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From: poser@csli.Stanford.EDU (Bill Poser)
Newsgroups: comp.lang.c
Subject: Re: strncpy
Message-ID: <11882@csli.Stanford.EDU>
Date: 23 Jan 90 23:41:03 GMT
References: <e-mail_to_poser> <11527@csli.Stanford.EDU> <000003Q@cdis-1.UUCP> <11616@csli.Stanford.EDU> <48314938.f69e@phobos.UUCP> <11864@csli.Stanford.EDU> <25BC3A32.3F5B@marob.masa.com> <11881@csli.Stanford.EDU>
Sender: poser@csli.Stanford.EDU (Bill Poser)
Reply-To: poser@csli.stanford.edu (Bill Poser)
Organization: Center for the Study of Language and Information, Stanford U.
Lines: 48


	As a further clarification of what strncpy(3) actually does,
let me point out that although it only COPIES up to the null byte in the
source string, it WRITES exactly N characters into the target string,
padding with nulls if necessary. So the bit about null-padding in the manual
is correct - what is erroneous is the bit about copying. To see this,
try a program like this:

#include <stdio.h>
main()
{
   char src[20];
   char tgt[20];

   sprintf(src,"%s","abcdefg");
   sprintf(tgt,"%s","abcdefg");
   printf("src = %s\n",src);
   printf("tgt = %s\n",tgt);
   src[3] = '\0';
   printf("src = %s\n",src);
   strncpy(tgt,src,5);
   printf("tgt = %s\n",tgt);
   printf("tgt[4] = %c\n",tgt[4]);
   printf("tgt[5+] = %s\n",&(tgt[5]));
   exit(0);
}

It initializes both src and tgt to "abcdefg", assigns a null to src[3],
does the strncpy, and then prints the various pieces of tgt. The
result is:

Script started on Tue Jan 23 15:34:49 1990
crystals-[1]/user2/poser
: foo
src = abcdefg
tgt = abcdefg
src = abc
tgt = abc
tgt[4] = 
tgt[5+] = fg
crystals-[2]/user2/poser
: 
script done on Tue Jan 23 15:34:57 1990

Notice that the fifth character of tgt, originally "e", has been
overwritten by a null, but that the remainder of tgt is unaffected.
So, what strncpy does is to COPY up to the null byte or N characters,
whichever comes first, and then null-pad out to the N character limit.