Re: Issue with communication of large data between Parent and child using Perl pipes

Pipes can only hold a limited number of bytes, and your pipe is getting filled.

Your child is waiting for the parent to start emptying the pipe so it can put more into it.
Your parent is waiting for the child to end.

You have a Deadlock.

Run the parent asynchronously instead of waiting for the child to end, or use qx// to launch the child.

By the way, it's probably not a good idea to use waitpid on a child launched using open. Closing the handle is the appropriate way of reaping the child.

By the way, please put computer text (code, data) in between <c>...</c> tags. It preserves the formatting, escapes characters that need escaping and provides a handy download link.

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Re^2: Issue with communication of large data between Parent and child using Perl pipes by waavman (Novice) on Jul 20, 2009 at 19:06 UTC
Thanks Ikegami. Your explanation of the concept that Pipes can hold only a limited number of bytes and how the deadlock is occurring made things clear. So now I made the parent read operation asynchronous by moving the read operation by the parent process BEFORE it checks to see if the child process has terminated. After it kills the child, I have made it read one last time again just in case child wrote something to the file handle PARENT_READ_HANDLE just before being killed. After making these changes, the code runs without any issues now else #parent process { # This is the section I added before checking for # child's status and it solved the problem while (<PARENT_READ_HANDLE>) { printf ("Parent received from the child : $_\n"); } # thIs section checks and makes sure child process if Terminated # if its hung for more than 30 seconds. $count =0; $childprocess = qx (ps -ef\|grep -v defunct\|grep -v grep \|grep $ +pid); while (($childprocess ne "") && ($count < 6)) { $count+=1; printf ("Found child process still running count=$count\n") +; sleep (5); $childprocess = qx (ps -ef\|grep -v defunct\|grep -v grep \|gr +ep $pid); } if ($childprocess ne "") { qx (kill -9 $pid); printf (STDERR "Child ssh process hung. So forcibly killed + it\n"); } waitpid($pid,0); # read the data printed by child while (<PARENT_READ_HANDLE>) { printf ("Parent received from the child : $_\n"); } } [download] As a note, there is documentation on opening pipes using "-\|" at the below URL under the section "Safe Pipe Opens" http://www.perl.com/doc/manual/html/pod/perlipc.html thanks waavman	[reply] [d/l]

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Re^2: Issue with communication of large data between Parent and child using Perl pipes
by waavman (Novice) on Jul 20, 2009 at 19:06 UTC

BEFORE

else #parent process 
{ 
#  This is the section I added before checking for 
#  child's status and it solved the problem

       while (<PARENT_READ_HANDLE>) 
       { 
             printf ("Parent received from the child :  $_\n"); 
       } 
# thIs section checks and makes sure child process if Terminated 
# if its hung for more than 30 seconds. 
       $count =0; 
       $childprocess = qx (ps -ef|grep -v defunct|grep -v grep |grep $
+pid); 
       while (($childprocess ne "") && ($count < 6)) 
       { 
           $count+=1; 
           printf ("Found child process still running count=$count\n")
+; 
           sleep (5);
           $childprocess = qx (ps -ef|grep -v defunct|grep -v grep |gr
+ep $pid); 
       } 
       if ($childprocess ne "") 
       { 
           qx (kill -9 $pid); 
           printf (STDERR "Child ssh process hung. So  forcibly killed
+ it\n"); 
       } 
        waitpid($pid,0); 
       # read the data printed by child 
       while (<PARENT_READ_HANDLE>) 
       { 
             printf ("Parent received from the child :  $_\n"); 
       } 
}
[download]

[reply]
[d/l]