Re: light weight user level semaphores

[R.E.Wolff@BitWizard.nl (Rogier Wolff)]

Alan Cox wrote:
> > > libc is entitled to, and most definitely does exactly that. Take a look at
> > > things like gethostent, getpwent etc etc.
> > 
> > Ehh.. I will bet you $10 USD that if libc allocates the next file
> > descriptor on the first "malloc()" in user space (in order to use the
> > semaphores for mm protection), programs _will_ break.
> > 
> > You want to take the bet?
> 
> Its not normally a good idea to take a Linus bet, but this time Im obviously
> missing something. fd0-2 will be passed in (and if not then shit already
> happens - see old bugtraq on the matter for setuid apps, glibc bugs)
> 
> So the C library gets fd 3
> My first fopen gets fd 4.

Code may 
	close (0);
	close (1);
	close (2); 
	... 
	malloc (); 

	/* Now open our controlling TTY/ stdin .. */ 
	fd = open (... ) ;

After taking care of this (*), problem I find the fd trick WAY more
appealing than Linus' magic numbers. With file descriptors we have a
"small integer which can be validated quickly". We also have storage
for a private pointer somewhere in the fd structure. 

If people are TOO afraid of breaking something, creating a new set of
small integers handled similarly as "fds" would do fine. (Maybe here
we'd allocate just a few, and reallocate when neccesary).

Roger. 

(*) I bet that 
	get_sem_fd () 
	{
	int rv;
	int fd; 
	fd = get_fd ();
	if (fd < 5) {
		rv = get_sem_fd ();
		close(fd);
	  	fd = rv; 
	  } 
	return fd; 
	}

will not break much. (UGLY coding. Don't tell me.)

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