Loosing signals even with sigqueue - High Level Programming

jonas.gabriel · #1 12-12-2006

Hi everyone,

I have a process that forks many times.At a random time point the children must send a SIGUSR1 to the parent.To do that I'm using a pair of sigaction & sigqueue.However many signals are getting lost.
Here are some code segments:
a)sigaction

Code:

sigset_t mask_set;	/* used to set a signal masking set. */
	sigfillset(&mask_set);


	struct sigaction act;
	act.sa_sigaction=catch_usr1;
	act.sa_mask=mask_set;
	act.sa_flags=SA_SIGINFO | SA_RESTART;

	sigaction(SIGUSR1,&act,NULL);

b)signal handler

Code:

void catch_usr1(int sig,siginfo_t *a,void *b )
{
	sigcount++;
	if (readers>0)
		readers--;
	else
		writing=false;
	printf("[LOG] Signal received. Readers now: %d Signal counter: %d\n",readers,sigcount);
	fflush(stdout);
}

c)sigqueue

Code:

tmp=sigqueue(getppid(),SIGUSR1,vsig);

Can anybody find an error

Perderabo · #2 12-12-2006

You didn't post the code surrounding your sigqueue call. The call should fail with EAGAIN if the queue is currently full.

jonas.gabriel · #3 12-12-2006

I just have this simple statements

Code:

tmp=sigqueue(getppid(),SIGUSR1,vsig);
close(handler.connfd);
printf("SERVER CHILD for %d: Out.Sigqueue returned %d\n",handler.id>0?handler.id:-handler.id,tmp);
fflush(stdout);
return 0;

I redirect my output to a file.The tmp variable has always the value 0.

Corona688 · #4 12-12-2006

Quote:

Originally Posted by Perderabo

You didn't post the code surrounding your sigqueue call. The call should fail with EAGAIN if the queue is currently full.

It can fail with an error status, but isn't required to. This has caused some rarely-seen and bothersome bugs on a few platforms.

Perderabo · #5 12-12-2006

Quote:

Originally Posted by Corona688

It can fail with an error status, but isn't required to. This has caused some rarely-seen and bothersome bugs on a few platforms.

Excuse me? If SA_SIGINFO is set, either the signal must be successfully queued or sigqueue() must return an error. And without SA_SIGINFO, sigqueue must behave at least like kill() and deliver the signal if it is not pending. This behavior is required by the Posix standard:

Quote:

Upon successful completion, the specified signal will have been queued, and the sigqueue() function returns a value of zero. Otherwise, the function returns a value of -1 and sets errno to indicate the error.

Corona688 · #6 12-12-2006

Apparently the linux kernel maintainers were misinformed, then. An old design flaw in the linuxthreads system is that, if the signal queue overflows and it's never informed of it, causing thread termination signals to stop being delivered, resulting in something very odd -- zombie threads. They couldn't fix it as it was a design flaw rather than a bug, and the kernel maintainers insisted that error status was not required. nptl doesn't have the same issues fortunately.

jonas.gabriel · #7 12-13-2006

So it isn't a fault of mine. I'm working already on a solution based on sockets using select.Should I reject signals as solution to my problem ? Reability of delivery is important to me.What is your opinion ?

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