Sync processes with signal in unix

Question

How can I sync 3 different processes with signals in Unix on C/C++? I need: first process starts second process. Second process starts third process. After third process is started I want kill all processes in order 1 - 2 - 3.

I have no idea about using wait, signal, pause etc functions for this. Could you help me? Thanks.

#include <iostream>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>

using namespace std;

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

    pid_t three_pid;
    pid_t second_pid;
    pid_t first_pid;

    cout << "child 1 is started" << endl;

    pid_t pid;

    if ((pid = fork()) == -1)
    {
        cout << "fork errror" << endl;
        exit(EXIT_FAILURE);
    }
    else if (pid == 0)
    {
        second_pid = getpid();

        cout << "child 2 is started" << endl;

        pid_t pid2;

        if ((pid2 = fork()) == -1)
        {
            cout << "fork 2 error" << endl;
            exit(EXIT_FAILURE);
        }
        else if (pid2 == 0)
        {
            three_pid = getpid();
            cout << "child 3 is started" << endl;

            cout << "child 3 is TERMINATED" << endl;
        }
        else
        {
            cout << "child 2 is TERMINATED" << endl;
        }
    }
    else
    {
        first_pid = getpid();

        cout << "child 1 is TERMINATED" << endl;
    }
}

Answer 1

To do this in a portable way let process 3 (the grandchild) call kill(<pid1>, SIGKILL) and use kill(<pid1>, 0) to test if the process is still running. If its gone kill() will fail with errno set to ESRCH .

Then let process 3 do the same for <pid2> .

Then let process 3 terminate.

Answer 2

You need to use waitpid in parent process to wait until child procsee is terminated. Read http://linux.die.net/man/2/waitpid for more details. Like this:

int status;
if (waitpid(cpid, &status, 0) < 0) { // where cpid is child process id
    perror("waitpid");
    exit(EXIT_FAILURE);
}

Also you need to correctly terminate child process using _exit(exitCode) . Read http://linux.die.net/man/2/exit for more details.

EDIT: If you want to to terminate processes in order 1-2-3 just wait for parent process id in child process.