#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
pid_t pid;
void sigHandler(int signo)
{
printf("[%d]signal received \n",getpid());
pid = fork();
if(pid == 0)
{
printf("[%d] I m child process \n",getpid());
signal(SIGHUP,sigHandler);
execl("./chainNew", "chainNew", (char *)0);
}
else if(pid > 0)
{
printf("[%d] I m Parent process \n",getpid());
sleep(60*10);
exit(0);
}
else
{
printf("[%d] Fork Failed \n",getpid());
}
}
int main ()
{
signal(SIGHUP,sigHandler);
signal (SIGCHLD, SIG_IGN);
while (1)
{
printf("[%d] Waiting for SIGHUP \n",getpid());
sleep(2);
}
}
When you install any signal handler, they are just installed and not executed untill the signal is received by them.
Now from your code its clear that you are not creating any child inside the main(). However, the SIGHUP is handled, what does it mean is that untill the calling shell program exits; this process is not going to get SIGHUP rather it runs in the while(1) only.
Now when you exit the bash (or whatever shell you use) itself , the SIGHUP handler gets called (you might not been able to see this after you exited) and here a child gets created which inturn calls execl() to replace the child process memory area with a new binary. In this process, only the PCB is kept by the scheduler but with the EIP reset as well as other execution stack pointers along with the binary text. Hence whatever signals this new binary code has handled only works and not the earlier haldlers hold good.
It means the parent now exits after a sleep of 60 seconds and the child is all togather a new executable with a overridden signal handlers.
Now lets examine a few more cases:
1) This new binary has not handled SIGHUP -means the default disposition would apply, and you won't see your sighandler executing.
2) This new binary has a different handler for SIGHUP -it'll execute only that and not the above signalhandler which you wrote.
3) This new binary is the same your executable -It should go in the execution scenario which you are expecting, however there is a default timeout is associated with the signal handlers when the Linux kernel executes them asychronously. Hence the fork() and the sleep of 60 seconds inside the handler is not advicable; instead use vfork() and a sleep of not more than a few hundred miliseconds.
I feel you should use a different terminal to see the results or call bash on your default the login shell prompt before starting your program in the background (& option) and then exit bash to see the results.
Last edited by Praveen_218; 10-15-2010 at 03:44 PM..
Reason: Typo error correction.
This User Gave Thanks to Praveen_218 For This Post:
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