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SubjectRe: Syslets, Threadlets, generic AIO support, v6
On Wed, 30 May 2007, Linus Torvalds wrote:

> On Wed, 30 May 2007, Davide Libenzi wrote:
> >
> > Here I think we are forgetting that glibc is userspace and there's no
> > separation between the application code and glibc code. An application
> > linking to glibc can break glibc in thousand ways, indipendently from fds
> > or not fds. Like complaining that glibc is broken because printf()
> > suddendly does not work anymore ;)
>
> No, Davide, the problem is that some applications depend on getting
> _specific_ file descriptors.
>
> For example, if you do
>
> close(0);
> .. something else ..
> if (open("myfile", O_RDONLY) < 0)
> exit(1);
>
> you can (and should) depend on the open returning zero.
>
> So library routines *must not* open file descriptors in the normal space.
>
> (The same is true of real applications doing the equivalent of
>
> for (i = 0; i < NR_OPEN; i++)
> close(i);
>
> to clean up all file descriptors before doing something new. And yes, I
> think it was bash that used to *literally* do something like that a long
> time ago.

Right. I misunderstood Uli and Ingo. I thought it was like trying to
protect glibc from intentional application mis-behaviour.



> Another example of the same thing: people open file descriptors and know
> that they'll be "dense" in the result, and then use "select()" on them.
>
> So it's true that file descriptors can't be used randomly by the standard
> libraries - they'd need to have some kind of separate "private space".
>
> Which *could* be something as simple as saying "bit 30 in the file
> descriptor specifies a separate fd space" along with some flags to make
> open and friends return those separate fd's. That makes them useless for
> "select()" (which assumes a flat address space, of course), but would be
> useful for just about anything else.

I think it can be solved in a few ways. Yours or Ingo's (or something
else) can work, to solve the above "legacy" fd space expectations.



- Davide


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