Hi Thomas,

On Thu, May 15, 2014 at 1:34 AM, Thomas Gleixner [off-list ref] wrote:

On Wed, 14 May 2014, Carlos O'Donell wrote:

quoted

On 05/14/2014 03:03 PM, Michael Kerrisk (man-pages) wrote:

quoted

quoted

However, unless I'm sorely mistaken, the larger problem is that glibc
removed the futex() call entirely, so these man pages don't describe

I don't think futex() ever was in glibc--that's by design, and
completely understandable: no user-space application would want to
directly use futex(). (BTW, I mispoke in my earlier mail when I said I
wanted documentation suitable for "writers of library functions" -- I
meant suitable for "writers of *C library*".)

I fully agree with Michael here.

The futex() syscall was never exposed to userspace specifically because
it was an interface we did not want to support forever with a stable ABI.
The futex() syscall is an implementation detail that is shared between
the kernel and the writers of core runtimes for Linux.

Nonsense.

If we change that interface (aside of adding functionality or some new
error return) it would break the world and some more, simply because
out of the blue glibc-2.xx would stop to work on linux-3.yy.

Aside of that the futex syscall is used as a bare interface without
any glibc interaction:

 - It's handy to implement user space wait queues

 - It's (ab)used in very interesting ways by data base apps

 - It's (ab)used by some Java monstrosities.

Thanks for the education about user-space uses of futexes. I was unaware.

Nothing you care about and you really don't want to see the gory
details, but you have to accept that there is an universe which is
happy to deal with the raw syscalls instead of going through some ill
defined posix interfaces.

And that universe would love to have your documentation of
FUTEX_WAKE_BITSET and FUTEX_WAIT_BITSET ;-),

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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