Implementing an ioctl call (for kernel versions above 2.6.39)

In the post "Creating an ioctl command in linux " we saw the basics of ioctl, and how we can create our own ioctl commands. The functions used to create ioctls have undergone a change from the kernel version 2.6.39,and the previous functions are no longer valid.

Before 2.6.39, the ioctl calls would lock the BKL (Big kernel lock) and then execute the required functions. This was unnecessary with the new fine grained locking available in the kernel. Thus to improve the implementation of ioctl, a new operation called unlocked_ioctl was introduced and all the existing ioctls were migrated to this new implementation.

Thus stating 2.6.39 the older implementation is no longer available.

The changes required in implementation of the ioctl are.

The function in the driver has changed from

int ioctl_funcs(struct inode *inode, struct file *filp,unsigned int cmd, unsigned long arg)

long ioctl(struct file *filp,unsigned int cmd, unsigned long arg)

The inode is no longer passed as an argument.
The fops uses the name unlocked_ioctl

struct file_operations fops = { open: open, unlocked_ioctl: ioctl_funcs, release: release };

The basic working of ioctl remains same.
Here is a code that implements the above mentioned changes if the kernel version is more than 2.6.39, else uses the older interface.

Module: ioctl_basic.c

#include <linux/module.h>#include <linux/kernel.h>#include <linux/fs.h> // required for various structures related to files liked fops. #include <linux/semaphore.h>#include <linux/cdev.h> #include "ioctl_basic.h" #include <linux/version.h>static int Major; #if LINUX_VERSION_CODE > KERNEL_VERSION(2,6,36) #define UNLOCKED 1 #endif int open(struct inode *inode, struct file *filp) { printk(KERN_INFO "Inside open \n"); return 0; } int release(struct inode *inode, struct file *filp) { printk (KERN_INFO "Inside close \n"); return 0; } #ifdef UNLOCKED long ioctl_funcs(struct file *filp,unsigned int cmd, unsigned long arg) { int ret=0; switch(cmd) { case IOCTL_HELLO: printk(KERN_INFO "Hello ioctl world"); break; } return ret; } struct file_operations fops = { open: open, unlocked_ioctl: ioctl_funcs, release: release }; #else int ioctl_funcs(struct inode *inode, struct file *filp, unsigned int cmd, unsigned long arg) { int data=10,ret; switch(cmd) { case IOCTL_HELLO: printk(KERN_INFO "Hello ioctl world"); break; } return ret; } struct file_operations fops = { open: open, ioctl: ioctl_funcs, release: release }; #endif struct cdev *kernel_cdev; int char_arr_init (void) { int ret; dev_t dev_no,dev; kernel_cdev = cdev_alloc(); kernel_cdev->ops = &fops; kernel_cdev->owner = THIS_MODULE; printk (" Inside init module\n"); ret = alloc_chrdev_region( &dev_no , 0, 1,"char_arr_dev"); if (ret < 0) { printk("Major number allocation is failed\n"); return ret; } Major = MAJOR(dev_no); dev = MKDEV(Major,0); printk (" The major number for your device is %d\n", Major); ret = cdev_add( kernel_cdev,dev,1); if(ret < 0 ) { printk(KERN_INFO "Unable to allocate cdev"); return ret; } return 0; } void char_arr_cleanup(void) { printk(KERN_INFO " Inside cleanup_module\n"); cdev_del(kernel_cdev); unregister_chrdev_region(Major, 1); } MODULE_LICENSE("GPL"); module_init(char_arr_init); module_exit(char_arr_cleanup);

The steps of compiling and testing the module are same as shown in the post "Creating an ioctl command in linux "