Path: utzoo!attcan!telly!nebulus!druid!darcy
From: darcy@druid.uucp (D'Arcy J.M. Cain)
Newsgroups: comp.unix.questions
Subject: Re: sparse files
Message-ID: <1989Dec10.170841.26798@druid.uucp>
Date: 10 Dec 89 17:08:41 GMT
References: <21581@adm.BRL.MIL> <235@dg.dg.com> <2700@auspex.auspex.com>
Reply-To: darcy@druid.UUCP (D'Arcy J.M. Cain)
Organization: D'Arcy Cain Consulting, West Hill, Ontario
Lines: 43

In article <2700@auspex.auspex.com> guy@auspex.auspex.com (Guy Harris) writes:
>>UNIX treats the "holes" as 0's when read. In fact, UNIX has only
>>minimal support for sparse files.  Backing up sparse files often
>>involves copying large amounts of nulls.  Once an area of a file is
>>written, it cannot be returned to its previous sparse state.
>
>Not in general, anyway.  At least the first version of AIX for the RT PC
>claimed, in its documentation, that it had an "fclear()" call to punch
>holes in files; I think this may show up in future releases of other
>UNIXes as well.
>
Seems simple enough to write a utility.  The core would be something like
the following:

	file_pointer = 0L;
	skip_space = 0;

	while ((c = fgetc(in_fp)) != EOF)
	{
		if (skip_space)
		{
			fseek(out_fp, file_pointer, SEEK_SET);
			skip_space = 0;
		}

		if (c)
			fputc(c, out_fp);
		else
			skipspace = TRUE;

		file_pointer++;

	}

Of course this is just off the cuff and probably needs some fleshing out
and optimizing but I think it would work on any system supporting sparse
files that return nulls for the empty parts.

-- 
D'Arcy J.M. Cain (darcy@druid)     |   "You mean druid wasn't taken yet???"
D'Arcy Cain Consulting             |                    - Everybody -
West Hill, Ontario, Canada         |
No disclaimers.  I agree with me   |