A signalling thread problem

Question

Hello -

I am attempting to get a thread running which will be responsible for detecting a signal from any other thread within the same process.

sigset_t    sigset;
sigfillset(&sigset);
pthread_sigmask( SIG_BLOCK, &sigset, &old );

Once i've done this, I then daemonize the process:

m_child = fork();
if (m_child == 0)
{
     // Start the thread responsible for detecting a signal
}

This is done within another class called "SignalHandler" where the kick-off to this thread is done as follows:

m_pThread   = new boost::thread( boost::bind(&SignalHandler::operator(), this ) );

and the functor operates in a tight loop as follows:

while (1)
{
    if ( sigtimedwait( &sigset, &info, &ts ) == -1 )
    {
        if ( errno == EAGAIN || errno == EINTR )
        {
            std::cout << "Time-out waiting for signal" << std::endl;
        }

        throw SystemError( "sigtimedwait error" );
    }
    else
    {
        std::cout << "Detected a signal [" << info.si_signo << "]" << std::endl;
    }
}

This works when I do a "kill -9 myProcess", but when I actually perform some illegal pointer manipulation (in any other thread), the process core dumps.

Why is the signal produced by the child thread not caught in my boost signal-catching thread?

Answer 1

Ah - I got it.

The problem is here:

sigset_t    sigset;
sigfillset(&sigset);
pthread_sigmask( SIG_BLOCK, &sigset, &old );

Using sigfillset for pthread_sigmask is dangerous because blocking the synchronous signals causes undesirable effects.

If any of the SIGFPE, SIGILL, SIGSEGV, or SIGBUS signals are generated while they are blocked, the result is undefined, unless the signal was generated by the kill() function, the sigqueue() function, or the raise() function.