Date: Sun, 1 Nov 1998 22:04:02 +0000 (GMT) From: Terry Lambert <tlambert@primenet.com> To: Arjan.deVet@adv.iae.nl (Arjan de Vet) Cc: hackers@FreeBSD.ORG Subject: Re: Possible bug in freopen()? Message-ID: <199811012204.PAA27908@usr05.primenet.com> In-Reply-To: <199810302139.WAA13889@adv.iae.nl> from "Arjan de Vet" at Oct 30, 98 10:39:02 pm
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> I may have found a bug in freopen() while testing INN 2.2-stable.
>
> Consider the following program:
>
> #include <stdio.h>
> #include <unistd.h>
>
> main () {
> FILE *f, *g;
> long i;
>
> g = fopen("/tmp/test", "a");
> f = freopen("/tmp/test", "a", g);
> i = ftell(f);
> printf("%d\n", i);
> fprintf(f, "test");
> i = ftell(f);
> printf("%d\n", i);
> close(f);
> }
>
> Start with an empty /tmp/test file and run the program three times
> consecutively. The results on BSD/OS 3.0, FreeBSD 2.2.7-stable and Solaris
> 2.6 are:
>
> BSDI 3.0 FreeBSD 2.2.7 Solaris
> -----------------------------------------------------------------------------
> 0 0 0
> 4 4 4
>
> 0 0 4
> 8 4 8
>
> 0 0 8
> 12 4 12
>
> Hmm... Quite different. I think Solaris shows the correct behaviour. In each
> case /tmp/test contains "testtesttest" after the running the program three
> times.
Realize that stdio buffers are not required to be flushed to disk
unless you explicitly call fclose on the FILE's.
Notice, also, that you are calling "close" on "f", which does nothing.
Also, you should not expect that an fopen and a freopen on the same
file will use the same user space stdio buffer space, merely because
the are connected to the same file. The buffer space is set
associative with the FILE, not with the underlying fd.
That said, if you correct your program to do the right fclose, it
looks as if:
``a'' Open for writing. The file is created if it does not
exist. The stream is positioned at the end of the file.
Is not being obeyed by the freopen() in either BSDI or FreeBSD, and
that the seek is delayed until after the write attempt, so the first
ftell fails on both FreeBSD and BSDI. So it looks like a missing
set of the offset to the file size.
It's interesting to note that both Solaris and BSDI do the write,
even though you don't properly close the file. This is probably
bogus, since it implies at best that they are resource tracking
the stdio buffers in an implementation dependent way, and at
worst that their stdio isn't very efficient.
Terry Lambert
terry@lambert.org
---
Any opinions in this posting are my own and not those of my present
or previous employers.
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