Re^6: Obtain the child process id in perl

Consider the following example. There I used setpgrp to make the process group and process group id as forked process

check.pl

print "process id: $$\n";
my $pid = fork;
my $FH;
unless($pid){
    setpgrp;
    exec('script');
}
print "Child process id is: $pid\n";
while(1) {sleep(5)}
[download]

script file:

use POSIX;

while(1) {
    sleep(1);
    print "I am running\n";
    my $pid = fork;
    unless($pid)
    {
       while(1) {  sleep(2);
                   print "I am child and I'am running\n";
       }
    }
}
[download]

In the script file once again fork the process and printing the line

From another terminal using kill -9 <Negative number of the child process id> command kill all the process belongs to this process group

The child process is running as a defunct process but the parent process is alive

Refer the ps command output
643 15141 9607 0 18:14 pts/1 00:00:00 perl check.pl
643 15142 15141 0 18:14 pts/1 00:00:00 script <defunct>

How could I avoid this problem

Also I need to redirect the script command output and error into one file . How can I achieve it?

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Re^7: Obtain the child process id in perl by almut (Canon) on May 26, 2010 at 14:37 UTC
Defunct ("zombie") processes arise when the respective process has terminated, but its (still alive) parent hasn't reaped the child's status by waiting for it. When you modify `check.pl` as follows `#!/usr/bin/perl use POSIX ":sys_wait_h"; print "process id: $$\n"; my $pid = fork; my $FH; unless($pid){ setpgrp; exec('./script'); } print "Child process id is: $pid\n"; for (1..5) { kill -9, $pid if $_ == 4; # kill children in the 4th round system "ps Tf -o etime,pid,pgrp,stat,cmd"; sleep(3); waitpid(-1, WNOHANG); }` [download] you'll get something like $ ./check.pl process id: 4609 Child process id is: 4610 ELAPSED PID PGRP STAT CMD 06:17 4569 4569 Ss bash 00:00 4609 4609 S+ \_ /usr/bin/perl ./check.pl 00:00 4610 4610 R \_ /usr/bin/perl ./script 00:00 4611 4609 R+ \_ ps Tf -o etime,pid,pgrp,stat,cmd I am running I am running I am child and I'am running I am running ELAPSED PID PGRP STAT CMD 06:20 4569 4569 Ss bash 00:03 4609 4609 S+ \_ /usr/bin/perl ./check.pl 00:03 4610 4610 R \_ /usr/bin/perl ./script 00:02 4612 4610 S \| \_ /usr/bin/perl ./script 00:01 4613 4610 S \| \_ /usr/bin/perl ./script 00:00 4614 4609 R+ \_ ps Tf -o etime,pid,pgrp,stat,cmd I am child and I'am running I am running I am child and I'am running I am child and I'am running I am running I am child and I'am running I am child and I'am running I am running ELAPSED PID PGRP STAT CMD 06:23 4569 4569 Ss bash 00:06 4609 4609 S+ \_ /usr/bin/perl ./check.pl 00:06 4610 4610 S \_ /usr/bin/perl ./script 00:05 4612 4610 S \| \_ /usr/bin/perl ./script 00:04 4613 4610 S \| \_ /usr/bin/perl ./script 00:03 4615 4610 S \| \_ /usr/bin/perl ./script 00:02 4616 4610 S \| \_ /usr/bin/perl ./script 00:01 4617 4610 S \| \_ /usr/bin/perl ./script 00:00 4618 4610 S \| \_ /usr/bin/perl ./script 00:00 4619 4609 R+ \_ ps Tf -o etime,pid,pgrp,stat,cmd I am child and I'am running I am child and I'am running I am child and I'am running I am running I am child and I'am running I am child and I'am running I am child and I'am running I am running I am child and I'am running I am child and I'am running I am child and I'am running I am child and I'am running I am running ELAPSED PID PGRP STAT CMD 06:26 4569 4569 Ss bash 00:09 4609 4609 S+ \_ /usr/bin/perl ./check.pl 00:09 4610 4610 Z \_ [script] <defunct> 00:00 4623 4609 R+ \_ ps Tf -o etime,pid,pgrp,stat,cmd ELAPSED PID PGRP STAT CMD 06:29 4569 4569 Ss bash 00:12 4609 4609 S+ \_ /usr/bin/perl ./check.pl 00:00 4624 4609 R+ \_ ps Tf -o etime,pid,pgrp,stat,cmd [download] As you can see in the next-to-last ps output, all children have been killed, but the `script` process is defunct, because its parent hasn't yet gotten around to reaping it (due to how I deliberately wrote the loop). Three secs later, in the last ps output, the process is no longer there, because waitpid has been called in the meantime.	[reply] [d/l] [select]

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Re^7: Obtain the child process id in perl
by almut (Canon) on May 26, 2010 at 14:37 UTC

Defunct ("zombie") processes arise when the respective process has terminated, but its (still alive) parent hasn't reaped the child's status by waiting for it.

When you modify check.pl as follows

#!/usr/bin/perl

use POSIX ":sys_wait_h";

print "process id: $$\n";
my $pid = fork;
my $FH;
unless($pid){
    setpgrp;
    exec('./script');
}
print "Child process id is: $pid\n";
for (1..5) {
    kill -9, $pid if $_ == 4;  # kill children in the 4th round
    system "ps Tf -o etime,pid,pgrp,stat,cmd";
    sleep(3);
    waitpid(-1, WNOHANG);
}
[download]

you'll get something like

$ ./check.pl 
process id: 4609
Child process id is: 4610
    ELAPSED   PID  PGRP STAT CMD
      06:17  4569  4569 Ss   bash
      00:00  4609  4609 S+    \_ /usr/bin/perl ./check.pl
      00:00  4610  4610 R         \_ /usr/bin/perl ./script
      00:00  4611  4609 R+        \_ ps Tf -o etime,pid,pgrp,stat,cmd
I am running
I am running
I am child and I'am running
I am running
    ELAPSED   PID  PGRP STAT CMD
      06:20  4569  4569 Ss   bash
      00:03  4609  4609 S+    \_ /usr/bin/perl ./check.pl
      00:03  4610  4610 R         \_ /usr/bin/perl ./script
      00:02  4612  4610 S         |   \_ /usr/bin/perl ./script
      00:01  4613  4610 S         |   \_ /usr/bin/perl ./script
      00:00  4614  4609 R+        \_ ps Tf -o etime,pid,pgrp,stat,cmd
I am child and I'am running
I am running
I am child and I'am running
I am child and I'am running
I am running
I am child and I'am running
I am child and I'am running
I am running
    ELAPSED   PID  PGRP STAT CMD
      06:23  4569  4569 Ss   bash
      00:06  4609  4609 S+    \_ /usr/bin/perl ./check.pl
      00:06  4610  4610 S         \_ /usr/bin/perl ./script
      00:05  4612  4610 S         |   \_ /usr/bin/perl ./script
      00:04  4613  4610 S         |   \_ /usr/bin/perl ./script
      00:03  4615  4610 S         |   \_ /usr/bin/perl ./script
      00:02  4616  4610 S         |   \_ /usr/bin/perl ./script
      00:01  4617  4610 S         |   \_ /usr/bin/perl ./script
      00:00  4618  4610 S         |   \_ /usr/bin/perl ./script
      00:00  4619  4609 R+        \_ ps Tf -o etime,pid,pgrp,stat,cmd
I am child and I'am running
I am child and I'am running
I am child and I'am running
I am running
I am child and I'am running
I am child and I'am running
I am child and I'am running
I am running
I am child and I'am running
I am child and I'am running
I am child and I'am running
I am child and I'am running
I am running
    ELAPSED   PID  PGRP STAT CMD
      06:26  4569  4569 Ss   bash
      00:09  4609  4609 S+    \_ /usr/bin/perl ./check.pl
      00:09  4610  4610 Z         \_ [script] <defunct>
      00:00  4623  4609 R+        \_ ps Tf -o etime,pid,pgrp,stat,cmd
    ELAPSED   PID  PGRP STAT CMD
      06:29  4569  4569 Ss   bash
      00:12  4609  4609 S+    \_ /usr/bin/perl ./check.pl
      00:00  4624  4609 R+        \_ ps Tf -o etime,pid,pgrp,stat,cmd
[download]

As you can see in the next-to-last ps output, all children have been killed, but the script process is defunct, because its parent hasn't yet gotten around to reaping it (due to how I deliberately wrote the loop). Three secs later, in the last ps output, the process is no longer there, because waitpid has been called in the meantime.

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