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SubjectRe: generic strncpy - off-by-one error
Date
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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