Timeout with SIG{ALRM} not working as expected

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

Hello everyone, I have a script (> 400 Lines) in wich I have implemented a timeout routine using SIG{ALRM}. The code used is the following:

    $cmd="myCommand";
    # Timeout evaluation:
    eval {
      # Set action to be done when alarm occurs:
      local $SIG{ALRM} = sub { die "ALARM_COMMAND_TIMEOUT" };
      # Schedule alarm in $CMD_TIMEOUT seconds:
      alarm($CMD_TIMEOUT);

      ### execute the command that can block
      $childPID = open(PIPE, "-|");

      ### Unable to create child process
      unless(defined $childPID) {
         close(PIPE);
         die "ERROR_FORK_PROCESS: $!\n";
      }

      ### Child Process Created ... execute command
      unless($childPID) {
         exec($cmd) or die "ERROR_OPEN_PIPE : $!\n";

      ### Unable to create child process
      }
      wait;
      alarm(0);

      close(PIPE);
    };
    alarm(0);
[download]

During the test period, I noticed something strange. When using this code, my whole script is 'respawned' instead of the single command I want to check for timeout. Maybe the code is wrong. All I need is to run a single command and wait for the result withing a given timeout. If the command takes too long I want to kill it. The script is running on AIX 4.3.3 & 5.2. Perl version is 5.8. Any suggestions ? Thank you very much.

Comment on Timeout with SIG{ALRM} not working as expected Download Code

Replies are listed 'Best First'.
Re: Timeout with SIG{ALRM} not working as expected by Random_Walk (Prior) on Jan 19, 2005 at 12:04 UTC
Why can you not do the relatively simple ? $cmd="myCommand"; # Timeout evaluation: eval { # Set action to be done when alarm occurs: local $SIG{ALRM} = sub { die "ALARM_COMMAND_TIMEOUT" }; # Schedule alarm in $CMD_TIMEOUT seconds: alarm($CMD_TIMEOUT); ### execute the command that can block $Results = `$cmd`; if ($?) {# optional error handling} } wait; alarm(0); close(PIPE); }; alarm(0); [download] The backticks spawn a child to run the command and block 'till it returns Update OK, I tried that, well this perl -le'$SIG{ALRM}=sub{die "TIMEOUT"};eval{alarm 1;`sleep 100`};print $@' and it did not work for me, this did however `perl -le'$SIG{ALRM}=sub {die "TIMEOUT"}; eval{alarm 10; open PIPE, "while [ 1 ] ;do echo \"foo\";sleep 5;done\|"; while (<PIPE>){print}}; print $@'` [download] If you trust the child to die on SIGPIPE then it will die when the eval exits Cheers, R. Pereant, qui ante nos nostra dixerunt!	[reply] [d/l] [select]
Re: Timeout with SIG{ALRM} not working as expected by Anonymous Monk on Jan 19, 2005 at 11:37 UTC
Not sure if that's your intention, but if the child process takes too long, you kill the parent (non-fatally, since it's done inside an eval). You make no attempt to kill the child. So, after a time-out, the parent goes on its merry way while neither closing the pipe, nor doing a wait. Bad things (for some value of bad) will happen: if the child has finished before the parent, the child will become a zombie (because you didn't wait for it) - and if the parent finished before the child, the child might die due to a SIGPIPE if it writes to the pipe after the parent has finished.	[reply]
Re^2: Timeout with SIG{ALRM} not working as expected by MorgothSauron (Initiate) on Jan 19, 2005 at 11:53 UTC
I forgot to add the code just after the eval: `if ($@ and $@ =~ /ALARM_COMMAND_TIMEOUT/) { &writeLog("Killing child process (pid = $childPID)"); kill(1, $childPID); }` [download]	[reply] [d/l]
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