LINUX NEWS
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Thursday, December 13, 2001TABLE OF CONTENTS
1) Sean’s Notes
2) Linux News
2.4 Maintainer Marcelo Tosatti Answers Your Questions
Red Hat Starts Clamping Down
Ximian to Offer Proprietary Exchange Connector
VA Linux is now VA Software3) Linux Resources
DNS For Dummies
Some Linux+ Practice Questions
Bynari Insight 2.6 Free Download
Multicast Video Server
What Are Your Expectations?4) App o’ the Week
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1) Sean’s Notes
Signals, in the Unix sense, are messages sent between processes, much like an interrupt is a signal sent from the hardware to the CPU. I’ve talked about signals in the past:
http://ertw.com/~sean/newsletter/June+28%2C+2001
With respect to killing off processes, the “kill” command is used to send a signal (SIGTERM) to a process, in this case, processid 1234
kill 1234
Upon receipt of the signal, the process gets the chance to exit gracefully. If it wasn’t expecting the signal, the operating system just turfs it.
Sometimes, processes misbehave. They either ignore SIGTERM, or maybe are stuck in IO and can’t get time on the CPU to quit. In this case, we can send a stronger signal, SIGKILL. There is no ignoring this one, and the kernel will do its best to get rid of the process.
kill -9 1234
However, there is a third class of processes that can’t be killed, no matter what signal you send them. These are called “zombies”. While they take up no CPU (because they’re technically dead), they’re usually a symptom of another problem with the application. They also take up resources, such as process table space. Thus, dealing with zombies is something a Unix admin will have to do at some point in their career. Rebooting is not an option, either! You can see zombies in the ps listing; they are marked with the <defunct> tag.
At this point, it’s probably a good thing if I explain parent and child processes. If a process wants to create a new process to handle some work, it does so by calling the fork() system call. The operating system then creates a new process, called a child process, which is exactly like the original, or parent process. Execution of both processes continues after the fork() call. Depending on the result of the call, the process knows if it is the parent or the child. A parent can have many children, but the children can only have one parent.
A more in depth look at fork() can be found in the July 12th edition of Linux News:
http://ertw.com/~sean/newsletter/July+12%2C+2001
This is commonly used in web servers. The parent process creates a bunch of children, and farms out the requests to them. This way, many requests can be handled simultaneously.
If the parent process quits before the child does, this is no problem. The child gets adopted by process #1, init. However, if the child dies before the parent, we have a problem. Create the following file, zombie1.c:
#include <stdlib.h>
void main(void) {
int pid;
printf("Parent's pid is %d\n", getpid());
if ((pid = fork()) > 0) { /* Parent */
while (1) sleep(10); /* Sit around */
} else { /* Child */
sleep(1);
printf("Child %d exiting\n", getpid());
exit;
}
}
Now compile, and execute it:
$ gcc zombie1.c -o zombie1 $ ./zombie1 Parent’s pid is 24751 Child 24752 exiting
zombie1 basically creates a process, tells you the processid, then creates a child. The parent then idles, while the child exits after printing its pid.
In another window, do a “ps -ef”: $ ps -ef … sean 24758 24675 0 18:09 pts/2 00:00:00 ./zombie1 sean 24759 24758 0 18:09 pts/2 00:00:00 [zombie1 <defunct>] …
There – process 24752 is a zombie. It was supposed to have quit, since it called exit(). Now, try to kill it.
$ kill -9 24759 $ ps -ef | grep zombie1 sean 24758 24675 0 18:09 pts/2 00:00:00 ./zombie1 sean 24759 24758 0 18:09 pts/2 00:00:00 [zombie1 <defunct>]
As many times as you try to kill the child, it isn’t going to go away.
So just how do you kill a zombie?
The answer isn’t as exciting as you’d think. To kill a zombie, kill its parent. The reason for this will shortly be evident.
$ kill 24758 $ ps -ef | grep zombie $
If a process is creating zombie children, there is a good chance that it’s not operating correctly in the first place. Left to its own devices, it’ll do so until it brings the system to its knees. I should mention, though, that some applications do create one or two zombies because of a design oversight, so you’ll have to become familiar with the natural behavior of your applications.
The reason the zombie gets created is because of a negligent parent. When a child process dies, the parent process is notified with the SIGCHLD signal. The parent process must either explicitly ignore the signal entirely, or install a signal handler that uses the wait() system call to acknowledge the child’s exit. As in life, there are responsibilities to becoming a parent. (OK, I’m done with the bad analogies for this week).
zombie2.c will fix the problem by ignoring the SIGCHLD signal.
#include <stdlib.h>
#include <signal.h>
void main(void) {
int pid;
/* Ignore SIGCHLD to prevent zombies */
signal(SIGCHLD, SIG_IGN);
printf("Parent's pid is %d\n", getpid());
if ((pid = fork()) > 0) { /* parent */
while (1) sleep(10); /* Sit around */
} else {
sleep(1);
printf("Child %d exiting\n", getpid());
exit;
}
}
$ gcc zombie2.c -o zombie2 $ ./zombie2 Parent’s pid is 24847 Child 24848 exiting
In my other window:
$ ps -ef | grep zombie sean 24847 24675 0 18:19 pts/2 00:00:00 ./zombie2
As you can see, the child (24848) got a clean exit, and the parent (24847) can idle freely.
The alternative, to handle the SIGCHLD signal, is a bit more work, but allows the parent to clean up after the child. If you’re interested, check out the man page for wait(2) to see how to properly acknowledge a child’s exit.
Creating zombie processes is the operating system’s way of handling the odd situation where the child dies before the parent. In a properly designed application, there should not be any zombie processes. Zombies tend to be indicators of a misbehaving application. In this case, you can clean up all the zombie children by killing the parent process (from ps -ef).
Long live the Penguin,
Sean swalberg@cramsession.com
2) Linux News
-
4 Maintainer Marcelo Tosatti Answers Your Questions
Now that 2.5 is started, Linus has turned the job of maintaining the 2.4 tree to Marcelo Tosatti, an 18-year-old programmer from Brazil. In the latest Slashdot interview, Marcelo answers some questions on his vision for 2.5, and of course, questions about his age. The answers are terse to say the least, but well worth the time to read.
http://slashdot.org/article.pl?sid/12/10/1656226&mode=nocomment
Red Hat Starts Clamping Down
Since it’s freely downloadable, there is nothing stopping someone from selling copies of Red Hat (or other distros, for that matter) for cheap. Red Hat has started to clamp down on this activity, insisting that if one redistributes the software that it not be called Red Hat, since the CDs will not come with any support. Personally, I think this is a perfectly fine way of protecting their trademark, though once you read the comments in this article, you’d think that the company had committed murder…
http://www.newsforge.com/article.pl?sid/12/10/2014239
Ximian to Offer Proprietary Exchange Connector
Ximian is that company that brings a great GNOME desktop to your Linux box. They have also put a lot of work into the Evolution project, which is to create an Outlook-like mail/calendar/etc. client for Linux. Their latest announcement is that they’ll soon be offering a connector that will let you use Evolution with an Exchange server. Single user licences are around $69, a pittance to pay if Outlook is the only application that’s keeping you running Windows.
http://techupdate.zdnet.com/techupdate/stories/main/0,14179,5100241 ,00.html
VA Linux is now VA Software
VA Linux used to be a big vendor of Linux-ready hardware. They’re also the parent company of SourceForge and Slashdot, among many others. To reflect the change of focus to software, they’ve changed their name appropriately.
http://www.valinux.com/about/pr/120501.php
3) Linux Resources
DNS For Dummies
Here’s another look at DNS, this time covering off everything from the local resolver, to the BIND server itself. It’s written with Solaris in mind, though everything mentioned is the same under Linux.
http://everythingsolaris.org/articles/dfd/frameset.html
Some Linux+ Practice Questions
Here are 25 Linux+ practice questions, complete with explanations. What’s good about these is that they’re intentionally ambiguous, which should help you get a feel for what you can expect on the real exam. The assumptions made are listed with the answer.
http://www.unixreview.com/documents/s81/urm0112b/0112b.htm
Bynari Insight 2.6 Free Download
I had to read this offer over a few times to figure out what it was they were giving away. Version 2.6 of the Insight client is an Microsoft Exchange compatible mail/calendar client for Linux. 2.6 is not the latest version, though, as 3.0 is coming out soon and that won’t be free. However, a good thing is a good thing, so I’ll be trying this one out at work next week!
http://www.bynari.net/Products/TradeXCH/body_tradexch.html
Multicast Video Server
When I was studying for the Cisco Switching exam, I really could have used some multicast software to test out multicast network configurations. The software on this page will stream out MPEG videos using multicasts, which cuts down on the bandwidth required to serve up multiple clients.
http://www.videolan.org/vlms/
What Are Your Expectations?
One of the driving forces behind Linux’s popularity is that it is free. This means that most of the development of Linux and its applications, is performed by volunteers. Sometimes we tend to forget this. The AbiWord project’s development team decided to write down their thoughts on it.
http://www.abisource.com/support/expectations.phtml
4) App o’ the week
Long time readers will recognize this link, but given the latest Goner virus, I thought it fitting to repost.
The Anomy Mailtool filters incoming (and outgoing, if you want) mail for possible viruses. You can hook in a commercial virus scanner, but Anomy’s strength lies in its ability to act on filenames using regular expressions. I use it myself, and it has caught new viruses without even touching the configuration.
http://mailtools.anomy.net/
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