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Date: Thu, 04 Jul 2013 09:11:29 +0200
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: Re: Use of size_t and ssize_t in mseek
Am Donnerstag, den 04.07.2013, 02:37 -0400 schrieb Rich Felker:
> > unless one would finally implement them separately, of course
>
> Yes but that's a huge maintenance burden (duplicate functionality) and
> while it's less bloat for individual static apps that don't use Annex
> K, it's much more bloat for libc.a and libc.so.
sure, I mentioned this merely for completeness, I don't think that
this is a viable option
> No, I mean just the sheer volume of interfaces to add.
The number of interfaces wouldn't scare me too much, and it would be
relatively easy to start supporting that incrementally.
> qsort_s can store the comparison function and context in TLS, and then
> pass to qsort a comparison function that grabs these from TLS and
> calls the original comparison function with the context pointer. This
> is valid assuming qsort does not run the comparisons in new threads.
sure, but for an execution of qsort_s this would have a lot of
indirections and a call to TLS for every comparison. For performance
sensible functions like this, this doesn't sound very attractive.
(In P99 I do that with inlining and gcc shows to be able to expand all
comparisons in place and to optimize that smoothly.)
> TLS is not guaranteed to exist when these functions are called;
> programs not using any multi-threaded functionality are supposed to
> "basically work" on Linux 2.4. I don't mind having the Annex K
> functions depend on TLS, since only new programs will use them anyway,
> but I don't want to break existing programs.
My guess would be that you can alias the TLS variable that would
control that behavior to a simple global variable in the case of
absence of threads.
> What I was saying is that, in library code, you can't rely on this.
> The application may have installed a handler that causes the functions
> to just return an error, or the default implementation-defined handler
> might do so.
sure, but I don't see any problem in this. continuing execution is
one of the permitted path that a constraint handler may take. these
are user interfaces, not meant to be used internally by the C library,
I think.
> > > My feeling is that we should hold off on a decision about them to
> > > see if any applications actually start using them.
> >
> > I think we have a hen and egg problem, here. Nobody will use them if
> > nobody provides an implementation.
>
> You presume we would want people to use them. :) I don't. I think
> they're very poorly designed interfaces that were crammed into the
> standards process by their sponsor's clout rather than any technical
> merit of existing practice.
AFAIK they had some existing practice in the MS world
I think there are some of these interfaces that are not too bad, from
a user perspective these interfaces are relatively simple to use.
Especially qsort_s is nice and I also see advantages in being able to
inhibit certain dangerous printf or scanf formats.
> _FORTIFY_SOURCE solves pretty much the same problems these functions
> were intended to solve, but does a much better job since it doesn't
> rely on the application developer to provide truthful information
> about object sizes, and instead gets the compiler to do it.
In C, applications *have* to have a consistent view of the size of
their buffers. So I don't see much of a burden, here. I started to use
them (through my P99 implementation) and I didn't find anything
remarkably heavy.
Jens
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