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Date: Sun, 9 Sep 2012 23:37:44 +0100
From: Justin Cormack <>
Subject: Re: capset() capget() syscalls

On 9 Sep 2012 23:26, "Rich Felker" <> wrote:
> On Sun, Sep 09, 2012 at 11:12:01PM +0100, Justin Cormack wrote:
> > On Sun, Sep 9, 2012 at 10:55 PM, Rich Felker <> wrote:
> > > With something like futex that's a trivial syscall, I'm not sure what
> > > qualifies it for a separate lib rather than inclusion in libc. The
> > > library would be <20 bytes of code on most archs... That's not to say
> > > I think we have to expose it in musl, but I think using a separate
> > > library (especially if there's any chance of it being built as a .so
> > > rather than just .a) is a worse choice than just writing
> > > syscall(SYS_futex, ...) in your source...
> >
> > The manpage says
> >
> >        To reiterate, bare futexes are not intended as an easy-to-use
> > abstraction for end-users.  (There
> >        is no wrapper function for this system call in glibc.)
> > Implementors are expected to be assembly
> >        literate and to have read the sources of the futex userspace
> > library referenced below.
> >
> > I thought (from memory) the non-contended case required architecture
> > specific assembly, the syscall just dealt with contended case. So the
> > library needs to provide the other stuff to make it usable. Its not a
> > lot, and must be there to use these for threading I guess, but doesnt
> > have standard names?
> "GNU C" (GCC-specific) or C11 atomics will work perfectly well in
> place of assembly; the whole point of exposing the futex syscall to
> the application would be for it to do its own waiting in conjunction
> with atomics (whether implemented with GNUC, C11, or asm). The futex
> API is mainly for waiting on locks or waking waiters; for the most
> part it does not even provide an API for obtaining locks, although the
> prio-inheritance stuff can provide full locking in kernelspace, albeit
> very inefficiently. We're talking 200x slower than calling
> pthread_mutex_lock, so it would be utterly useless..
> Rich

I should make myself a working example then. It sounds rather simpler than
the wording on the man page...


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