Re: Quick close of all the open files.

[Valdis . Kletnieks]

On Wed, 07 Jun 2006 03:32:58 +0530, vamsi krishna said:
> > For bonus points, how did you verify that all the open files were closed?
> >
> I checked as follows I did printf("lowest fd = %d",fileno(tmpfile()));
> it prints 3

Which proves that file descriptor 3 was closed, so tmpfile() was able to
open it.  This certainly implies that fd 0, 1, 2 (connected to stdin,
stdout, and stderr streams of stdio) are still open, contrary to your
statement that *all* of them are closed.

If file descriptor 3 is closed, but 4 is open, what does tmpfile()
do? Hint - tmpfile() ends up invoking open(), and the manpage for that says:

       Given a pathname for a file, open() returns a file descriptor, a small,
       non-negative integer for  use  in  subsequent  system  calls  (read(2),
       write(2), lseek(2), fcntl(2), etc.).  The file descriptor returned by a
       successful call will be the lowest-numbered file  descriptor  not  cur-
       rently open for the process.

So.. explain why you think that "all files were closed"?  We know that 0..2
were open, and we know nothing about 4..1023.

This doesn't look like a kernel bug, you may want to continue the discussion
on one of the various "beginning Linux C programming" lists.

On Tue, 06 Jun 2006 23:03:38 BST, =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= said:
> > (Hint - what does that fp->_chain = stderr *really* do? ;)
> 
> As it touches the innards of the FILE type, it invokes undefined
> behavior.  Nothing that follows can be considered a bug.

Invoking undefined behavior is often considered a bug itself.  And it's
certainly happening in userspace.. so there's a userspace bug. ;)

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