Re: D.E. Knuth's strlen

[Keith Thompson]

jacob navia <ja***@nospam.comwrites:
[...]

Knuth's strlen then, looks like this.

#include <stdio.h>
#include <string.h>
#define H 0x8080808080808080ULL
#define L 0x0101010101010101ULL

H and L are used only within myStrlen; I'd declare them inside the
function. Yes, I know macro definitions aren't scoped that way, but
it's useful for documentation.

size_t myStrlen(char *s)
{

[snip]

}

#ifdef TEST
int main(int argc,char *argv[])
{
char *str = "The lazy fox jumped over the slow dog";

if (argc 1) {
str = argv[1];
}
printf(
"Strlen of '%s' is %d (%d)\n",
str,strlen(str),myStrlen(str));

You're using a %d format for a size_t value.

And I'd add a "return 0;" here, even though it isn't strictly
necessary in C99.

}
#endif

As I mentioned elsethread, the algorithm (which I snipped here) is not
portable due to alignment issues, and I believe there's no way to make
it 100% portable. But there are *mostly* portable tricks that can
detect pointer alignment on *most* systems (and testing can detect
systems where the tricks don't work). And, of course, an
implementation of strlen() doesn't have to be written in 100% portable
C; it can use whatever system-specific tricks it likes.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Ian Collins]

Keith Thompson wrote:

jacob navia <ja***@nospam.comwrites:
[...]

>Knuth's strlen then, looks like this.

#include <stdio.h>
#include <string.h>
#define H 0x8080808080808080ULL
#define L 0x0101010101010101ULL

H and L are used only within myStrlen; I'd declare them inside the
function. Yes, I know macro definitions aren't scoped that way, but
it's useful for documentation.

So make them const unsigned long long.

--
Ian Collins

[Keith Thompson]

Ian Collins <ia******@hotmail.comwrites:

Keith Thompson wrote:

>jacob navia <ja***@nospam.comwrites:
[...]

>>Knuth's strlen then, looks like this.

#include <stdio.h>
#include <string.h>
#define H 0x8080808080808080ULL
#define L 0x0101010101010101ULL

H and L are used only within myStrlen; I'd declare them inside the
function. Yes, I know macro definitions aren't scoped that way, but
it's useful for documentation.

So make them const unsigned long long.

Yes, of course. There are cases where a declaration of a const object
can't replace a macro definition; I should have realized this isn't
one of them.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"