Using signals to unblock calls to msgrcv() in a multi-threaded process (Cygwin v1.7.33)

Corinna Vinschen
Mon Jan 19 19:10:00 GMT 2015

On Jan 19 11:38, Corinna Vinschen wrote:
> On Jan 19 08:29, SCOTT Damien wrote:
> > I have a multi-threaded process running under Cygwin that receives
> > messages from several IPC message queues (built using Cygwin's gcc,
> > version 4.8.3).  The listener for each queue runs in its own thread,
> > each of which is created from the main thread.  Each listener is using
> > a blocking call to msgrcv().  I want the main thread to be able to use
> > pthread_kill() to send a SIGUSR1 signal to each listening thread to
> > cause it to abandon its call to msgrcv() and then exit.  
> > 
> > My approach has been to create a signal mask containing just SIGUSR1
> > and associate this with a signal handler (although the signal handler
> > contains no functionality, since all I need is for the blocking call
> > to msgrcv() to be abandoned).  I initially set the signal mask to
> > block in the main thread, which should then be inherited by each of
> > the created listening threads.  I only unblock the signal mask around
> > the call to msgrcv() in each listening thread.
> > 
> > I have included a simplified version of my code below.  I usually
> > don't see any evidence of the SIGUSR1 signal being received by either
> > of the listener threads.  Occasionally the first thread unblocks as
> > expected but I have never seen both threads unblock.  If I change my
> > code so that there is only a single listener thread, then everything
> > works correctly every time.  Changing the msgrcv() call to a sleep
> > call also allows unblocking to occur correctly.  Note that this
> > simplified code does not include removal of the created queues (since
> > it never manages to exit anyway) so I manually remove these after each
> > run using the ipcrm command.
> > 
> > Example output:
> > 
> > Running thread 2 with queue Id = 1441792
> > Running thread 1 with queue Id = 1179649
> > About to kill thread 1
> > 
> > Is there an issue with the Cygwin implementation of signal handling,
> > or is there a problem with my code?  Any help would be much
> > appreciated.
> It's an issue with the Cygwin code, probably.  The msgrcv code ultimately
> hangs in a blocking ReadFile call on a named pipe, and this call is
> non-interruptible.  Changing that requires a bigger change in Cygwin
> which will take time.

...or not.  I'm not sure I can fix that for the upcoming 1.7.34 release,
but it seems cygserver is using the wrong signal handle.  At one point
the signal handling got changed a lot to improve per-thread signalling,
and, as far as I can see, the cygserver code still assumes a global
per-process signal handle, which is the old version from before the

Just to let you know I'm looking into that.  Stay tuned.

Oh, btw., there's a bug in your testcase:

  int id = msgget(key, IPC_CREAT|IPC_EXCL);

This call is missing the permission bits, so the permissions are set
to 000.  The permission bits should be added:

  int id = msgget(key, IPC_CREAT|IPC_EXCL|S_IWUSR|S_IRUSR);


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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