Creating a misc characther device and reading from it

Linux has misc devices which meant for small device drivers. According to the article written by Alessandro Rubini Miscellaneous Character Drivers :

Sometimes people need to write “small” device drivers, to support custom hacks—either hardware or software ones. To this end, as well as to host some real drivers, the Linux kernel exports an interface to allow modules to register their own small drivers. The misc driver was designed for this purpose.

miscdevice is declared as the structure in miscdevice.h

struct miscdevice { int minor; const char *name; const struct file_operations *fops; struct list_head list; struct device *parent; struct device *this_device; const struct attribute_group **groups; const char *nodename; umode_t mode; };

For this example we will just use the first three fields.

minor: Minor number assigned to the device name: The name by which the device will be known fops: The file_operations structure mapped to the the device.

The name of the device can be any name that the user wants. The minor number can be statically assigned by the user , if it has to be dynamically assigned by the user , the value has to be set to `MISC_DYNAMIC_MINOR.

Thus our device structure will look as below.

struct miscdevice my_misc = { .minor = MISC_DYNAMIC_MINOR, .name="mymisc", .fops = &myfops, };

To register a misc device the function used is

int misc_register ( struct miscdevice * misc); where struct miscdevice * misc: device structure

We can create a characther string which will hold the data for now which will be sent out everytime the device is read. Thus we create a function to fill the data for the device.

void fill_data(void) { msg=kmalloc(10*sizeof(char),GFP_KERNEL); msg="hello"; len=strlen(msg); size=sizeof(char)*len; return 0; }

The init function for the device will have to register the device. Unlike a charachter device, we dont need to get a major number for a misc device as the default major number for misc devices is 10. To see the minor number which will be assigned dynamically we will add a print statement to get the minor number.

int misc_init(void) { misc_register(&my_misc); printk(KERN_INFO "registered device %d",my_misc.minor); return 0; }

We will create two file operations for this, one to open the device and one to read from the device. Thus the file operations structure will be

struct file_operations myfops = { .open = misc_open, .read = misc_read, };

The open function will fill data into the device,which we can read using the read function.

static int misc_open(struct inode *inode, struct file *file) { printk(KERN_INFO "Opening misc device\n"); fill_data(); return 0; }

The read function will copy the contents of the device to the userspace.

static ssize_t misc_read(struct file *file, char __user *buf, size_t count, loff_t *offp){ int temp,ret=size; if(size) { temp=copy_to_user(buf,msg,size); size=size-(size-temp ); printk(KERN_INFO "Reading device"); } return ret; }

Thus the full code misc device will be



Save the above code as misc_device_read.c

Use the following makefile to compile the code.

ifneq ($(KERNELRELEASE),) obj-m := misc_device_read.o else KERNELDIR ?= /lib/modules/$(shell uname -r)/build PWD := $(shell pwd) default: $(MAKE) -C $(KERNELDIR) M=$(PWD) modules clean: $(MAKE) -C $(KERNELDIR) M=$(PWD) clean endif

Compile and insert the code using

$ make $ insmod misc_device_read.ko

To see the minor number allocated to the device, use dmesg

$ dmesg | tail -1 [ 515.426861] registered device 55

We can also view the minor number and find if the device got registered succssfully by looking into the file /proc/misc

55 mymisc 229 fuse 56 vsock 57 rfkill 58 vmci 235 autofs 59 memory_bandwidth 60 network_throughput 61 network_latency 62 cpu_dma_latency 1 psaux 175 agpgart 228 hpet 231 snapshot 63 vga_arbiter

The device will automatically get listed under /dev which we can view using the ls command.

$ ls -l /dev/mymisc crw------- 1 root root 10, 55 Mar 30 04:08 /dev/mymisc1

We can see that the major number is 10 and the minor number is 55.

To read the device, we can use the cat command.

$ cat /dev/mymisc hello