Re: Signal delivered while blocked

[Houder <houder at xs4all dot nl>]

On Fri, 4 Aug 2017 00:44:45, Noah Misch wrote:
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> The attached demonstration program blocks signals (with sigprocmask()) to
> achieve mutual exclusion between signal handlers.  It aborts upon receipt of a
> blocked signal.  On "CYGWIN_NT-10.0 2.7.0(0.306/5/3) 2017-02-12 13:18 x86_64",
> signals regularly arrive despite being blocked.  Essential parts of the
> program include handling two signal numbers and having handlers run for at
> least 1-2ms; this problem goes away if I remove one of those attributes.
> GNU/Linux, AIX, Solaris, and "CYGWIN_NT-6.0 1.7.27(0.271/5/3) 2013-12-09 11:57
> i686" never deliver a blocked signal to this program.  I think this Cygwin
> behavior is non-conforming.

Hi Noah,

I do not think that Cygwin is the problem here; your code is the problem
here, I believe.

Please, study, for example, chapters 20 and 21 of LPI (Linux Programming
Interface by Michael Kerrisk).

(20.10 The Signal Mask (Blocking Signal Delivery)
(20.13 Changing Signal Dispositions: sigaction())

You cannot use sigprocmask() like you do; you cannot use SIG_SETMASK as
a parameter in sigprocmask() within the context of a handler.

Cygwin exhibits misbehaviour in case of your code, because it is slower
than Linux; however, the code is also wrong for Linux.

The misbehaviour occurs as result of nested interrupts in case of your
code (yes, nested interrupts are possible with Linux/Unix!).
However your code does not experience nesting under Linux, because, as
I said, Linux is faster than Cygwin.

-----
The simplest way to exclude one signal from another, is to specify the
signal (or signals) in the sa_mask of the sigaction parameter ...

    see sigaction()

-----
However if you desire 'control' during the execution of a handler, you
have to resort to sigprocmask(), and use SIG_BLOCK and SIG_UNBLOCK, in
order to add and remove a specific signal to/from the mask.

    see sigprocmask()

You cannot use SIG_SETMASK in that context.

Regards,

Henri

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