LINUX NEWS
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Thursday, June 28, 2001
Read By Over 6,000 Linux Enthusiasts Weekly!TABLE OF CONTENTS
1) Sean’s Notes
2) Linux News
Loki MPEG Player Update
RedHat Enters Database Market
Putting a Bad API on a Good OS
BOFH Meets the Linux Evangelist3) Linux Resources
SAIR Linux Security Certification
Linux on the Desktop: A Guide For Windows Users
Installing a Custom Kernel
Fundamentals of Multithreading
The CGI Bin4) App o’ the week
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1) Sean's Notes
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It's been a while since we looked at some of the stuff that
goes on behind the scenes. This week, we're getting back
into it by brushing up on signals.
Signals are messages that are sent to a process. Messages
might be from you (quit), or from the kernel itself (hey,
you just stomped on someone else's memory!). In the Windows
world, programs cycle around an event loop. In UNIX, they
are allowed to go on their merry way, but can optionally trap
certain signals. Signals have a default action, so if the app
doesn't trap the signal, it may continue as normal, or perhaps
be stopped by the system.
You're probably already familiar with signals, if you've
ever done:
# kill 1234
# killall -HUP named
then you've sent signals to processes. The first example
sends the SIGTERM signal to process 1234, which will cause it
to stop execution. The second sends the SIGHUP signal to all
the named processes. Uncaught, SIGHUP (hangup) will also
cause the application to terminate, but common usage is to
trap it and cause the application to re-read its configuration
files. There are about 30 signals used in Linux, but you'll
only need to know a few of them.
To give you an idea of how this all operates, I'm going to
have to write some code. If you want to follow along with me,
you can call this t1.c.
<div class="highlight"><pre><code class="c"><span class="cm">/* Program execution starts here */</span>
<span class="kt">int</span> <span class="nf">main</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"My PID is %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">getpid</span><span class="p">());</span>
<span class="k">while</span> <span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="p">{</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"Good night!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="n">sleep</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"Good morning!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
Compile this with:
gcc t1.c -o t1
which will create an executable called t1. In C, execution
starts in the main() function. The code prints it's current
processid, and then enters an infinite loop. Run the program:
./t1
and you'll see that you get "Good night!", a 10 second pause
(that's the sleep() function in action), followed by "Good
morning!", then it repeats.
Still with me? Pop open a new window, and send a HUP signal
to the process. If you saw:
My PID is 21317
then in your other window you'll type:
kill -HUP 21317
On my screen, I get:
$ ./t1
My PID is 21317
Good night!
Hangup
$
So, even though the program never had any logic to get out
of that infinite loop, the operating system terminated the
process (and wrote "Hangup"). Let's catch that signal now.
Call this file t2.c:
<div class="highlight"><pre><code class="c"><span class="cp">#include &lt;signal.h></span>
<span class="cm">/* This is the signal handler */</span>
<span class="kt">void</span> <span class="nf">handler</span><span class="p">(</span><span class="kt">int</span> <span class="n">signum</span><span class="p">)</span> <span class="p">{</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"Inside handler (signal %d)</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">signum</span><span class="p">);</span>
<span class="p">}</span>
<span class="cm">/* Program execution starts here */</span>
<span class="kt">int</span> <span class="nf">main</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"My PID is %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">getpid</span><span class="p">());</span>
<span class="n">signal</span><span class="p">(</span><span class="n">SIGHUP</span><span class="p">,</span> <span class="o">&</span><span class="n">handler</span><span class="p">);</span>
<span class="k">while</span> <span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="p">{</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"Good night!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="n">sleep</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"Good morning!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
The first line imports all the definitions for the signal
handling functions. The next few lines define a signal
handler, simply called "handler". A signal handler has a
known form -- it can return nothing, and accepts an integer
(signal.h has all that stuff in there). Inside handler, I
simply print out the value of that integer, and return.
The last difference is that I call the signal() function
before I start the loop, which associates the signal (SIGHUP)
with my handler. Build the program and then run it:
$ gcc t2.c -o t2
$ ./t2
My PID is 21343
Good night!
Send the HUP signal again, and see what happens:
$ kill -HUP 21343
Inside handler (signal 1)
Good morning!
and execution continues. Why? t2.c caught the HUP signal,
and decided to continue execution. You can kill it by
sending a TERM (actually, that's the default) signal to it:
$ kill 21343
Terminated
In fact, there is no reason we can't capture SIGTERM and
ignore it, though in practice we would want to exit
gracefully instead of just dying. To make sure that people
don't do stuff like that, some signals can not be ignored,
such as SIGKILL. SIGKILL is a heavy handed version of
SIGTERM, it has signal number 9, and you can do stuff like...
kill -9 21343
kill -KILL 21343
...to get rid of that process that just won't die. I use
this one a lot on Netscape:
killall -9 netscape-navigator
because sometimes it just hangs and there is no way to get
rid of it.
There are some processes that can't be killed with good ol'
signal 9, these are called "zombies". I won't get into what
a zombie is today, but they show up on a "ps -ef" with the
word zombie on the line. To get rid of those, you have to
kill its parent (third column in the ps listing).
You can get a list of all the signals through the signal(7)
man page:
man 7 signal
Those interested in learning more about this kind of stuff
should pick up "UNIX Network Programming" by W Richard
Stevens. A necessity if you're going to get into network
programming, and inter-process communications.
Long live the Penguin,
Sean
mailto:swalberg@brainbuzz.com
Visit the Linux News Board at
http://boards.brainbuzz.com/boards/vbt.asp?b2
-------------------------
2) Linux News
-------------------------
-----------------------
Loki MPEG Player Update
-----------------------
Loki, the porters of many fine games, developed an MPEG
library a while ago to handle playback of movies. This most
recent update fixes a bunch of bugs and adds some features.
If you're looking for an MPEG player, here is a great start.
http://www.lokigames.com/development/smpeg.php3
-----------------------------
RedHat Enters Database Market
-----------------------------
RedHat, "the leader in developing, deploying and managing
open source solutions, today announced Red Hat Database.
The Red Hat Database product is an open source database
solution, based on PostgreSQL 7.1, and optimized with Red
Hat Linux 7.1, for mid-sized organizations and corporate
workgroups and departments."
http://www.redhat.com/databasesolution
------------------------------
Putting a Bad API on a Good OS
------------------------------
Some might say that OS/2's WIN32 compatibility was part of
its downfall since it offered vendors no incentive to target
it specifically. So, are projects like WINE, which implement
the WIN32 API under Unix, doing the same?
http://www.osopinion.com/perl/story/11297.html
-------------------------------
BOFH Meets the Linux Evangelist
-------------------------------
Everyone's favourite system administrator, the BOFH, matches
wits with a luser...I mean user...who doesn't quite get the
hang of his new Linux system. "Where's Word?" he asks. See
how the BOFH handles this call.
http://www.theregister.co.uk/content/30/19898.html
-------------------------
3) Linux Resources
-------------------------
---------------------------------
SAIR Linux Security Certification
---------------------------------
If you were looking to write the SAIR Linux Security exam,
you'd be wise to check out Brainbuzz user ironpaw's view.
He wrote the exam and was, shall we say, less than impressed.
http://boards.brainbuzz.com/boards/vbm.asp?m13946
-----------------------------------------------
Linux on the Desktop: A Guide For Windows Users
-----------------------------------------------
This article (watch out for line wrapping in the URL) is a
nice little introduction on what has to be done when
installing a desktop version of Linux, and what you can
expect. Those that have done it once or twice might find
their time better spent elsewhere, but for the complete
newbie it's a worthwhile read.
http://www.extremetech.com/article/0,2299,s%253D1028%2526a%253D5549
,00.asp
--------------------------
Installing a Custom Kernel
--------------------------
Though I wrote about this topic a while back, this article
puts the whole process of kernel compilation and installation
into a nice stepped process. It is geared toward the people
who are recompiling the existing kernel, instead of doing an
upgrade, so take some extra care if you're doing the latter.
http://www.linux.com/develop/newsitem.phtml?sid=1&aid439
------------------------------
Fundamentals of Multithreading
------------------------------
Threads are otherwise known as light weight processes, they
are one of the methods that allow applications to take
advantage of multi-processor machines. Within this article,
a lot of threading and MP topics are examined. Warning - this
article gets pretty technical.
http://www.systemlogic.net/articles/01/6/multithreading/
-----------
The CGI Bin
-----------
CGI, the Common Gateway Interface, allows a web server to
interact with other processes to produce dynamic web pages.
This site has several tutorials that introduce you to the
topic using some pretty handy examples.
http://www.thecgibin.com/
-------------------------
4) App o' the week
-------------------------
This looks like a handy one...If your server is so locked up
that you can't log in to fix things, but can still be pinged,
this kernel patch will let you send a special ping to reboot.
Don't worry, there is a password mechanism in place!
http://nail.itapac.net/ricmp/
-------------------------
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