Continue output after parent child termination , i am unable to perform the operation-CodePudding

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

static void sigusr(int iSig)   //SIGUSER are the user defined signals
  {
        if (iSig == SIGUSR1)
        {
                printf("Received SIGUSR1 signal, going to sleep for 2 seconds\n");
                sleep(2);
                
        }
  }
int main ()
  {
        int pid;
        signal(SIGUSR1, sigusr);
        pid = fork();
        if (pid > 0)   //Parent process created
        {
                for(int i=0; i<=1000;i  )
                {
                        printf("%d\n",i);
                    usleep(70);
                }
        }
        else           //Child process created
        {
                sleep(5);
                kill(pid,SIGUSR1);
                exit(0);
        }
   }

Create 2 processes, a parent and a child using fork(). The parent prints the value of ‘i’ from 0 to 1000 and then exits. Meanwhile the child process sleeps for 5 seconds after it is created, sends a SIGUSR1 signal to the parent and then exits.
The parent should catch that signal, print on standard output “Received SIGUSR1 signal, going to sleep for 2 seconds”, sleep for two seconds and then continueprinting the numbers. But i am unable to continue the process after child process termination.

CodePudding user response：

For me everything works fine if I increase usleep time, without it parent process terminates before child process send signal. The problem is with kill call, else statement is executed only in child process, so it means that pid value is 0, kill with pid 0 sends signal to whole group, in this case to parent and a child, you should change it to

kill(getppid(), SIGUSR1);

CodePudding user response：

In addition to the @complikator 's answer, you shoudl print and sleep outside the signal handler. There are remaining questions like "main finishes before the signal is received", but this is really depending on you use case...

would look like this:

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


volatile static bool called = false;

static void sigusr(int iSig)
{
    called = true;
}

void check_signaled(void) {
    if (called) {
        called = false;
        printf("Received SIGUSR1 signal, going to sleep for 2 seconds\n");
        sleep(2);
    }
}
int main(void)
{
    int pid;
    pid = fork();
    if (pid > 0)   //Parent process created
    {
        signal(SIGUSR1, sigusr);
        for(int i=0; i<=1000;i  )
        {
            check_signaled(); /* if signal come while iterating */
            printf("%d\n",i);
            usleep(70);
        }
        wait(NULL); /* wait child completion */
        check_signaled(); /* signal may happen "too late" */
    }
    else           //Child process created
    {
        sleep(1);
        kill(getppid(),SIGUSR1);
    }
}