Re: sigqueue() losing signals

Re: sigqueue() losing signals

[Robert Hancock]

James Renton wrote:
> I needed to kick off a bunch of background threads and receive
> notification if one failed due to fatal error.  I was hoping waitpid()
> would work; but found out quickly that although each thread has a pid_t;
> threads and processes were not interchangeable.  Without a timed
> pthread_join() over multiple threads (like WIN32
> WaitForMultipleObjects), I formulated a seemingly dead simple
> implementation using signals.  I hacked out a quick prototype to ensure
> I could get the signals to work as desired.  
> 
> When I ran the prototype, I experienced behavior looking very bug-like;
> but I wanted to post it here in case I am misunderstanding unix signals
> and just doing something silly.  I am requesting for comments or
> [hopefully clever] suggestions... Code follows:

For normal signals, you're not guaranteed that the signal handler will 
be called once for every time the signal is raised. You may get only one 
signal handler call for multiple events if they happen in rapid 
succession. Maybe if you use realtime signals it will work. But this 
seems like a bizarre way to accomplish the task.. better to just set 
some pthread condition in your threads when they exit and do a 
pthread_cond_timedwait on that condition in the main thread.

IMHO, in general, when signals are the answer, chances are the question 
is wrong..

Re: sigqueue() losing signals

[Samuel Thibault]

Mmm, isn't that just because non-realtime signals can not stack?

Samuel

sigqueue() losing signals

[James Renton]

I needed to kick off a bunch of background threads and receive
notification if one failed due to fatal error.  I was hoping waitpid()
would work; but found out quickly that although each thread has a pid_t;
threads and processes were not interchangeable.  Without a timed
pthread_join() over multiple threads (like WIN32
WaitForMultipleObjects), I formulated a seemingly dead simple
implementation using signals.  I hacked out a quick prototype to ensure
I could get the signals to work as desired.  

When I ran the prototype, I experienced behavior looking very bug-like;
but I wanted to post it here in case I am misunderstanding unix signals
and just doing something silly.  I am requesting for comments or
[hopefully clever] suggestions... Code follows:

<CODE>
#include <pthread.h>
#include <sys/types.h>
#include <linux/unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <errno.h>

siginfo_t signal_info;
pthread_t thread_main;
pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;

_syscall0(pid_t,gettid)

void *fThrd( void *context )
{
    union sigval val;
    val.sival_int = gettid();
    printf( "{%d} TID started OK\n", gettid() );
    sleep( 5 );
    printf( "{%d} TID exiting...\n", gettid() );

    //xxpthread_mutex_lock( &mtx );
    while( 0!=sigqueue( getpid(), SIGCHLD, val ) )
    {
        printf( "ERR=[%d]\n", errno );
        if( EAGAIN!=errno )
            break;
        //xxusleep( 10000 );
    }
    //xxusleep( 10000 );
    //xxpthread_mutex_unlock( &mtx );
};

void fSigChld( int signal, siginfo_t *pInfo, void* context )
{
    printf( "{%d} SIGCHLD called...\n", gettid() );
    if( getpid()!=gettid() || signal!=SIGCHLD )
        return;
    memcpy( &signal_info, pInfo, sizeof(siginfo_t) );
    printf( "{%d} SIGCHLD executed OK\n", gettid() );
}

int main( int argc, char* argv[] )
{
    pthread_t hT = 0;
    struct sigaction act;
    int rc = -1,i;
    const pid_t ppid = getppid();
  
    thread_main = pthread_self();

    memset( &signal_info, 0x00, sizeof(signal_info) );
    printf( "{%d} Parent PID={%d}\n", gettid(), ppid ); 

    for( i=0;i<5;i++ )
         rc = pthread_create( &hT, NULL, &fThrd, NULL );

    memset( &act, 0x00, sizeof(act) );   
    act.sa_flags = SA_SIGINFO;
    act.sa_sigaction = &fSigChld;
    rc = sigaction( SIGCHLD, &act, NULL );

    for( ;; )
    {
       printf( "{%d} PID sleeping...\n", gettid() );
       sleep(11);
       if( 0!=signal_info.si_pid )
           printf( "  SIGNAL SRC={%d} OK\n",
signal_info.si_value.sival_int);
       memset( &signal_info, 0x00, sizeof(signal_info) );     
    };

    printf( "{%d} PID exiting...\n", gettid() );
    return rc;    
};
</CODE>

When I execute it in 2.6.9-42.EL, I get the following output:
{10321} Parent PID={10235}
{10321} PID sleeping...
{10322} TID started OK
{10323} TID started OK
{10324} TID started OK
{10325} TID started OK
{10326} TID started OK
{10322} TID exiting...
{10323} TID exiting...
{10324} TID exiting...
{10325} TID exiting...
{10326} TID exiting...
{10321} SIGCHLD called...
{10321} SIGCHLD executed OK
  SIGNAL SRC={10322} OK
{10321} PID sleeping...
{10321} PID sleeping...

As you can see, 4/5 of the signals are lost.

When I uncomment the other lines above, (in fThrd); the output looks
better:
{10355} Parent PID={10235}
{10355} PID sleeping...
{10356} TID started OK
{10357} TID started OK
{10358} TID started OK
{10359} TID started OK
{10360} TID started OK
{10356} TID exiting...
{10357} TID exiting...
{10358} TID exiting...
{10359} TID exiting...
{10360} TID exiting...
{10355} SIGCHLD called...
{10355} SIGCHLD executed OK
  SIGNAL SRC={10356} OK
{10355} PID sleeping...
{10355} SIGCHLD called...
{10355} SIGCHLD executed OK
  SIGNAL SRC={10357} OK
{10355} PID sleeping...
{10355} SIGCHLD called...
{10355} SIGCHLD executed OK
  SIGNAL SRC={10358} OK
{10355} PID sleeping...
{10355} SIGCHLD called...
{10355} SIGCHLD executed OK
  SIGNAL SRC={10359} OK
{10355} PID sleeping...
{10355} SIGCHLD called...
{10355} SIGCHLD executed OK
  SIGNAL SRC={10360} OK
{10355} PID sleeping...
{10355} PID sleeping...

James
Beverly, MA USA