Re: generic strncpy - off-by-one error

Nick Piggin
2003-08-13T03:57:54Z

Albert Cahalan wrote:

>On Tue, 2003-08-12 at 22:47, Erik Andersen wrote:
>
>>On Tue Aug 12, 2003 at 10:18:21PM -0400, Albert Cahalan wrote:
>>
>>>You're all wrong. This is some kind of programming
>>>test for sure!
>>>
>>>Let us imagine that glibc has a correct version.
>>>By exhaustive testing, I found a version that works.
>>>Here it is, along with the test code:
>>>
>>>//////////////////////////////////////////////////////
>>>#define _GNU_SOURCE
>>>#include <string.h>
>>>#include <stdlib.h>
>>>#include <stdio.h>
>>>
>>>// first correct implementation!
>>>char * strncpy_good(char *dest, const char *src, size_t count){
>>> char *tmp = dest;
>>> memset(dest,'\0',count);
>>> while (count-- && (*tmp++ = *src++))
>>> ;
>>> return dest;
>>>}
>>>
>>char *strncpy(char * s1, const char * s2, size_t n)
>>{
>> register char *s = s1;
>> while (n) {
>> if ((*s = *s2) != 0) s2++;
>> ++s;
>> --n;
>> }
>> return s1;
>>}
>>
>
>That's excellent. On ppc I count 12 instructions,
>4 of which would go away for typical usage if inlined.
>Annoyingly, gcc doesn't get the same assembly from my
>attempt at that general idea:
>
>char * strncpy_5(char *dest, const char *src, size_t count){
> char *tmp = dest;
> while (count--){
> if(( *tmp++ = *src )) src++;
> }
> return dest;
>}
>
>I suppose that gcc could use a bug report.
>

Its has different semantics though. Well taken as a whole they
are the same. When your loop finishes, count will be -1.