Re: exec sometimes changes pid

[moritz]

It might related to the fact that you don't pass a simple command to exec, but rather a string with shell meta characters.

So when you call exec it overrides the current (child) program with the shell from /bin/sh which interprets the string for you, which in turns starts another perl script which prints a PID (which is not the same as the one you forked off originally).

Maybe your other system is smart enough to detect that it doesn't really need to spawn a shell, but can execvp directly the new perl process. I'd guess that you'll get the same result on both systems if you use the LIST form of exec instead.

[doom]

Some preliminary trials show you're probably right (though I still don't see what could be different on these two systems... maybe a different version of some IPC::* module?). In any case, I was experimenting with the list form of exec for other reasons, and I finally made it through the gotchas:

my @cmd =
  ('perl',
   '-e
    print "child: id after exec: $$\n";
    print "child: sees processes...\n " ,
    grep { m/perl/ }
    `ps ax` , "\n";
  ',
          );

print "parent: pre-fork id $$\n";

if ( my $pid = fork ) {
  # this is parent code
  print "parent: post-fork id $$\n";
  print "parent: return from fork is: $pid\n";
  sleep 3;
  print "parent: still here, with id $$\n";
  print "parent: sees processes...\n " ,
     grep { m/perl/ }
     `ps ax` , "\n";
} else {
  # this is child code
  die "cannot fork: $!" unless defined $pid;
  print "child believes it is: $$\n";
  exec {$cmd[0]} @cmd;
}

The gotchas:

• You use the command name first ("indirect object" syntax, ala printing to a file handle), *and* have the command name again inside the list. The second one is what appears in the process listing, the first is the program that's actually run.
• From the shell, you typically do a perl -e 'some code', but if you're going around the shell, those single quotes on the string fed to "-e" just get in the way: @cmd = ('perl', q{-e some code }).