linux-kernel - Re: [for-next][PATCH 23/24] string.h: Add strncmp

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Message-ID: <20181221155435.38a9a221@gandalf.local.home>
Date:   Fri, 21 Dec 2018 15:54:35 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [for-next][PATCH 23/24] string.h: Add strncmp_prefix() helper
 macro

On Fri, 21 Dec 2018 12:46:47 -0800
Joe Perches <joe@...ches.com> wrote:


> > @@ -14,6 +14,28 @@ extern void *memdup_user(const void __user *, size_t);
> >  extern void *vmemdup_user(const void __user *, size_t);
> >  extern void *memdup_user_nul(const void __user *, size_t);
> >  
> > +/**
> > + * have_prefix - Test if a string has a given prefix  
> 
> There is a naming mismatch of have/has here
> and has_prefix is probably too generic a name.
> 
> How about str_has_prefix?

Sure.

> 
> > + * @str: The string to test
> > + * @prefix: The string to see if @str starts with
> > + *
> > + * A common way to test a prefix of a string is to do:
> > + *  strncmp(str, prefix, sizeof(prefix) - 1)
> > + *
> > + * But this can lead to bugs due to typos, or if prefix is a pointer
> > + * and not a constant. Instead use has_prefix().
> > + *
> > + * Returns: 0 if @str does not start with @prefix
> > +         strlen(@prefix) if @str does start with @prefix
> > + */
> > +#define has_prefix(str, prefix)						\
> > +	({								\
> > +		const char *____prefix____ = (const char *)(prefix);	\
> > +		int ____len____ = strlen(____prefix____);		\
> > +		strncmp(str, ____prefix____, ____len____) == 0 ?	\
> > +			____len____ : 0;				\
> > +	})   
> 
> I think all the underscores are unnecessary and confusing.
> 

Well, perhaps I can just remove the ending ones. I get paranoid with
macro variables, and tend to over do it so that there's no question.

I don't find them confusing ;-)  But I tend to use a lot of macros.

-- Steve