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I/O port accessing

Posted on 2003-12-07
Medium Priority
Last Modified: 2010-04-22

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.

Question by:gautam_venkatesh
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LVL 44

Accepted Solution

Karl Heinz Kremer earned 80 total points
ID: 9891880
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.


Author Comment

ID: 9896516
Thanks for the response.
Why do I get undefined reference to __check_region?

LVL 44

Expert Comment

by:Karl Heinz Kremer
ID: 9896686
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.
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Author Comment

ID: 9904541
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.
LVL 44

Expert Comment

by:Karl Heinz Kremer
ID: 9904980
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.

Expert Comment

ID: 9971278
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 :)
LVL 44

Expert Comment

by:Karl Heinz Kremer
ID: 9971534
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().

Expert Comment

ID: 9989683
If u have some more doubts post it


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