NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/816

Internet Message Format | 1996-08-05 | 2.4 KB

Path: mail2news.demon.co.uk!genesis.demon.co.uk
From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: Problem with stringcopy
Date: Tue, 09 Jan 96 13:57:59 GMT
Organization: none
Message-ID: <[e-mail]>
References: <4clguu$[e-mail]> <[e-mail]> <4cq9dr$[e-mail]> <4cqppv$[e-mail]>
Reply-To: [e-mail]
X-NNTP-Posting-Host: genesis.demon.co.uk
X-Newsreader: Demon Internet Simple News v1.27
X-Mail2News-Path: genesis.demon.co.uk

In article <4cqppv$[e-mail]> [e-mail] "William Hutto" writes:

>In article <4cq9dr$[e-mail]>,
>   [e-mail] (Ray Dunn) wrote:
>>In referenced article, Lawrence Kirby says...
>>>Morten Brun writes:
>>>
>>>>How do i do a partial stringcopy i.e. copy from a specific position
>>>>in a string and a certain numbers of bytes. I am looking for a
>>>>function like target = Stringcopy(source, startpos, length)
>>>
>>>target[0] = '\0';
>>>strncat(target, source+startpos, length);
>>
>>....but use strncpy if you are overwriting some existing characters in
>>target and don't want the target string to terminate after the copied
>>characters.

memcpy is more commonly used in situations like this (when you are dealing
with possibly non null-terminated sequences of characters you usually
have the length available separately).

>Considering strncpy()'s shortcomings, I amend my code:
>
>char destination[ > max_length];
>
>        strncpy(destination,&source[startpos],max_length);
>        destination[max_length]='\0';

But once you've got to that you're probably better off with the strncat
version unless you explicitly want the destination filled with trailing
null characters all the way up to destination+max_length.

strncpy works well when used for its intended purpose (writing to a
fixed width array where 'strings' shorter than the array size are padded
to the full length with null characters). Once such a 'semi-string' is
created it is a fixed width datastructure and is naturally copied around
with memcpy, compared with memcmp etc. C also directly supports outputting
such datastructures with printf("%.*s", width, semistring);

strncpy is rarely useful for anything else.

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
-----------------------------------------