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How a Web server actually works ~ with C source code

Do you wonder how to write a program that accepts incoming messages with a network socket? Have you ever just wanted your own Web server to experiment and learn with?

Have you ever wondered how a Web server actually works? Experiment with nweb -- a simple Web server with only 200 lines of C source code. In this article, Nigel Griffiths provides a copy of this Web server and includes the source code as well. You can see exactly what it can and can't do.

Well, look no further -- nweb is what you need. This is a simple Web server that has only 200 lines of C source code. It runs as a regular user and can't run any server-side scripts or programs, so it can't open up any special privileges or security holes.

This article covers:

  • What the nweb server program offers
  • Summary of C functions features in the program
  • Pseudo code to aid understanding of the flow of the code
  • Network socket system calls used and other system calls
  • How the client side operates
  • C source code

nweb only transmits the following types of files to the browser :

  • Static Web pages with extensions .html or .htm
  • Graphical images such as .gif, .png, .jgp, or .jpeg
  • Compressed binary files and archives such as .zip, .gz, and .tar
  • If your favorite static file type is not in this list, you can simply add it in the source code and recompile to allow it.

Read more at IBM developerworks...

Recently I received a question via email:

How do I build Linux kernel module against installed or running Linux kernel? Do I need to install new kernel source tree from kernel.org?

To be frank you do not need a new full source tree in order to just compile or build module against the running kernel i.e an exploded source tree is not required to build kernel driver or module. The instruction outlined below will benefit immensely to a developer and power user.

Linux Kernel headers

This is essential because if you just want to compile and install driver for new hardware such as Wireless card or SCSI device etc. With following method, you will save the time, as you are not going to compile entire Linux kernel.

Please note that to work with this hack you just need the Linux kernel headers and not the full kernel source tree. Install the linux-kernel-headers package which provides headers from the Linux kernel. These headers are used by the installed headers for GNU glibc and other system libraries as well as compiling modules. Use following command to install kernel headers:
# apt-get install kernel-headers-2.6.xx.xx.xx

Replace xx.xx with your actual running kernel version (e.g. 2.6.8.-2) and architecture name (e.g. 686/em64t/amd64). Use uname -r command to get actual kernel version name. Please note that above command will only install kernel headers and not the entire kernel source-code tree.

A more generic (recommend) and accurate way is as follows:
# apt-get install kernel-headers-$(uname -r)

All you need to do is change Makefile to use current kernel build directory. You can obtain this directory name by typing following command:
$ ls -d /lib/modules/$(uname -r)/build
Sample output:

/lib/modules/2.6.27-7-generic/build

Let, say you have .c source code file called hello.c. Now create a Makefile as follows in the directory containing hello.c program / file:
$ vi Makefile
Append following text:

obj-m := hello.o
KDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
default:
        $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

Save and close the file. Type the following command to build the hello.ko module:
$ make

To load Linux kernel module type the command:
# modprobe hello

A complete example

Create test directory (you can download following Makefile and .c file here):
$ mkdir ~/test
$ cd ~/test

Create hello.c kernel module file:

#include <linux/module.h>	
#include <linux/kernel.h>	
 
int init_module(void)
{
	printk(KERN_INFO "init_module() called\n");
	return 0;
}
 
void cleanup_module(void)
{
	printk(KERN_INFO "cleanup_module() called\n");
}
 

Create a Makefile:

obj-m += hello.o
 
all:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
 
clean:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clea

To build kernel module enter:
$ make
Sample output:

make -C /lib/modules/2.6.27-7-generic/build M=/tmp/test2 modules
make[1]: Entering directory `/usr/src/linux-headers-2.6.27-7-generic'
  CC [M]  /tmp/test2/hello.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /tmp/test2/hello.mod.o
  LD [M]  /tmp/test2/hello.ko
make[1]: Leaving directory `/usr/src/linux-headers-2.6.27-7-generic'

Run ls command to see newly build kernel module:
$ ls
Sample output:

hello.c  hello.ko  hello.mod.c	hello.mod.o  hello.o  Makefile	Module.markers	modules.order  Module.symvers

hello.ko is kernel module file. To see information about module, enter:
$ modinfo hello.ko
Sample output:

filename:       hello.ko
srcversion:     4F856ABA1F3290D5F81D961
depends:
vermagic:       2.6.27-7-generic SMP mod_unload modversions 586 

To load kernel module, enter:
$ sudo insmod hello.ko
OR
$ sudo modprobe hello
To list installed Linux kernel module, enter:
$ lsmod
$ lsmod | grep hello

To remove hello Linux kernel module, enter:
$ rmmod hello
This module just logs message to a log file called /var/log/messages (/var/log/syslog), enter:
$ tail -f /var/log/messages
Sample output:

Nov  5 00:36:36 vivek-desktop kernel: [52488.923000] init_module() called
Nov  5 00:36:50 vivek-desktop kernel: [52503.065252] cleanup_module() called

Download above examples in zip format

See a detailed and completeexample with C source code about Compiling Linux kernel module for more information.