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http waits on process

by hotshot (Prior)
on Jan 13, 2004 at 17:21 UTC ( [id://321042] : perlquestion . print w/replies, xml ) Need Help??

hotshot has asked for the wisdom of the Perl Monks concerning the following question:

Hello all!

I have a cgi script that calls an external script to run on background, for example:
`$myScript &`;
The problem is that the apache probably waits on this script and returns only after the script ends (I wrote such a script that only sleeps for 30 sec and the web page returned to me after 30 sec and not immediatelly as expected).

when I run a regular perl scripts that calls $myScript in the background, I have no problem, the prompt returns immediatelly and the script runs in the background.
Anyone has an idea what's the problem here and how to solve it?


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Re: http waits on process
by waswas-fng (Curate) on Jan 13, 2004 at 18:14 UTC
    Back ticks ` are used in perl to execute and use the output -- so perl is waiting for the output of the command even though it is in the background. I hate to beat merlyn to the punch but he has an article about this. =)

Re: http waits on process
by etcshadow (Priest) on Jan 13, 2004 at 20:57 UTC
    The problem is that when you call
    `$myscript &`;
    You are essentially calling
    `sh -c '$myscript &'`;
    In other words... you are waiting for the shell process to terminate... but the shell process (if it were interactive) would have returned a prompt imediately after firing off $myScript.

    If you really want to fire off a background job from a perl process, and don't care about the output (btw: backticks imply that you do care about the output, but continuing imediately imply that you do not care about the output)... then you should do something more like this:

    my $pid = fork; die "Could not fork: $!" unless defined $pid; if (!$pid) { exec $myScript; }
    Which is somewhat akin to what the shell does when you launch the program foo by typing:
    [user@host]$ foo &
    ------------ :Wq Not an editor command: Wq
      As I said, the problem happened only when I did it from cgi script through the web page, when I tried it from a regular script, I had no problem.
      Additionally, I already tried the solution you suggested (fork and exec), and still I get the same problem, I think it something related to apache that waits for stdout or someting (closing stdout, didn't work also).
      Do you have another idea?