Re: generic strncpy - off-by-one error

[Nick Piggin]

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.


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