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@aerifal.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
>
>     ¶4. 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
>