Re: generic strncpy - off-by-one error

[Anthony Truong <Anthony.Truong@mascorp.com>]

On Wed, 2003-08-13 at 13:18, Willy Tarreau wrote:

On Tue, Aug 12, 2003 at 11:38:31PM -0400, Albert Cahalan wrote:
> 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.

I often noticed that using '++' and '--' within or just before
assignments
and/or comparisons often break the code and make it suboptimal. C
provides
enough flexibility to code what you think nearly at the instruction
level.
Since 'while' loops often start with a jump to the end, you can
sometimes help
the compiler by enclosing them within an 'if' statement such as below.
BTW, in
your case, count ends with -1.

I've absolutely not tried this one, but it could produce different code
on your
PPC, and can trivially be derived to cleaner constructs. I proceeded the
same
way when I wrote my own optimized strlcpy() implementation which is 45
bytes
long and copies 1 char per CPU cycle on i686.

char *strncpy(char *dest, const char *src, size_t count)
{
   if (count) {
      char *tmp = dest;
      while (1) {
         *tmp = *src;
         if (*src) src++;
         tmp++;
         if (!count--) break;
      }
   }
   return dest;
}

Cheers,
Willy


Hello,
This reminds me of my days in school learning how to program in C/C++. 
I first learnt that in
  while (count-- && (*dest++ = *src++));
  the first operand of && will be evaluated first, and then the second
  iff the first is evaluated to TRUE.
  count-- : count will be checked to see if it is not 0 first, and if it
  is not, it will get decremented.  If it is, the while loop is ended.
  Therefore, how could count go to -1?
If there is a bug in the gcc, then should we fix it rather than messing 
with our coding trying to please it?
We don't have to do if (count) before the loop because a count of 0 is 
already caught in while (count-- ......).

Again, maybe what I learnt in school about C/C++ programming is all
wrong.:-)

:-)
Anthony Dominic Truong.




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