Date: Mon, 22 Nov 1999 18:42:32 -0500 (EST) From: Brian Fundakowski Feldman <green@FreeBSD.org> To: Zhihui Zhang <zzhang@cs.binghamton.edu> Cc: freebsd-hackers@FreeBSD.org Subject: Re: A file with holes - a bug? Message-ID: <Pine.BSF.4.10.9911221841190.34116-100000@green.myip.org> In-Reply-To: <Pine.GSO.3.96.991122133521.4039A-100000@sol.cs.binghamton.edu>
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On Mon, 22 Nov 1999, Zhihui Zhang wrote:
>
> Please take a look at the following piece of code that creates a large
> hole in a file named hole.dat. It tries to write 0x30-0x39 both at the
> front and the tail of that file, the hole is therefore in the middle.
>
> main()
> {
> char c;
> FILE * fp;
>
> fp = fopen("hole.dat", "w");
>
> for (c=0x30; c<0x3a; c++) fputc(c, fp);
> fputc('\n',fp);
> fflush(fp); /* XXX */
> lseek(fileno(fp), 3 * 8192, SEEK_CUR);
This should be fseek() and not lseek().
> for (c=0x30; c<0x3a; c++) fputc(c, fp);
> fputc('\n',fp);
> fclose(fp);
> }
>
> If I remove the fflush(fp), then the characters 0x30-0x39 will be all
> written at the end of the file (use hexdump to find out), not as expected
> (one at the beginning and the other at the end). It seems to me that the
> first for loop happens AFTER the lseek() statement without fflush(). Can
> anyone explain this to me? I am using FreeBSD 3.3-Release.
That's because you're not using fseek() like your should be using
for FILE * IO. Don't mix FILE *fp and int fd operations callously.
>
> By the way, I also find out if you copy a file with holes into another
> file, the holes in the first file will be replaced with 0s in the second
> file, taking more disk space (check with du). Is there a better solution
> for this?
>
> Any help is appreciated.
>
> -Zhihui
>
--
Brian Fundakowski Feldman \ FreeBSD: The Power to Serve! /
green@FreeBSD.org `------------------------------'
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