Question

In the below code, when system function get executed (in Linux and compiled with g++), it will send the SIGCHLD signal to process (from which it is called) which doesn’t happen in Solaris code. I mean, if I run the same code in Solaris, the system function doesn’t send any signal to the process from which it is called. As a result, my process in Linux hangs.

#include <iostream>
using namespace std;
#include <cstdlib>
#include <signal.h>

void sigHandler(int sgnNbr)
{
    cout <<"Signal " << sgnNbr << "caught" << endl;
    system("echo $HOSTNAME");
    exit(1);
}

int main()
{
    signal(SIGCHLD,sigHandler);
    system("echo $HOSTNAME");
    return 0;
}  

My questions are:

1. Why does the process hang in the above case?
2. why does the system function not send any signal in Solaris?

… and my last question, which is related to my real application:

Actually, in my real application (in which single signal handler is there for some four signals), some other process sending the SIGABRT signal to my process and in signal handler I am killing the one process using system function as a result it got hanged(which is similar to the above scenario). Is there any way to overcome this?

Answer 1

From http://pubs.opengroup.org/onlinepubs/009604499/functions/system.html:

The system() function shall ignore the SIGINT and SIGQUIT signals, and shall block the SIGCHLD signal, while waiting for the command to terminate.

Solaris is correct; Linux is incorrect.

The most likely reason that your program is hanging is that it is not safe to call system within a signal handler. See http://pubs.opengroup.org/onlinepubs/009604499/functions/xsh_chap02_04.html#tag_02_04 for the list of async-signal-safe functions.

To kill another process, you can use kill, which is async-signal-safe.