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# 1  
Old 04-13-2010
Explain output
Code:
#include<signal.h>
#include<stdio.h>
#include<stdlib.h>
sigcatcher()
{
    printf("pid=%d",getpid());
    signal(SIGINT,sigcatcher);     //line1
}
main()
{
    int ppid;
    signal(SIGINT,sigcatcher);
    if(fork()==0)
    {
        sleep(5);
        ppid=getppid();
        while(1)
            if(kill(ppid,SIGINT)==-1)
                exit(1);
    }
    nice(10);
    while(1){}
}

why this program is printing pid infinite times if line1 is eliminated????
Last edited by Franklin52; 04-13-2010 at 03:40 PM.. Reason: Please use code tags!
# 2  
Old 04-14-2010
Quote:
Originally Posted by gol007
Code:
#include<signal.h>
#include<stdio.h>
#include<stdlib.h>
sigcatcher()
{
    printf("pid=%d",getpid());
    signal(SIGINT,sigcatcher);     //line1
}
main()
{
    int ppid;
    signal(SIGINT,sigcatcher);
    if(fork()==0)
    {
        sleep(5);
        ppid=getppid();
        while(1)
            if(kill(ppid,SIGINT)==-1)
                exit(1);
    }
    nice(10);
    while(1){}
}

why this program is printing pid infinite times if line1 is eliminated????
You mean this does NOT print the pid over and over if line1 IS present? Although this depends on the signal handling on your system, I would expect it to either always print your PID again and again, or to print it once if you deleve line1, not the other way around.

Either way, this is where you tell the program to send itself SIGINT (and hence call sigcatcher()) again and again:

Code:
while (1)
    if(kill(ppid,SIGINT)==-1)
                exit(1);

If you want some other behaviour, change it.
# 3  
Old 04-16-2010
i was expecting the same output as you said but on running the program..its output is independent of line1 means it has no effect and pid is being printed again and again...
# 4  
Old 04-16-2010
Quote:
Originally Posted by gol007
i was expecting the same output as you said but on running the program..its output is independent of line1 means it has no effect and pid is being printed again and again...
Ah, I see what you mean - so it prints the PID indefinitely, even if line1 is commented out.

Simply put, some systems reset the handler for a signal to the "default" once it's been called, some don't - yours obviously doesn't, so your signal handler is always the one called.

Portable programs should assume the default handler is reinstated, so:

  1. If you want your signal handler to remain in-place, keep line1 there "just in case".
  2. If you want the default signal handler reinstated, do so explicitly.
# 5  
Old 04-17-2010
thanks i got it...
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