Date: Mon, 22 Nov 1999 16:26:44 -0800 (PST) From: Alfred Perlstein <bright@wintelcom.net> To: Brian Fundakowski Feldman <green@FreeBSD.ORG> Cc: Zhihui Zhang <zzhang@cs.binghamton.edu>, freebsd-hackers@FreeBSD.ORG Subject: Re: A file with holes - a bug? Message-ID: <Pine.BSF.4.21.9911221621320.4557-100000@fw.wintelcom.net> In-Reply-To: <Pine.BSF.4.10.9911221841190.34116-100000@green.myip.org>
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On Mon, 22 Nov 1999, Brian Fundakowski Feldman wrote:
> 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.
Brian is right, a long while back it took me _forever_ to figure
out that the reason I was having a ton of trouble (nulls appearing
in a file) was that one part of the program was using stdio and
creating the file, then it would hand it off to another part which
used direct io, however I wasn't fflush()'ing or fclose()'ing the
FILE before handing it off. Did I mention that the problem was
difficult to reproduce because the dataset would change thereby
masking the problem more often than not?
*doh*
yah, don't mix stdio and direct io. :)
-Alfred
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