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Message-ID: <CAKHv7pgYaJhreKAVX84c79=jMMX+d_MRv+=AZBmJ5w5eAauQhQ@mail.gmail.com>
Date: Wed, 24 Oct 2012 23:54:17 +0200
From: Paul Schutte <sjpschutte@...il.com>
To: musl@...ts.openwall.com
Subject: Re: Possible file stream bug
Thanks Rich !
I was thinking in terms of file descriptors which is only integers and not
whole data structures.
Sorry for wasting your time.
BTW
Musl is really awesome !
On Wed, Oct 24, 2012 at 11:25 PM, Rich Felker <dalias@...ifal.cx> wrote:
> On Wed, Oct 24, 2012 at 11:22:13PM +0200, Paul Schutte wrote:
> > Hi,
> >
> > It is not my code, but I can not see why it is invalid.
>
> C99 7.19.3 Files
>
> ΒΆ4. A file may be disassociated from a controlling stream by
> closing the file. Output streams are flushed (any unwritten buffer
> contents are transmitted to the host environment) before the
> stream is disassociated from the file. The value of a pointer to a
> FILE object is indeterminate after the associated file is closed
> (including the standard text streams). Whether a file of zero
> length (on which no characters have been written by an output
> stream) actually exists is implementation-defined.
>
> Since the value of the pointer-to-FILE is indeterminate after fclose,
> any use of it results in undefined behavior.
>
> The error you've encountered is analogous to the error of calling
> free() on memory obtained by malloc(), then later calling realloc() on
> the already-freed pointer to try to get it back. This is not the
> purpose of realloc (or freopen), and fundamentally cannot work in
> general, since the pointed-to memory could already have been reclaimed
> for another use. In both cases (realloc and reopen), the function must
> be used on an object that is still valid, not one which has already
> been closed/freed.
>
> The fact that it happens to work on glibc is purely luck (good or bad
> luck, depending on your perspective). This is the nature of undefined
> behavior - it can lead to your program doing what you wanted it to,
> even though the program is wrong.
>
> Rich
>
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