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


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