[newlib-cygwin] Cygwin: signalfd: implement non-polling select

[Václav Haisman]

On Mon, 14 Jan 2019 at 17:19, Corinna Vinschen <corinna@sourceware.org> wrote:
>
> https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=f42776fa781de858a927bc03aa966a0f3096b581
>
> commit f42776fa781de858a927bc03aa966a0f3096b581
> Author: Corinna Vinschen <corinna@vinschen.de>
> Date:   Mon Jan 14 17:19:37 2019 +0100
>
>     Cygwin: signalfd: implement non-polling select
>
>     Allow the signal thread to recognize we're called in consequence of
>     select on a signalfd.  If the signal is part of the wait mask, don't
>     call any signal handler and don't remove the signal from the queue,
>     so a subsequent read (or sigwaitinfo/sigtimedwait) still gets the
>     signal.  Instead, just signal the event object at
>     _cygtls::signalfd_select_wait for the thread running select.
>
>     The addition of signalfd_select_wait to _cygtls unearthed the alignment
>     problem of the context member again.  To make sure this doesn't get lost,
>     improve the related comment in the header file so that this (hopefully)
>     doesn't get lost (again).
>
>     Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
> [...]
>
> diff --git a/winsup/cygwin/cygtls.h b/winsup/cygwin/cygtls.h
> index 39dba13..65a905c 100644
> --- a/winsup/cygwin/cygtls.h
> +++ b/winsup/cygwin/cygtls.h
> @@ -189,8 +189,11 @@ public:
>    stack_t altstack;
>    siginfo_t *sigwait_info;
>    HANDLE signal_arrived;
> +  HANDLE signalfd_select_wait;
>    bool will_wait_for_signal;
> -  long __align;                        /* Needed to align context to 16 byte. */
> +  /* context MUST be aligned to 16 byte, otherwise RtlCaptureContext fails.
> +     If you prepend cygtls members here, make sure context stays 16 byte
> +     aligned. */
>    ucontext_t context;
>[...]
Would it not better to do this with `ucontext_t context
__attribute__((__aligned__(16)));` instead?