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For 2023 - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Perl - multiprocess applications

There are times that you'll be writing an application that you want to go off and perform task "x" and AT THE SAME TIME have it perform task "y", co-ordinating task "y" with task "x". Threading (NOT the subject covered here) is one possible approach. And another possible approach is forking. How does that work?

Let's say that you want to write a process that receives requests from a number of different clients and handles them sequentially - but quite slowly. A print driver would be a good example ...

1. Set up the main (to be monitoring) process, open a port, wait for requests.

2. When a request is received, note you process ID (the $$ variable), set up a pipe then fork.

The effect of the fork is to split your single process into a parent and a child each of which has copies of all your variables and each of which can continue on its own way.

You recognise that you're in the child process because the fork function returns a false value, whereas in the parent it returns the child's process ID.

The parent can then resume its wait loop, and the child can process the data.

3. When the child wants to talk to the parent, or vice versa, it can write to the pipe. A pipe opened before a fork connects any writes on the parent to reads on the child, and any writes on the child to reads on the parent.

4. If the child or parent needs to communicate with the other asyncronously (i.e. when the other is NOT waiting for an input), you use a signal. Signals are sent by the alarmingly named kill function, and collected by a sub that you've named in the %SIG hash in the receiver.

If you're writing a Perl program that forks, have a look too at the $| or autoflush special variable, and both of the select functions that are built in - one to set the default channel, and the other to check whether there's any data in your read buffers without having to wait if there isn't. Note also the use of temporary files to transfer information, and perhaps look at shared memory.

There's a simple fork example and also a more complex talker written in Perl available on our site. Training (with practicals) is available if you're attending Perl on the Web or Perl for larger projects.
(written 2006-02-13, updated 2006-06-05)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P224 - Perl - Intersystem Communications
  [1073] Heartbeat script in Perl - (2007-02-09)
  [1918] Perl Socket Programming Examples - (2008-12-02)
  [2402] Automated Browsing in Perl - (2009-09-11)
  [2695] TCP v UDP / Client v Server - Python examples - (2010-03-25)
  [2876] Different perl examples - some corners I rarely explore - (2010-07-18)
  [2970] Perl - doing several things at the same time - (2010-09-25)

P223 - Perl - Interprocess Communication
  [2694] Multiple processes (forking) in Python - (2010-03-25)
  [3010] Children, zombies, and reaping processes - (2010-10-23)
  [3011] What are .pid files? - (2010-10-23)
  [3412] Handling binary data in Perl is easy! - (2011-08-30)
  [3940] Run other processes from within your Perl program - (2012-12-03)

P201 - Perl - Introduction
  [25] Release numbers - (2004-08-23)
  [317] Programming languages - a comparison - (2005-05-20)
  [382] Central London Courses - Perl, PHP, Python, Tcl, MySQL - (2005-07-18)
  [577] Learning to program in Perl or PHP - (2006-01-26)
  [594] Twice is a co-incidence and three times is a pattern - (2006-02-07)
  [629] Choosing the right language - (2006-03-01)
  [691] Testing you Perl / PHP / MySQL / Tcl knowledge - (2006-04-19)
  [743] How to debug a Perl program - (2006-06-04)
  [846] Is Perl being replaced by PHP and Python? - (2006-08-27)
  [924] The LAMP Cookbook - Linux, Apache, MySQL, PHP / Perl - (2006-11-13)
  [1717] Q - Should I use Perl or Python? - (2008-07-23)
  [1753] Perl v PHP, choosing the right language - (2008-08-14)
  [1852] Perl and Blackberries - (2008-10-23)
  [2070] Converting to Perl - the sort of programs you will write - (2009-03-08)
  [2812] What is Perl? - (2010-06-15)
  [2832] Are you learning Perl? Some more examples for you! - (2010-06-27)

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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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