device number registration Post: 302673633

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Top Forums UNIX for Advanced & Expert Users device number registration Post 302673633 by dr_mabuse on Wednesday 18th of July 2012 07:35:01 AM

07-18-2012

Registered User

device number registration

I would THANK if you answer my doubts in DETAILS please.

Here's a sample code to create a device:

Code:

if(scull_major) {
  dev=MKDEV(scull_major, scull_minor);
  result=register_chrdev_region(dev, scull_nr_devs, "scull");
} else{
   result=alloc_chrdev_region(&dev, scull_minor, scull_nr_devs,  "scull");
 scull_major=MAJOR(dev);
}

if(result < 0)
{
   printk(KERN_WARNING "scull:can't get major %d\n", scull_major);
   return result;
}

1. I didn't understand the first line! In the second and third lines, a device is created using the given minor and major numbers (I am sure this is a static allocation) and register_chrdev creates multiple instances of it.
what about the first line? I can't understand what that if(major number) doing there, what's so relevant to it?

2. if the major number is not created it goes to while n creates a dynamic major number. Major number means the driver ID to which the device associates with, OK? but so far, how would the kernel know about which driver it shall connect it? it just allocates one dynamically!

3.

Quote:

register_chrdev_region works well if you know ahead of time exactly which device numbers you want. Often, However, you will not know which major numbers your device will use.

!!! what's so relevant between major number and device numbers in 3? major number is the driver number the device associates with and it's not the device number (that's minor number). So, why 3 notes that?

4. Why do we need a seperate script to read /proc/devices file?

dr_mabuse

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LEARN ABOUT FREEBSD

major

MAKEDEV(3)						   BSD Library Functions Manual 						MAKEDEV(3)

NAME

     makedev, major, minor -- device number conversion

SYNOPSIS

     #include <sys/types.h>

     dev_t
     makedev(int major, int minor);

     int
     major(dev_t dev);

     int
     minor(dev_t dev);

DESCRIPTION

     The makedev() macro allows a unique device number to be generated based on its major and minor number.  The major() and minor() macros can be
     used to obtain the original numbers from the device number dev.

     In previous implementations of FreeBSD all block and character devices were uniquely identified by a pair of major and minor numbers.  The
     major number referred to a certain device class (e.g. disks, TTYs) while the minor number identified an instance within the device class.
     Later versions of FreeBSD automatically generate a unique device number for each character device visible in /dev/.  These numbers are not
     divided in device classes.

     On FreeBSD these macros are only used by utilities that need to exchange numbers with other operating systems that may use different encod-
     ings for dev_t, but also applications that present these numbers to the user in a more conventional way.

RETURN VALUES

     The major() macro returns a device major number that has a value between 0 and 255.  The minor() macro returns a device minor number whose
     value can span the complete range of an int.

SEE ALSO

     mknod(2), devname(3), devfs(5)

BSD
								September 28, 2008							       BSD