Re: [PATCH 1/2] lib/string: add memrchr function

[Randy Dunlap]

On 2/13/19 7:48 PM, Xiang Xiao wrote:
> Here is the detailed description for memrchr:
> 
> void *memrchr(const void *s, int c, size_t n);
> 
> The memrchr() function is like the memchr() function, except
> that it searches backward from the end of the n bytes pointed
> to by s instead of forward from the beginning.
> 
> The memrchr() functions return a pointer to the matching byte
> or NULL if the character does not occur in the given memory
> area.
> 
> Signed-off-by: Xiang Xiao <xiaoxiang@xxxxxxxxxx>
> ---
>  include/linux/string.h |  1 +
>  lib/string.c           | 21 +++++++++++++++++++++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/include/linux/string.h b/include/linux/string.h
> index 7927b87..f380f4b 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -167,6 +167,7 @@ static inline void memcpy_flushcache(void *dst, const void *src, size_t cnt)
>  	memcpy(dst, src, cnt);
>  }
>  #endif
> +void *memrchr(const void *s, int c, size_t n);
>  void *memchr_inv(const void *s, int c, size_t n);
>  char *strreplace(char *s, char old, char new);
>  
> diff --git a/lib/string.c b/lib/string.c
> index 38e4ca0..92914f6 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -964,6 +964,27 @@ void *memchr(const void *s, int c, size_t n)
>  EXPORT_SYMBOL(memchr);
>  #endif
>  
> +/**
> + * memrchr - Find a character in an area of memory.

This should say up front that this is a reverse search, from the end of
the memory area backwards.

> + * @s: The memory area
> + * @c: The byte to search for
> + * @n: The size of the area.
> + *
> + * returns the address of the last occurrence of @c, or %NULL

    * Return: the address of the last occurrence of @c, or %NULL

> + * if @c is not found
> + */
> +void *memrchr(const void *s, int c, size_t n)
> +{
> +	const unsigned char *p = s + n;
> +
> +	while (n-- != 0) {
> +		if ((unsigned char)c == *--p)
> +			return (void *)p;
> +	}
> +	return NULL;
> +}
> +EXPORT_SYMBOL(memrchr);
> +
>  static void *check_bytes8(const u8 *start, u8 value, unsigned int bytes)
>  {
>  	while (bytes) {
> 


-- 
~Randy