Re: light weight user level semaphores

Rogier Wolff
2001-04-19T22:36:53Z

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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