Re: Syslets, Threadlets, generic AIO support, v6

[Davide Libenzi <davidel@xmailserver.org>]

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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