Re^2: question about running a system and a perl function background

[Anonymous Monk]

Hello graff,

I have been attempting to do the same thing using threads according to BrowserUK's solution. But due to some complicated issues with the system I am using, I am trying to attempt the same thing using fork and exec instead.

Is it possible to have some arrangement whereby the original process sets up a separate STDOUT/STDIN prior to forking each command and then uses select to read the output from the child processes back into the parent for further possessing?

BrowserUK also is interested in seeing a working example using fork and exec.

So after the process forks, can I read each line that the process outputs, see if that line contains a search word I am looking for and if it does, stop all other forks immediately.

So for example if I am forking three processes: Dir C:, Dir I:, Dir P:

Then if any of those contain the searchword “Desktop”, then I woul
+d like to immediately stop all the other forks.

If you could provide a working example, me and BrowserUK would learn a lot.

Thank you.

[graff]

If I understand right, you want a form of IPC such that some number of processes can run in parallel, and as soon as any one of them comes up with "the answer", that job and all the others should immediately stop, and the parent should proceed to do stuff with the answer.

In this sort of case, playing with the stdin/stdout of all the children, and making the parent read all output from all children, strikes me as being more complicated than necessary (unless there's really something in your setup that makes it essential for the parent to see/process all the input from all children, and/or makes it impossible for the children to write to a commonly accessible disk).

Anyway, if you're actually just talking about a case where a few children need to run the same sort of search in parallel to cover a few different spaces, just set up the children so that each one will write whatever information the parent needs into a file. The parent should set the file name in advance (one that does not exist), and pass it along to all the children; then, after starting the kids going, it should just loop, watching for the file to come into existence. As soon as the file is there, "bye-bye, kids!"

If there's any chance of a race condition among children (i.e. two or more could come up with "the answer"), and especially if writing the file is more than just "open, write a record, close", it will probably be best for each child to write to its own personal temp file; once that file is complete and closed, check for existence of the parent's designated file, and if it does not exist, rename the temp file to the designated file name.

If the child processes involve something like "dir" or "find", i.e. some general-purpose tool that doesn't provide a way to stop as soon as some special condition is met, you could write a wrapper script to serve as the child process for the fork/exec; the child wrapper just needs to do a pipeline open of the general-purpose tool, and do the right thing when the target output shows up (closing the pipe file handle stops the tool).

(Sorry about not providing a working example, but I hope it's simple enough to work out on your own.)

[Mr. Muskrat]

First you post question about running a system and a perl function background and then this reply with more details today. No problem. So why did you feel the need to post forking and monitoring processes and now fork and exec as well?