Re: [for-next][PATCH 23/24] string.h: Add strncmp_prefix() helper macro

Joe Perches
2018-12-21T20:46:54Z

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.