Signal Handler Behavior

Question

I have just some questions about the system calls signal, kill and the signal handler. I have this code:

int figlio=-1;
int main(int argc,char* argv[])
    int pid1,pid2,dormi,numero;
    float reciproco;
    signal(SIGUSR1,gestore);
    signal(SIGUSR2,gestore);
    numero=atoi(argv[1]);
    printf("Numero %d\n\n",numero);
    fflush(stdout);
    pid1=fork();
    if(pid1>0)  //PADRE
    {
        pid2=fork();
        if(numero%2 == 0)
            kill(pid1,SIGUSR1);
        else
            kill(pid2,SIGUSR2);
        wait(NULL);
        wait(NULL);
    }
    if(pid1 == 0)   //FIGLIO1
    {   
        //sleep(1);
        printf("%d\n",figlio);
        if(figlio == 1)
        {
            numero=numero*numero*numero;
            fflush(stdout);
            printf("Ho eseguito il cubo %d\n",numero);
        }
        else
        {
            pause();
            if(figlio == 1) //Se il gestore di SIGUSR1 è partito
            {
                fflush(stdout);
                printf("Ciao dal figlio 1\n");
            }
        }
        printf("Figlio1 termina\n\n");
        exit(0);
    }
    if(pid2 == 0)   //FIGLIO2
    {
        if(figlio == 2)
        {
            dormi=numero;
            reciproco=(float)numero;
            reciproco=1/reciproco;
            fflush(stdout);
            printf("Ho eseguito il reciproco %f\n",reciproco);
            sleep(dormi);
            fflush(stdout);
            printf("Mando un segnale a mio fratello %d\n",pid1);
            kill(pid1,SIGUSR1);
        }
        else
            printf("Arrivederci e grazie\n");
        printf("Figlio2 termina\n\n");
        exit(0);
    }   
    return 0;
}

void gestore(int signo)
{
    if(signo == SIGUSR1)
        figlio=1;
    else
        figlio=2;
}

1) I don't understand why the first printf in the first child, return -1 without a sleep(1) before...seems that the handler is executed after the beginning of the child.
2) When there is a kill to a child, after this, the scheduler works on the child or continue on the father?
3) The handler is executed when the kill is sent or when the scheduler works on the child?

jxh jxh · Accepted Answer · 2018-04-25T20:46:17

After the fork(), the parent and child processes are in a race. There is no guarantee the parent will have a chance to issue the call to kill() to the child before the child reaches the printf() call.
If there are multiple processors, both processes can be running simultaneously. Otherwise, if the child is not currently running, Linux will set some internal state that the signal should be delivered to the child process when it next runs, and it will mark the process as runnable if it is not already.
If the child is currently running, the signal gets processed immediately. Otherwise, it is processed when the child next runs. The handler is run as a result of processing the signal.

Signal Handler Behavior

1 Answers