Re: Syslets, Threadlets, generic AIO support, v6

[Eric Dumazet <dada1@cosmosbay.com>]

Linus Torvalds a écrit :
> 
> 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.
> 

Fix the application, and not adding kernel bloat ?

> 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.

Then you can also exclude multi-threading, since a thread (even not inside 
glibc) can also use socket()/pipe()/open()/whatever and take the zero file 
descriptor as well.

Frankly I dont buy this fd namespace stuff.

The only hardcoded thing in Unix is 0, 1 and 2 fds.
People usually take care of these, or should use a Microsoft OS.

POSIX mandates that open() returns the lowest available fd.
But this obviously works only if you dont have another thread messing with 
fds, or if you dont call a library function that opens a file.

Thats all.

> 
> 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);

Quite buggy IMHO

This hack was to avoid bugs coming from ancestors applications, 
forking/execing a shell, and at times where one process could not open more 
than 20 files (AT&T Unix, 21 years ago)

Unix has fcntl(fd, F_SETFD, FD_CLOEXEC). A library should use this to make 
sure fd is not propagated at exec() time.

> 
> 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.
> 
> 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.

poll() is nice. Even AT&T Unix had it 21 years ago :)

> 
> 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.
> 

Please dont do that. Second class fds.

Then what about having ten different shared libraries ? Third class fds ?