Thursday, October 29, 2009

"hello world" program without a main() function

Hi Friends,

This short article is on request of my friend Viswanath who wanted to run a C program without main() function on his computer .

I have done this on Ubuntu OS , with the simple Linux module , taken form guide :
"The Linux Kernel Module Programming Guide"

note : Aim of this article is not to teach you how to write a Linux kernel module , but to simply write a C program which runs on your Linux system and prints "Hello world", without use of main() function

only thing worth mentioning is "output" from "printk" goes to by default "/var/log/messages" , so your "hello world" would be printed their instead of terminal.

Step 1 :

create a file named hello-1.c with code given below :

* hello−1.c − The simplest kernel module.
#include /* Needed by all modules */
#include /* Needed for KERN_INFO */
int init_module(void)
printk(KERN_INFO "Hello world 1.\n");
* A non 0 return means init_module failed; module can't be loaded.
return 0;
void cleanup_module(void)
printk(KERN_INFO "Goodbye world 1.\n");

//code ends here

Step 2 :

create a Makefile , with following code : (ie a file name Makefile )

//code starts here

obj-m += hello-1.o

"press tab"make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
"press tab"make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

//code ends here

Step 3:

from terminal with sudo premission run following command :

$ make

you will see output like given below if successful :

hostname:~/lkmpg−examples/02−HelloWorld# make
make −C /lib/modules/2.6.11/build M=/root/lkmpg−examples/02−HelloWorld modules
make[1]: Entering directory `/usr/src/linux−2.6.11'
CC [M] /root/lkmpg−examples/02−HelloWorld/hello−1.o
Building modules, stage 2.
CC /root/lkmpg−examples/02−HelloWorld/hello−1.mod.o
LD [M] /root/lkmpg−examples/02−HelloWorld/hello−1.ko
make[1]: Leaving directory `/usr/src/linux−2.6.11'

Step 4:

to load your module run command :

$ insmod ./hello-1.ko

now your module should be added in the module list of kernel. For checking this type following command :

$ cat /proc/modules

the first entry should be hello-1 , and thats your module.

Step 5:

to unload your module use following command :

$ rmmod hello-1

Step 6 :

wondering where did your printk " Hello world 1." and " Goodbye world 1."

went just and check the file :

$ cat /var/log/messages

towards the end you will find the two entries ,

Hello world 1.
Goodbye world 1.

Thats it.
Thanx for reading.

Note: if you really want to understand how all this worked please go through the article "The Linux Kernel Module Programming Guide" , which is easily available on web.

1 comment:

  1. hey guys nw u can see hw to run c program without using main function...
    to see visit


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Hi Friends, I am Samarth currently pursuing my Masters degree in Information Technology from International Institute of Information Technology ,Bangalore .