pointer or 
integer encodings of errors, etc.

Thanks,

	Ingo