Re: [Libevent-users] Forked processes and signals

[Christophe Fillot <cf@xxxxxx>]

Nick Mathewson wrote:

Hello Nick,

> In theory, this approach is supposed to work: what problems are you
> seeing in the master process?  (event_base_free sounds like what you
> want if you do not want to use the original event_base in the
> subprocess. )
>
> If I understand right, you start with a master process with an
> event_base with some signal events pending in it.  Then you fork, and
> create new event_bases in the subprocesses, and you want to add
> signals to them instead.  Given that (IIUC) you do not plan to use the
> original event_base in the subprocesses, so it should be okay to free
> it.
That's exactly what I want to do.

> Of course, if this isn't working for you, then something has gone
> wrong.  What failure mode have you found when you try to do this?
I've attached an example program which shows the problem.
With event_base_free(master_base) called in the subprocesses,
the master process cannot receive anymore the SIGHUP signal
(whereas it's fine for the subprocesses).

If you remove event_base_free(master_base), all the processes
handle the SIGHUP signal correctly, but there are warning
messages. In fact, with evsig_set_base() in the subprocesses,
the warning messages are not displayed, and everything seems
to work fine (all processes handle the signal correctly).
I guess this behavior is related to the epoll mechanism.

Thanks for your help,

Christophe

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <assert.h>

#include <event2/event-config.h>
#include <event2/event.h>

static struct event_base *master_base;

static void subproc_sighup_handler(evutil_socket_t fd,short ev_type,
                                   void *arg)
{
   long id = (long)arg;
   printf("Subprocess %ld: SIGHUP received!\n",id);
}

static void subproc_work(int id)
{
   struct event_base *base;
   struct event *ev_sighup;

   //event_base_free(master_base);

   printf("Subprocess %d: creating event base\n",id);
   base = event_base_new();
   assert(base != NULL);

   evsig_set_base(base);

   ev_sighup = evsignal_new(base,SIGHUP,subproc_sighup_handler,
                            (void *)(long)id);
   assert(ev_sighup != NULL);
   evsignal_add(ev_sighup,NULL);

   event_base_dispatch(base);
}

static void subproc_create(int id)
{
   pid_t pid;

   switch((pid = fork())) {
      case -1:
         abort();
      case 0:
         printf("Subprocess %d created\n",id);
         subproc_work(id);
         break;
      default:
         printf("Master: subprocess %d created with PID=%ld\n",
                id,(long)pid);
   }
}

static void master_sighup_handler(evutil_socket_t fd,short ev_type,void *arg)
{
   printf("Master: SIGHUP received!\n");
}

int main(void)
{
   struct event *ev_sighup;

   printf("Master: PID = %ld\n",(long)getpid());

   printf("Master: creating event base and events...\n");
   master_base = event_base_new();
   assert(master_base != NULL);

   ev_sighup = evsignal_new(master_base,SIGHUP,master_sighup_handler,NULL);
   assert(ev_sighup != NULL);
   evsignal_add(ev_sighup,NULL);

   printf("Master: creating sub-processes:\n");
   subproc_create(0);
   subproc_create(1);

   printf("Master: running event loop...\n");
   event_base_dispatch(master_base);
   return(0);
}