Re: [iproute PATCH 1/6] utils: Implement strlcpy() and strlcat()

On Wed, 6 Sep 2017 13:59:27 +0000
David Laight [off-list ref] wrote:

From: Stephen Hemminger [mailto:stephen@networkplumber.org]

quoted

Sent: 04 September 2017 19:25
On Mon, 4 Sep 2017 17:00:15 +0200
Phil Sutter [off-list ref] wrote:
  

quoted

On Mon, Sep 04, 2017 at 02:49:20PM +0000, David Laight wrote:  

quoted

From: Phil Sutter  

quoted

Sent: 01 September 2017 17:53
By making use of strncpy(), both implementations are really simple so
there is no need to add libbsd as additional dependency.
 

...  

quoted

+
+size_t strlcpy(char *dst, const char *src, size_t size)
+{
+	if (size) {
+		strncpy(dst, src, size - 1);
+		dst[size - 1] = '\0';
+	}
+	return strlen(src);
+}  

Except that isn't really strlcpy().
Better would be:
	len = strlen(src) + 1;
	if (len <= size)
		memcpy(dst, src, len);
	else if (size) {
		dst[size - 1] = 0;
		memcpy(dst, src, size - 1);
	}
	return len - 1;  

Please elaborate: Why isn't my version "really" strlcpy()? Why is your
proposed version better?

Thanks, Phil  

Linux kernel:
size_t strlcpy(char *dest, const char *src, size_t size)
{
	size_t ret = strlen(src);

	if (size) {
		size_t len = (ret >= size) ? size - 1 : ret;
		memcpy(dest, src, len);
		dest[len] = '\0';
	}
	return ret;
}

FreeBSD:
size_t
strlcpy(char * __restrict dst, const char * __restrict src, size_t dsize)
{
	const char *osrc = src;
	size_t nleft = dsize;

	/* Copy as many bytes as will fit. */
	if (nleft != 0) {
		while (--nleft != 0) {
			if ((*dst++ = *src++) == '\0')
				break;
		}
	}

	/* Not enough room in dst, add NUL and traverse rest of src. */
	if (nleft == 0) {
		if (dsize != 0)
			*dst = '\0';		/* NUL-terminate dst */
		while (*src++)
			;
	}

	return(src - osrc - 1);	/* count does not include NUL */
}


They all give the same results for some basic tests.
Test			FreeBSD		Linux		Iproute2
"",0:           	0 "JUNK"      	0 "JUNK"      	0 "JUNK"
"",1:           	0 ""          	0 ""          	0 ""
"",8:           	0 ""          	0 ""          	0 ""
"foo",0:        	3 "JUNK"      	3 "JUNK"      	3 "JUNK"
"foo",3:        	3 "fo"        	3 "fo"        	3 "fo"
"foo",4:        	3 "foo"       	3 "foo"       	3 "foo"
"foo",8:        	3 "foo"       	3 "foo"       	3 "foo"
"longstring",0: 	10 "JUNK"     	10 "JUNK"     	10 "JUNK"
"longstring",8: 	10 "longstr"  	10 "longstr"  	10 "longstr"  

You need to look at the contents of the destination buffer after the
first '\0'.
strlcpy() shouldn't change it.

	David

Zeroing the bytes after the first null character should not be a big issue
other than a few nanoseconds extra work.