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Thursday, July 12, 2001
Read By Over 6,000 Linux Enthusiasts Weekly!TABLE OF CONTENTS
1) Sean’s Notes
2) Linux News
Caldera OpenLinux to Require Licencing
CerfBoard
Interview with a PHP Developer
Microsoft to Allow Some Rebranding in XP3) Linux Resources
HTTP Benchmarking
Security Manuals
SNMP FAQ
PostgreSQL Book
XPilot Newbie Guide4) App o’ the week
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1) Sean’s Notes
One of the fundamentals of UNIX is how processes are created. Sure, we know init does all the dirty work, but how do processes themselves call for new processes? If you’re running a web server you’ll see that there are a bunch of httpd processes going, but you only started one of them. Likewise, inetd (or xinetd) can fire off your ftp server if a connection comes in.
Well, folks, it’s all done through the fork() system call. Fork, like the name implies, denotes a split of paths. A simple example (fork1.c) will illustrate (or confuse):
int main(void) { printf(“Hello (%d)\n”, getpid()); fork(); printf(“Goodbye (%d)\n”, getpid()); }
Remember from a previous article that the main function is where everything starts. We’re going to print out “Hello” along with our processid. Then we call fork(), and then another silly message along with the PID. Should be simple, eh?
$ gcc fork1.c -o fork1 $ ./fork1 Hello (3169) Goodbye (3169) Goodbye (3170)
Huh? I only told it to print out Goodbye once! Look at the PID, though. The first and second are from different processes. The fork() actually returns twice, but after creating a second process. 3169 is the parent process, 3170 is the child process.
So if fork() returns twice, what kind of value does it return? Here is fork2.c:
include <sys/types.h>
include <unistd.h>
int main(void) { pid_t pid; printf(“My pid is %d\n”, getpid()); pid = fork(); printf(“Fork returned %d in pid %d\n”, pid, getpid()); }
The differences here are that I included a couple of files that the man page for fork() recommended. This gets me the pid_t type which stores the result of a fork(). I then captured the return value of the system call, and printed it out along with the PID.
$ gcc fork2.c -o fork2 $ ./fork2 My pid is 3235 Fork returned 3236 in pid 3235 Fork returned 0 in pid 3236
Now this is pretty interesting. The parent (3235) was given the PID of its child (3236). The child was given a big fat zero. But this isn’t a problem, because a child can always find its parent with getppid(), but it is difficult for a parent to find all its children.
Now, on to the practical part of the whole exercise. Forks are generally used as worker tasks. Client server programming is helpful in systems administration, you can write a quick little daemon that waits around for connections and does some small tasks. When the connection comes in, you don’t want the process to be busy until the work is done, you want to wait for another connection. The solution? fork() off another process. If it returns 0, you’re in the child process, so do the work. If you get a positive number, you’re the parent, so go back to listening.
On that thread (pardon the pun), you could have a program that is interacting with the user. Rather than making the user wait while some action is performed (like a backup), fork() a child process to do the work. If you recall a couple of weeks ago, we talked about signals. Well, when the child process dies (finishes), the parent process is notified with SIGCHLD.
http://www.ertw.com/~sean/newsletter/June+28%2C+2001
The fork() system call isn’t only accessible in C, many other languages let you use it in the same manner, such as PERL.
I’m going to give you one more example that has no useful application, except to demonstrate where init fits into all of this. See if you can answer the question before trying this out.
“If the parent process dies before the child process, who is the child’s parent?”
Here’s fork3.c, which will attempt to answer that:
int main(void) {
printf("My pid is %d\n", getpid());
if (fork() == 0 ) { /* child */
printf("I am %d, my parent is %d\n", getpid(), getppid());
sleep(5);
printf("I am %d, my parent is %d\n", getpid(), getppid());
} else { /* parent */
sleep(1);
printf("I am the parent %d, but I'm leaving now\n", getpid());
}
}
I’m using the sleep() library call to make each process wait so that the child can display its parent before and after it dies. This example also demonstrates the way that programmers use fork() to decide who is the child and who is the parent.
$ gcc fork3.c -o fork3 $ ./fork3 My pid is 3353 I am 3354, my parent is 3353 I am the parent 3353, but I’m leaving now I am 3354, my parent is 1
The child, 3354, was picked up by PID 1 – our friend init.
So that’s process creation in a nutshell. Even if you never use it, understanding fork() is almost essential in order to understand the rest of the system. Every time you run a command, your shell is fork()ing off a new process. Following the parent-child relationship in the process tree gives you a better handle on the state of the system. This whole business of parents dying before their children is what causes zombies (and was going to be today’s topic, but I got sidetracked).
I’ve posted a poll on the Linux news board, please give your opinion on the use of C code and the discussion of system calls in general:
http://boards.brainbuzz.com/boards/vbm.asp?rpg=1&wpg=1&sb=0&pvmlse &m21050
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
Caldera OpenLinux to Require Licencing
I can’t believe anyone thinks this is a good idea. Users of Caldera 3.1 OpenLinux Workstation will be required to buy a licence for each computer they run it on. I’m thinking that within six months, they’ll either reverse this decision or go bust. How about you?
http://linuxtoday.com/news_story.php3?ltsn 01-06-25-006-20-PS
CerfBoard
Besides a cool name, this device features a 192 MHz StrongARM 1110 with 32 MB of RAM, 16 MB of non-volatile flash memory, ethernet, and a lot of IO. Not too useful as a general purpose computer, but this embedded device prototype fits into 0.015 cubic foot!
http://www.linuxdevices.com/articles/AT2683549967.html
Interview with a PHP Developer
PHP (http://www.php.net) is an ultra cool, super powerful, web development language, along the lines of ASP. Linux.com interviews one of the developers here, and he offers some insight into the future of the language, along with the reasoning behind ActiveState’s commercial interest in the software. ActiveState is the company that ported PERL to WIN32.
http://www.linux.com/develop/newsitem.phtml?sid&aid466
Microsoft to Allow Some Rebranding in XP
I guess all the antitrust suits must be paying off. In this MS press release, our favourite monopoly says they’ll allow OEMs to place icons on the desktop, and remove some Start menu items. My favourite is that they promise to make the end user parts of IE easily removable, but then again they’ve always claimed you could do that.
http://www.microsoft.com/presspass/press/2001/jul01/07-11OEMFlexibi lityPR.asp
3) Linux Resources
HTTP Benchmarking
If you’re doing any sort of dynamic web pages, system administrators everywhere will thank you if you take performance into consideration. Systems administrators might also want to see how well the latest upgrade improves speed on the web server. In both cases, you’ll want to do some benchmarking. This HOWTO explains how it should be done so that you get reliable data.
http://www.xenoclast.org/doc/benchmark/HTTP-benchmarking-HOWTO/
Security Manuals
The Idea Hamster, besides being a funny name, is the title of a project that is developing open-sourced manuals for security-related topics. There is one on how to perform security tests, how to write secure code, and much more.
http://www.ideahamster.org/
SNMP FAQ
The Simple Network Management Protocol can be used to get information on systems such as a Linux box or a network switch. Despite the “Simple” in the title, the whole concept can be a bit confusing, so this FAQ will help you out. Network and Systems people alike should be well versed in this protocol …used properly, it can make your job a lot easier.
http://www.pantherdig.com/snmpfaq/
PostgreSQL Book
This online book is all about PostgreSQL, a free relational database. PGSQL is very powerful, having many more features than MySQL. This comes at a cost of resources and more skills to administer it. Luckily, you’ll have this book handy to get you through it.
http://www.ca.postgresql.org/docs/aw_pgsql_book/index.html
XPilot Newbie Guide
Ever since finding this game, I’ve been hooked. XPilot is a multiplayer game where you control a little spaceship, and fly around a map fighting with other people. There is even a team play mode, where the object is to get the other team’s treasure back to your own base. The Newbie Guide here will show you how to control your ship so you don’t get blown out of the skies quite so fast.
http://www.j-a-r-n-o.nl/Xpilot/Newbie/Unix/newbie.shtml
4) App o’ the week
TPC stands for “The Phone Company”, which is a project designed to build a worldwide fax over email service. I didn’t even notice that RedHat has the client for this on the distribution CDs until the other day! If your recipient is in an area covered by this, you might just save a few bucks on long distance. If you live in an uncovered area, it might be worthwhile to set up a node. Not only are you giving something to the community, but it’s a pretty good way to get into UNIX administration if you’re looking for a project to do.
http://www.tpc.int
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