I/O port accessing


I am a beginner at writing Linux dev drivers. I am facing problem while trying to compile and install the below simple module. The error is, "inb","outb" and
"--check_region" can't be resolved.

The code is

#define __KERNEL__
#define MODULE
#include <linux/module.h>
#include <linux/ioport.h>
int init_module()
    check_region(0x23c, 0x04);
    outb((unsigned char)"a", 0x23c));
    printk("<1> The data at port is %d",inb(0x23c));
    return 1;

void cleanup_module()


I am using gcc -O to compile. and insmod to install the module using the object code. Please remember that I am a beginner and would appreciate an explaination as detailed as possible. All I am trying to do is get familiar with the idea of reading and writing from IOPorts. If you can provide some information on this, it will be much appreciated. I eventually want to learn how to write a device driver in Linux and I am using O'reilley publications' "Linux Device Drivers" and it is not particularly helpful for rank beginners.

Who is Participating?
Karl Heinz KremerCommented:
If the book is not too helpful, try the "Linux Kernel Module Programming Guide" at http://www.tldp.org/LDP/lkmpg/

In your case, you have to also add a line
#include <asm/io.h>

You should also return 0 from your init_module function: Anything but 0 will report flag an error.

gautam_venkateshAuthor Commented:
Thanks for the response.
Why do I get undefined reference to __check_region?

Karl Heinz KremerCommented:
This worked for me. Which kernel version are you using? If I remember correctly, the use of check_region is discouraged, but it should still be in your header files. It should be declared in ioport.h. I'm not at my Linux box, so I cannot verify this.
Cloud Class® Course: Amazon Web Services - Basic

Are you thinking about creating an Amazon Web Services account for your business? Not sure where to start? In this course you’ll get an overview of the history of AWS and take a tour of their user interface.

gautam_venkateshAuthor Commented:
I am using kernel version 2.4.2. If check_region usage is discouraged, what is the alternative that I could use?
Can you provide a code snippet as well.
Karl Heinz KremerCommented:
Not right now (I'm not at my Linux machine). Do you have to use a kernel that old? I'm using 2.4.20, and it does handle check_region correctly.
Hi Gautham

First of all ,I don't understand the reason for using the check_region () call ,Basically it is done to check ports are available to transreceiver bits,I mean used to serial,parallel or etc...

go to this link that will provide u the port address that u interact with.

If this call succeds it means ports are free
then u got to go for request region before
request_region(port,range,device name);

But if u want to right pseudo driver,Basic driver check region is not required i guess,This explianation may help u

chiku :)
Karl Heinz KremerCommented:
The reason for check_region is to see if a different driver already has allocated the ports. Older kernels did not provide a return value for request_region(), therefore it was necessary to check first (because request_region was not able to flag an error).

I have to admit that I have not done any driver development with 2.4, so I'm not up do date on the changes in regards to check_region().
If u have some more doubts post it

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

All Courses

From novice to tech pro — start learning today.