Which is the best way for accessing device files in C?

halturata · 09-06-2005, 04:09 AM

Hi there,

I have to write a small programm in C which talks to a VMEbus and receives respond, but I don't know which is the best way for accessing the particular vme_* device files. Is there a special C function for doing this or the standart I/O functions fopen(), fclose() etc. will also do? Can anyone give me some hints?

Thanks.

rjlee · 09-06-2005, 05:26 AM

You can use standard I/O routines to access device files. That's open()/close()/read()/write()/seek()/tell()/ioctl() or fopen()/fclose()/fread()/fwrite()/fseek()/ftell() depending on if you want software buffering or not.

The only thing you can't do with buffering is ioctl(), which sends special messages to the hardware device. It doesn't really make much sense to do this on buffered streams, because you don't know where the hardware is up to in processing the stream; although you can still use ioctl() by digging the file descriptor out of the FILE structure.

The reason for this is that Linux is a UNIX system, which was originally designed in C. C only provides functions for accessing files, not specific hardware devices, and the device file was created as a way of abstracting away all the hardware-specific code and making a standard form of access method that was so simple people could actually stick to it.

halturata · 09-06-2005, 05:55 AM

Quote:

Originally posted by rjlee
The only thing you can't do with buffering is ioctl(), which sends special messages to the hardware device.

What kind of special messages are those, and what they are used for? I need just an overview, that's all.
Thanks a lot!

keefaz · 09-06-2005, 07:04 AM

check the man page for ioctl_list

halturata · 09-06-2005, 08:59 AM

And what about this - when I create a new device file with the command

Code:

[root@host]# mknod -m755 testdev c 240 0

and then try to access it, I get the following message:

Code:

[root@host]# less -f testdev
testdev: No such device

Whats wrong with it? Or there is another way to see what's inside a device file?

halturata · 09-06-2005, 09:51 AM

An still the device file won't open and I don't know why:

Code:

int main(void){
...   
if((fopen("/dev/testdevl", "r+"))==NULL){
    perror("fopen()");
    printf("Cannot open file.\n");
    exit(1);
   }
...
}

This code returns:

Code:

fopen(): No such file or directory
Cannot open file.

And the problems is NOT that the 'testdev' device does not exist. It is present in /dev. Any ideas what is wrong with it?

exvor · 09-06-2005, 05:45 PM

If you create a device node that points to a device that is not really there what else
were you expecting?

computers only do what there told to.

halturata · 09-07-2005, 05:25 AM

I see my stupid mistake now.

And how you make it so that the device node points to a particular device? I mean is there a command for this or so?
Thanks.

rjlee · 09-07-2005, 08:17 AM

Each device has a “major” and “minor” device number. You can pass these numbers as arguments to mknod when you create the device file node in the first place.

You can get the numbers to use from the source code for the device driver of the device you're using; many of the standard numbers are listed in the file /usr/src/linux/Documentation/devices.txt

halturata · 09-07-2005, 12:31 PM

The major and minor numers are OK, the problem is that when I try to load the driver module I get the following messages:

Code:

[root@host]# insmod ca91c042.o
Warning: ca91c042.o will taint the kernel: no license
ca91c042.o: init_module: Device or resource busy
Hint: insmod errors can be caused by incorrect module parameters, including invalid IO or IRQ parameters.

What's that "Device or resource busy"? I have created all the necessary devices...
And what about these IO and IRQ parameters? Are they to be changed somehow?
Any idea what is wrong (except me being a luser)?

rjlee · 09-07-2005, 12:58 PM

A device is a physical object that plugs (or wires) in to the computer. You've created a device-special file.

The IO and IRQ parameters are parameters passed to the kernel module to tell it which I/O port and which IRQ line to use. You'll find out the values to use from the documentation and/or jumper settings on the hardware, although most devices for the last few years will allow you to auto-detect these values. See the insmod man-page for details on passing command-line parameters to modules.

Make sure that no other module to drive this piece of hardware (device) are loaded into the kernel when you try to load this module.

halturata · 09-08-2005, 05:18 AM

Quote:

Originally posted by rjlee
Make sure that no other module to drive this piece of hardware (device) are loaded into the kernel when you try to load this module.

How can I check this? I mean when I try to insert the module and the system complains that the device is busy, does this mean that there is already a module loaded into the kernel for that particular device?

Siddhi Verma · 10-09-2018, 03:09 AM

Quote:

Originally Posted by halturata

I see my stupid mistake now.

And how you make it so that the device node points to a particular device? I mean is there a command for this or so?
Thanks.

Could you kinldy tell, how did you solve this issue? Even I am facing the same problem

astrogeek · 10-09-2018, 02:58 PM

You have replied to a thread which has been abandoned for 13 years, please do not do that.

To get exposure to currently active members please post your own question with the details specific to your use case in a new thread.

Please review the Site FAQ for guidance in asking well formed questions.

Welcome to LQ!

NevemTeve · 10-09-2018, 03:44 PM

(Anyways it would be good idea to explain what kind of hardware you have -- I mean device drivers usually work in conjuction with some piece of hardware. (Actully, these questions don't strictly belong to programming.))