Question

Simple code:

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

using namespaces td;                                                             
bool unblock = false;                                                  
long int ile = 0;                                                 

void ouch(int sig) {
    cout << "signal " <<  sig << endl;                               
    unblock = true;                                                  
}                                                                
int main(){                                                          
    struct sigaction act;                                           
    act.sa_handler = ouch;                                           
    sigemptyset(&act.sa_mask);                                  
    act.sa_flags = 0;                                             
    sigaction(SIGINT, &act, 0);                                    

    do {                                                         
        cout << "." << endl;                                         
    } while(!unblock);                                                          

    cout << "EXIT" << endl;                                          
}

Now i compile the code and i get the “a.out”.
When i run the a.out like this:

[przemek@localhost test]$ ./a,out

it runs just as expected and when i press CTRL+C the program exits just as needed
But when i run the program like this (which is needed in my project):

[przemek@localhost test]$ ./a.out&

The os passes the control to the shell and i am unable to break the loop by CTRL+C

I need to run this code in bash script and i need to run it in background. Is it possible to somehow catch the signals in such situation?

Answer 1

When you press ctrl-c you send signal 2 (SIGINT) to current process (the process that runs in foreground on the terminal). You can send the signal to process that runs in background (was started with &) using kill:

$ kill -2 %%

When you say %%you mean the last background process. Of course you can specify another one. You need to know its PID (see ps(1)) or JobID (bash(1)/jobs).

I want to note also that you can use %-notation only inside shells with jobs management (e.g. in bash). When you are not in such a shell you can use only PIDs.

The PID of the last started process is in $!. That may be useful in some scripts.