[PATCH 3/6] string: reduce strjoin runtime, drop trailing separator

[Sascha Hauer]

On Wed, Oct 26, 2022 at 08:42:02AM +0200, Ahmad Fatoum wrote:
> The implementation of strjoin is a bit suboptimal. The destination
> string is traversed from the beginning due to strcat and we have a
> left-over separator at the end, while it should only be in-between.
> 
> Fix this.

Rather than fixing a just introduced function I would expect a patch
introducing strjoin() and then another one converting the time command
over to use it.

> +void *mempcpy(void *dest, const void *src, size_t count)
> +{
> +	return memcpy(dest, src, count) + count;
> +}
> +EXPORT_SYMBOL(mempcpy);
>  
>  #ifndef __HAVE_ARCH_MEMMOVE
>  /**
> @@ -943,7 +948,7 @@ char *strjoin(const char *separator, char **arr, size_t arrlen)
>  {
>  	size_t separatorlen;
>  	int len = 1; /* '\0' */
> -	char *buf;
> +	char *buf, *p;
>  	int i;
>  
>  	separatorlen = strlen(separator);
> @@ -951,12 +956,14 @@ char *strjoin(const char *separator, char **arr, size_t arrlen)
>  	for (i = 0; i < arrlen; i++)
>  		len += strlen(arr[i]) + separatorlen;
>  
> -	buf = xzalloc(len);
> +	p = buf = xmalloc(len);
>  
>  	for (i = 0; i < arrlen; i++) {
> -		strcat(buf, arr[i]);
> -		strcat(buf, separator);
> +		p = stpcpy(p, arr[i]);
> +		p = mempcpy(p, separator, separatorlen);
>  	}
>  
> +	p[-separatorlen] = '\0';

That's a bit hard to read. How about:

	for (i = 0; i < arrlen; i++) {
		p = stpcpy(p, arr[i]);
		if (i < arrlen - 1)
			p = stpcpy(p, separator);
	}

That would also allow you to optimize the allocated buffer size above.

Sascha

-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |