How to use sem_timedwait with CLOCK_MONOTONIC?

Alexandre Bique bique.alexandre@gmail.com
Thu Feb 16 12:41:00 GMT 2017


On Thu, Feb 16, 2017 at 1:09 PM, Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
> On 16/02/2017 09:06, Alexandre Bique wrote:
>> Hi,
>>
>> I'm working on real-time audio project, and very often the libc
>> syscalls only supports CLOCK_REALTIME, where I would need
>> CLOCK_MONOTONIC to avoid issues with NTP time changes.
>>
>> Do you know any workaround sem_timedwait()?
>>
>> Many thanks,
>>
>
> POSIX sem_timedwait is not really implemented as a direct wrapper to a syscall,
> but it rather uses a combination of atomic operation and futex syscall. The main
> problem is 'sem_timedwait' is POSIX explicit states timeout argument should
> be explicit handled as CLOCK_REALTIME [1].
>
> It is already being tracked by BZ#14717 [2] and a possible solution is not that
> hard since futex syscall with FUTEX_WAIT_BITSET (which is used on sem_timedwait)
> without FUTEX_CLOCK_REALTIME (default value) is handled as CLOCK_MONOTONIC by
> kernel.  The main issue is *how* to expose this interface to user.
>
> It was noted on [3] that we can either support QNX 'sem_timedwait_monotonic'
> or add an interface like 'sem_timedwait_np'.
>
> Now for a workaround I would advise use another API that does support clock
> such pthread_cond_timedwait [4].  GLIBC implementation will internally
> convert the absolute timeout to a relative one.
>
> You can also try to mimic internal glibc implementation on sem_timedwait
> and change futex call to omit the FUTEX_CLOCK_REALTIME.
>
> [1] http://pubs.opengroup.org/onlinepubs/9699919799/
> [2] https://sourceware.org/bugzilla/show_bug.cgi?id=14717
> [3] https://bugzilla.kernel.org/show_bug.cgi?id=112521
> [4] http://stackoverflow.com/questions/29153310/how-would-i-cope-with-changes-to-the-system-time-while-doing-sem-timedwait

If I provide a patch which implements sem_timedwait_np(sem, timeout,
clock), is there any chances that it gets merged for the next release?

-- 
Alexandre Bique



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