Re^4: System Function...script does not move until process ends.

[curtisb]

Ok, excuse my ingnorance on this fork issue. I have never used it before. Could someone please take a look at the code I wrote (above) and suggest something or how I should do it.

After I start the database again, I want to be able make a connection to it using the dbi module in perl. I've done that, but I need to know more or shown the suggestion on how fork works.

Thanks,

Bobby

[RazorbladeBidet]

try something like:

 use POSIX ":sys_wait_h";

 sub startTheDB {
   (defined $pid = fork()) or die "Unable to fork!\n";
   if ( !$pid ) { 
     exec "StartTheDB" or die "Can't start the db!\n";
   } else {
     do {
       $kid = waitpid(-1, WNOHANG);
     } until $kid > 0;    
   }
 }

Note that you'll want to test the db from the parent after you return from the sub.

Updated: It still waits for the child - as pointed out by frodo72

Just a note that the scary looking stuff (e.g. the waitpid) is explained in the docs mentioned throughout this node and here

[polettix]

Sorry, I don't see the point.

If my synapses are not too cluttered due to hanger (which could be, it's dinner time here :), your sub keeps the parent waiting for child's death, which is exactly what system does. But while system does this "efficiently", putting the parent process to sleep (I hope this happens in Windows as well, can't swear on it >;), this function does a quite active wait, given the WNOHANG option. So, it seems to me that not only this does not solve the problem, but increases CPU load!

What am I missing (apart a good dinner)?

[RazorbladeBidet]

What you're missing is that exec abandons the process after it starts it. So it will only wait until the child dies, which should be right after it starts the database :)

I had reservations about this when I wrote it, and I confirmed them:

#!/usr/bin/perl -w

use strict;
use POSIX ":sys_wait_h";

defined (my $pid = fork()) or die "D'oh";

if ( $pid ) {
  print "$$ gonna wait...\n";
  my $kid;
  do {
   $kid = waitpid $pid, WNOHANG;
  } until ( $kid > 0 );
  print "Ok, done waiting\n";
} else {
    exec 'sleep 10; echo "Done sleeping\n"';
}

What I had thought it would do was to start the execution of the command and then bail. Well.. it didn't. It waited.

Now go get that dinner.

No dinner for you :)