Subject: Re: generic strncpy - off-by-one error
From: Albert Cahalan <albert@users.sf.net>
To: linux-kernel mailing list <linux-kernel@vger.kernel.org>
Cc: bernd@firmix.at, Anthony.Truong@mascorp.com, alan@lxorguk.ukuu.org.uk,
       schwab@suse.de, ysato@users.sourceforge.jp, willy@w.ods.org,
       Valdis.Kletnieks@vt.edu, william.gallafent@virgin.net
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Message-Id: <1060741101.948.245.camel@cube>
Mime-Version: 1.0
Date: 12 Aug 2003 22:18:21 -0400
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Sender: linux-kernel-owner@vger.kernel.org
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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;
}

static char ref[32];
static char hmm[32];

static char s0[] = "";
static char s1[] = "1";
static char s2[] = "12";
static char s6[] = "123456";
static char s7[] = "1234567";
static char s8[] = "12345678";
static char s9[] = "123456789";

static void tester(const char *src, size_t count){
  memset(ref,'%',sizeof ref);
  memset(hmm,'%',sizeof hmm);
  strncpy     (ref+2,src,count);
  strncpy_good(hmm+2,src,count);
  if(memcmp(ref,hmm,sizeof hmm)){
    printf("error @ size %d\n",(int)count);
  }
}  
   
int main(int argc, char *argv[]){
  int i = 15;
  while(i--){
    tester(s0,i);
    tester(s1,i);
    tester(s2,i);
    tester(s6,i);
    tester(s7,i);
    tester(s8,i);
    tester(s9,i);
  }
  return 0;
}
////////////////////////////////////////////////////////////


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