Linux character driver skeleton

Driver file:

1. /*
2.  * simplechrdriver.c
3.  *
4.  * Copyright (C) 2008 Breathomn <breathomn@sohu.com>
5.  *
6.  */
7. #include <linux/module.h>
8. #include <linux/kernel.h>
9. #include <linux/fs.h>
10. #include <linux/init.h>
11. #include <asm/uaccess.h>    /* copy_*_user */
13. static unsigned int major_num = 0;
15. static int global_var = 0;
17. static ssize_t simplechr_read(struct file *filp, char *buf, ssize_t size, loff_t *off)
18. {
19.     if (copy_to_user(buf, &global_var, sizeof(int)))
20.         return -EFAULT;
21.     return sizeof(int);
22. }
24. static ssize_t simplechr_write(struct file *filp, const char *buf, ssize_t size, loff_t *off)
25. {
26.     if (copy_from_user(&global_var, buf, sizeof(int)))
27.         return -EFAULT;
28.     return sizeof(int);
29. }
31. static struct file_operations simplechrdev_ops = 
32. {
33.     .owner  = THIS_MODULE,
34.     .read   = simplechr_read,
35.     .write  = simplechr_write,
36. };
39. static int __init simplechr_init(void)
40. {
41.     int ret;
42.     if ((ret = register_chrdev(major_num, "simplechrdev", &simplechrdev_ops)) < 0)
43.         printk(KERN_ALERT "simplechrdev register failed!/n");
44.     else
45.     {
46.         major_num = ret;
47.         printk(KERN_ALERT "simplechrdev register success!/n");
48.     }
49.     return ret;
50. }
52. static void __exit simplechr_exit(void)
53. {
54.     int ret;
55.     if ((ret = unregister_chrdev(major_num, "simplechrdev")))
56.         printk(KERN_ALERT "simplechrdev unregister failed!/n");
57.     else
58.         printk(KERN_ALERT "simplechrdev unregister success!/n");
59. }
61. module_init(simplechr_init);
62. module_exit(simplechr_exit);

User file:

1. /*
2.  * simplechrsrc.c
3.  *
4.  * Copyright (C) 2008 Breathomn <breathomn@sohu.com>
5.  *
6.  */
7. #include <sys/types.h>
8. #include <sys/stat.h>
9. #include <stdio.h>
10. #include <fcntl.h>
12. int main()
13. {
14.     int fd, num;
15.     if ((fd = open("/dev/simplechrdev", O_RDWR, S_IRUSR | S_IWUSR)) != -1)
16.     {
17.         read(fd, &num, sizeof(int));
18.         printf("Read: %d/n", num);
20.         printf(" Write a num: ");
21.         scanf("%d", &num);
22.         write(fd, &num, sizeof(int));
24.         read(fd, &num, sizeof(int));
25.         printf("Raed again: %d/n", num);
27.         close(fd);
28.     }
29.     else
30.         printf("Open device failed!/n");
31. }

Makefile file:

 

obj-m := simplechrdriver.o

KERNELDIR ?= /lib/modules/$(shell uname -r)/build

PWD       := $(shell pwd)

all:
            $(MAKE) -C $(KERNELDIR) M=$(PWD)

clean:
            rm -rf *.o *~ core .depend .*.cmd *.ko *.mod.c .tmp_versions

 

Compile our driver into kernel:
(1) Create a new directory named simplechrdev in "$(KERNELSOURCE)/drivers/char" directory.
(2) Copy the driver file "simplechrdriver.c" to the new directory.
(3) Create a Kconfig file in "simplechrdev" directory with contents below:
    config SIMPLECHRDEV
    tristate "SimpleChrDev"
(4) Modify the Kconfig file of its parent directory "char", add contents:
    source "drivers/char/simplechrdev/Kconfig"
    Then we can find a item named "SimpleChrDev" in "Device Driver->Character devices" when we use "make menuconfig/xconfig".
(5) Create a Makefile file in "simplechrdev" directory with contents below:
    obj-$(CONFIG_SIMPLECHRDEV)      += simplechrdriver.o
(6) Modify the Makefile file of its parent directory "char", add contents:
    obj-$(CONFIG_SIMPLECHRDEV)      += simplechrdev/
    All things done! Remake kernel and reboot computer, the driver simplechrdriver works probably. Input the command "cat /proc/devices", our device simplechrdev is in the Character devices list.
(7) To load our module automatically
    We can choose to compile our driver as a module, but sometimes the kernel can not load our module automatically. We can find a solution that add some contents to "/etc/rc.d/rc.sysinit" file to the problem.
    # SimpleChrDev
    if [ -d /lib/modules/$unamer/kernel/drivers/char/simplechrdev ]; then
    for module in /lib/modules/$unamer/kernel/drivers/char/simplechrdev/*; do
    module=${module##*/}
    module=${module%.ko}
    modprobe $module >/dev/null 2>&1
    done
    fi