hi there,
I'm trying to use fork in a script that batch converts my old taped Simpsons
episodes to dvd.
This is the first time I've used fork and I'm a bit bewildered by the behaviour
exhibited
when a child process dies.
Basically, I want the main process to fork a list of children(one for each
video file)
but not to wait until the child dies before the next fork.
Then when the child dies I want it logged.
As you can see from my code I've installed a SIG{CHLD} handler
(which I found in the perlipc manpage):
#!/usr/bin/perl -w
use strict;
use POSIX ":sys_wait_h";
$SIG{CHLD} = \&REAPER;
sub REAPER
{
my $stiff;
while(($stiff = waitpid(-1, WNOHANG)) > 0)
{
print LOG "Child process no. $stiff exited with status $?\n";
}
$SIG{CHLD} = \&REAPER;
}
open LOG, ">> /home/rools/Video/simpsons/log" or die "Can't open log
file: $!";
my $dir = "/home/rools/Video/simpsons";
my @files;
opendir DH, $dir or die "Can't open Video dir: $!";
for (readdir DH)
{
(/^Simpsons_\d+$/)? push @files, $_ : next;
}
closedir DH;
print LOG "Simpsons episodes:\n", map "$_\n", @files;
for my $file(@files)
{
defined(my $pid = fork) or die "Can't fork for $file: $!";
unless($pid)
{
$file =~ /^Simpsons_(.+)$/;
my $episode = "Sim_".$1;
exec("tovid", "-pal", "-in", "$dir/$file", "-out", "$episode");
die "Can't execute tovid for process no $$ on file $file: $!";
}
else
{
print LOG "Forked child process no. $pid for file $file.\n";
}
$SIG{CHLD} = \&REAPER;
}
Tovid executes successfully for both files but when the encoding is finished
the parent process just hangs there.
The program is forking the children in parallel as can be seen from the tovid
output
as well as from tailing log
The SIG{CHLD} handler doesn't get called at all since the output from
print LOG "Child process no. $stiff exited with status $?\n";
is missing.
As you can see I've tried putting in another $SIG{CHLD} at the end but to no
avail.
I have found the following text in the perlfork manpage:
Calling exec() within a pseudo-process actually spawns the requested
executable in a separate
process and waits for it to complete before exiting with the same exit status
as that process.
This means that the process ID reported within the running executable will be
different from
what the earlier Perl fork() might have returned.
but then in Randal and Tom's Learning Perl(3rd edition) about exec "bedrock" on
p.196:
When we reach the exec operation, Perl locates bedrock and "jumps into it".
At that point, there is no Perl process any more*, just the process running
the
bedrock command.
*Actually, it's the same process, having performed the Unix exec(2) system
call.
The process ID remains the same.
Since the exec'ed process doesn't retain the same PID as what forked returned,
how can my waitpid
ever reap the correct child process ?
I'm trying hard to get my head around this and would really appreciate your
input.
Cornelis
--memento mori
--
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
<http://learn.perl.org/> <http://learn.perl.org/first-response>
