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Subject: Re: [for-next][PATCH 23/24] string.h: Add strncmp_prefix() helper
 macro
From:   Joe Perches <joe@perches.com>
To:     Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.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>,
        Tom Zanussi <zanussi@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Date:   Fri, 21 Dec 2018 12:46:47 -0800
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On Fri, 2018-12-21 at 15:35 -0500, Steven Rostedt wrote:
> At first I thought this would have issues, but with a slight change...
> 
> #define have_prefix(str, prefix) ({ \
> 	const char *__pfx = (const char *)prefix; \
> 
> 
> And the rest the same, it appears to work.
> 
> Need the cast because if for some reason someone passed in something
> like "const unsigned char" then it wouldn't work. But that's just a nit.
> 
> So something like this then?
> 
> -- Steve
> 
> diff --git a/include/linux/string.h b/include/linux/string.h
[]
> @@ -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?

> + * @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.