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I am having difficulty in understanding IPC in multiprocess system. I have this system where there are three child processes that send two types of signals to their process group. There are four types of signal handling processes responsible for a particular type of signal.

There is this monitoring process which waits for both the signals and then processes accordingly. When I run this program for a while, the monitoring process doesn't seem to pick up the signal as well as the signal handling process. I could see in the log that the signal is only being generated but not handled at all.

My code is given below

#include <cstdlib>
#include <iostream>
#include <iomanip>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/time.h>
#include <signal.h>
#include <unistd.h>
#include <fcntl.h>
#include <cstdio>
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>

using namespace std;

double timestamp() {
  struct timeval tp;
  gettimeofday(&tp, NULL);
  return (double)tp.tv_sec + tp.tv_usec / 1000000.;
}

double getinterval() { 
  srand(time(NULL));
  int r = rand()%10 + 1;
  double s = (double)r/100;
}

int count;
int count_1;
int count_2;
double time_1[10];
double time_2[10];


pid_t senders[1];
pid_t handlers[4];
pid_t reporter;

void catcher(int sig) {
  printf("Signal catcher called for %d",sig);
}

int main(int argc, char *argv[]) {

  void signal_catcher_int(int);

  pid_t pid,w;
  int status;

  if(signal(SIGUSR1, SIG_IGN) == SIG_ERR) {
    perror("1");
    return 1;
  }

  if(signal(SIGUSR2 ,SIG_IGN) == SIG_ERR) {
    perror("2");
    return 2;
  }

  if(signal(SIGINT,signal_catcher_int) == SIG_ERR) {
    perror("3");
    return 2;
  }

  //Registering the signal handler
  for(int i=0; i<4; i++) {
    if((pid = fork()) == 0) {
      cout << i << endl;
      //struct sigaction sigact;
      sigset_t sigset;
      int sig;
      int result = 0;

      sigemptyset(&sigset);

      if(i%2 == 0) {

        if(signal(SIGUSR2, SIG_IGN) == SIG_ERR) {
          perror("2");
          return 2;
        }

        sigaddset(&sigset, SIGUSR1);
        sigprocmask(SIG_BLOCK, &sigset, NULL);
      } else {
            if(signal(SIGUSR1, SIG_IGN) == SIG_ERR) {
          perror("2");
          return 2;
        }

        sigaddset(&sigset, SIGUSR2);
        sigprocmask(SIG_BLOCK, &sigset, NULL);
      }

      while(true) {
        int result = sigwait(&sigset, &sig);
        if(result == 0) {
          cout << "The caught signal is " << sig << endl;
        }
      }
      exit(0);

    } else {
      cout << "Registerd the handler " << pid << endl;
      handlers[i] = pid;
    }
  }

  //Registering the monitoring process
  if((pid = fork()) == 0) {
    sigset_t sigset;
    int sig;
    int result = 0;

    sigemptyset(&sigset);
    sigaddset(&sigset, SIGUSR1);
    sigaddset(&sigset, SIGUSR2);

    sigprocmask(SIG_BLOCK, &sigset, NULL);

    while(true) {
      int result = sigwait(&sigset, &sig);
      if(result == 0) {
        cout << "The monitored signal is " << sig << endl;
      } else {
        cout << "error" << endl;
      } 
    }

  } else {
    reporter = pid;
  }


  sleep(3);
  //Registering the signal generator
  for(int i=0; i<1; i++) {
    if((pid = fork()) == 0) {

      if(signal(SIGUSR1, SIG_IGN) == SIG_ERR) {
        perror("1");
        return 1;
      }

      if(signal(SIGUSR2, SIG_IGN) == SIG_ERR) {
        perror("2");
        return 2;
      }
      srand(time(0));
      while(true) {
        volatile int signal_id = rand()%2 + 1;
        cout << "Generating the signal " << signal_id << endl;
        if(signal_id == 1) {
          killpg(getpgid(getpid()), SIGUSR1);
        } else {
              killpg(getpgid(getpid()), SIGUSR2);
        }     
        int r = rand()%10 + 1;
        double s = (double)r/100;  
        sleep(s);
      }

      exit(0); 
    } else {
      cout << "Registered the sender " << pid << endl;
      senders[i] = pid;

    } 

  }


  while(w = wait(&status)) {
    cout << "Wait on PID " << w << endl;
  }

}



void signal_catcher_int(int the_sig) {
  //cout << "Handling the Ctrl C signal " << endl;
  for(int i=0; i<1; i++) {
    kill(senders[i],SIGKILL);
  }

  for(int i=0; i<4; i++) {
    kill(handlers[i],SIGKILL);
  }

  kill(reporter,SIGKILL);

  exit(3);
}

Any suggestions?

Here is a sample of the output as well

In the beginning

Registerd the handler 9544
Registerd the handler 9545
1
Registerd the handler 9546
Registerd the handler 9547
2
3
0
Registered the sender 9550
Generating the signal 1
The caught signal is 10
The monitored signal is 10
The caught signal is 10
Generating the signal 1
The caught signal is 10
The monitored signal is 10
The caught signal is 10
Generating the signal 1
The caught signal is 10
The monitored signal is 10
The caught signal is 10
Generating the signal 1
The caught signal is 10
The monitored signal is 10
The caught signal is 10
Generating the signal 2
The caught signal is 12
The caught signal is 12
The monitored signal is 12
Generating the signal 2
Generating the signal 2
The caught signal is 12
The caught signal is 12
Generating the signal 1
The caught signal is 12
The monitored signal is 10
The monitored signal is 12
Generating the signal 1
Generating the signal 2
The caught signal is 12
Generating the signal 1
Generating the signal 2
10
The monitored signal is 10
The caught signal is 12
Generating the signal 1
The caught signal is 12
The monitored signal is GenThe caught signal is TheThe caught signal is 10
Generating the signal 2

Later on

The monitored signal is GenThe monitored signal is 10
Generating the signal 1
Generating the signal 2
The caught signal is 10
The caught signal is 10
The caught signal is 10
The caught signal is 12
Generating the signal 1
Generating the signal 2
Generating the signal 1
Generating the signal 1
Generating the signal 2
Generating the signal 2
Generating the signal 2
Generating the signal 2
Generating the signal 2
Generating the signal 1
The caught signal is 12
The caught signal is 10
The caught signal is 10
Generating the signal 2
Generating the signal 1
Generating the signal 1
Generating the signal 2
Generating the signal 1
Generating the signal 2
Generating the signal 2
Generating the signal 2
Generating the signal 1
Generating the signal 2
Generating the signal 1
Generating the signal 2
Generating the signal 2
The caught signal is 10
Generating the signal 2
Generating the signal 1
Generating the signal 1

As you can see initially, the signal was generated and handled both by my signal handlers and monitoring processes. But later on the signal was generated a lot, but it was not quite processes in the same magnitude as before. Further I could see very less signal processing by the monitoring process

Can anyone please provide some insights. What's going on?

share|improve this question
    
That's a lot of code and output. Can you be any more specific in your question? –  deadly Oct 9 '12 at 15:10

1 Answer 1

Are you certain that the monitoring code is not being executed?

In your section above: Later On

The monitored signal is GenThe monitored signal is 10

Appears that the std::out that the monitoring process is being overwritten by your other forked process. I would suggest that you write your output to separate files, perhaps named with the PIDs and timestamps to fully understand the behavior you are seeing.

The second issue that I could see happening, is that the forked monitoring process has (for some reason) unexpectedly quit. For this to see what is happening in the OS, I would suggest running top and watching the behavior of your program and its forked processes.

share|improve this answer
    
Did you try to run this program? –  user34790 Sep 20 '12 at 8:24
    
No I haven't. Are you still seeing the same behavior after writing to separate output files? –  cacopela Sep 21 '12 at 14:36
    
@user34790 Did you try the suggestion? –  deadly Oct 10 '12 at 15:37

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