From:	Erik Max Francis <max AT alcyone DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: strncpy question
Date:	Sat, 25 Oct 1997 13:46:54 -0700
Organization:	Alcyone Systems
Lines:	35
Message-ID:	<34525ABE.54074B5A@alcyone.com>
References:	<199710240221 DOT MAA29326 AT rabble DOT uow DOT edu DOT au>
NNTP-Posting-Host:	newton.alcyone.com
Mime-Version:	1.0
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Brett Porter wrote:

> It started to overwrite other memory.
> I realise what I had done here was bad coding, and I've fixed it up,
> but I
> was curious what was going on here. IS the definition of strncpy in
> DJGPP to
> zero out up to the limit specified, or does it stop at the NULL
> character? I
> thought it was the latter but this example would seem to indicate
> otherwise.
> 
> No urgency in replying, I'm just curious

strncpy, contrary to what it might sound like from the name (namely that
it begins with str) does _not_ always give you a NUL-terminated,
possibly truncated, string.

strncpy (and its counterpart, strncat) is intended for padding strings
into fixed-size arrays.  The remainder of the string is NUL padded. 
ANSI C even has a footnote which mentions this (7.11.2.4, footnote 134):

    Thus, if there is no null character in the first n characters of the
    [string to be copied], the result will not be null-terminated.

Basically, don't use strncpy or strncat except in cases where you're
trying to fill a fixed-sized buffer, and do not care whether or not the
result is NUL-terminated (but _do_ care if it is NUL-padded).
-- 
         Erik Max Francis, &tSftDotIotE / mailto:max AT alcyone DOT com
                       Alcyone Systems / http://www.alcyone.com/max/
  San Jose, California, United States / icbm://+37.20.07/-121.53.38
                                     \
  "After each war there is a little / less democracy to save."
                                   / Brooks Atkinson

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