How do I use ioctl() to manipulate my kernel module?

So I'm trying to write a kernel module that uses the linux/timer.h file. I got it to work inside just the module, and now I am trying to get it to work from a user program.

Here is my kernel module:

//Necessary Includes For Device Drivers.
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/errno.h>
#include <linux/proc_fs.h>
#include <asm/uaccess.h>
#include <linux/timer.h>
#include <linux/ioctl.h>

#define DEVICE_NAME "mytimer"
#define DEVICE_FILE_NAME "mytimer"
#define MAJOR_NUM 61
#define MINOR_NUM 0


static struct timer_list my_timer;

struct file_operations FileOps = 
    //No File Operations for this timer.

//Function to perform when timer expires.
void TimerExpire(int data)
    printk("Timer Data: %d\n", data);

//Function to set up timers.
void TimerSetup(void)
    setup_timer(&my_timer, TimerExpire, 5678);
    mod_timer(&my_timer, jiffies + msecs_to_jiffies(5000));

//Module Init and Exit Functions.
int init_module(void)
    int initResult = register_chrdev(MAJOR_NUM, "mytimer", &FileOps);

    if (initResult < 0)
        printk("Cannot obtain major number %d\n", MAJOR_NUM);

        return initResult;

printk("Loading MyTimer Kernel Module...\n");

return 0;
void cleanup_module(void)
    unregister_chrdev(MAJOR_NUM, "mytimer");
    printk("Unloading MyTimer Kernel Module...\n");

More specifically, I want my user program to call the TimerSetup() function. I know that I'll need to use ioctl() but I'm not sure how to specify in my MODULE FILE that TimerSetup() should be callable via ioctl().

Also, my second question: I was able to insmod my module and also mknod into /dev/mytimer with the correct major number. But when I tried to open() it so that I can get the file descriptor from it, it kept returning -1, which I'm assuming is wrong. I made sure the permissions were fine (in fact, I made it 777 just to be sure)... It still doesn't work... Is there something I'm missing?

Here is the user program just in case:

#include <stdio.h>

int main(int argc, char* argv[])
    int fd = open("/dev/mytimer", "r");
    printf("fd: %d\n", fd);

    return 0;


The example code you need can be found in drivers/watchdog/softdog.c (from Linux 2.6.33 at the time this was written), which illustrates proper file operations as well as how to permit userland to fill a structure with ioctl().

It's actually a great, working tutorial for anyone who needs to write trivial character device drivers.

I dissected softdog's ioctl interface when answering my own question, which may be helpful to you.

Here's the gist of it (though far from exhaustive) ...

In softdog_ioctl() you see a simple initialization of struct watchdog_info that advertises functionality, version and device information:

    static const struct watchdog_info ident = {
            .options =              WDIOF_SETTIMEOUT |
                                    WDIOF_KEEPALIVEPING |
            .firmware_version =     0,
            .identity =             "Software Watchdog",

We then look at a simple case where the user just wants to obtain these capabilities:

    switch (cmd) {
            return copy_to_user(argp, &ident, sizeof(ident)) ? -EFAULT : 0;

... which of course, will fill the corresponding userspace watchdog_info with the initialized values above. If copy_to_user() fails, -EFAULT is returned which causes the corresponding userspace ioctl() call to return -1 with a meaningful errno being set.

Note, the magic requests are actually defined in linux/watchdog.h , so that the kernel and userspace share them:

#define WDIOC_GETSUPPORT        _IOR(WATCHDOG_IOCTL_BASE, 0, struct watchdog_info)
#define WDIOC_GETTEMP           _IOR(WATCHDOG_IOCTL_BASE, 3, int)

WDIOC obviously signifying "Watchdog ioctl"

You can easily take that a step further, having your driver do something and place the result of that something in the structure and copy it to userspace. For instance, if struct watchdog_info also had a member __u32 result_code. Note, __u32 is just the kernel's version of uint32_t.

With ioctl(), the user passes the address of an object, be it a structure, integer, whatever to the kernel expecting the kernel to write its reply in an identical object and copy the results to the address that was provided.

The second thing you are going to need to do is make sure your device knows what to do when someone opens, reads from it, writes to it, or uses a hook like ioctl(), which you can easily see by studying softdog.

Of interest is:

static const struct file_operations softdog_fops = {
        .owner          = THIS_MODULE,
        .llseek         = no_llseek,
        .write          = softdog_write,
        .unlocked_ioctl = softdog_ioctl,
        .open           = softdog_open,
        .release        = softdog_release,

Where you see the unlocked_ioctl handler going to ... you guessed it, softdog_ioctl().

I think you might be juxtaposing a layer of complexity that really doesn't exist when dealing with ioctl(), it really is that simple. For that same reason, most kernel developers frown on new ioctl interfaces being added unless they are absolutely necessary. Its just too easy to lose track of the type that ioctl() is going to fill vs the magic you use to do it, which is the primary reason that copy_to_user() fails often resulting in the kernel rotting with hordes of userspace processes stuck in disk sleep.

For a timer, I agree, ioctl() is the shortest path to sanity.

You are missing a .open function pointer in your file_operations structure to specify the function to be called when a process attempts to open the device file. You will need to specify a .ioctl function pointer for your ioctl function as well.

Try reading through The Linux Kernel Module Programming Guide, specifically chapters 4 (Character Device Files) and 7 (Talking to Device Files).

Chapter 4 introduces the file_operations structure, which holds pointers to functions defined by the module/driver that perform various operations such as open or ioctl.

Chapter 7 provides information on communicating with a module/drive via ioctls.

Linux Device Drivers, Third Edition is another good resource.

Minimal runnable example

Tested in a fully reproducible QEMU + Buildroot environment, so might help others get their ioctl working. GitHub upstream: kernel module | shared header | userland.

The most annoying part was understanding that some low ids are hijacked: ioctl is not called if cmd = 2 , you have to use _IOx macros.

Kernel module:

#include <asm/uaccess.h> /* copy_from_user, copy_to_user */
#include <linux/debugfs.h>
#include <linux/module.h>
#include <linux/printk.h> /* printk */

#include "ioctl.h"


static struct dentry *dir;

static long unlocked_ioctl(struct file *filp, unsigned int cmd, unsigned long argp)
    void __user *arg_user;
    union {
        int i;
        lkmc_ioctl_struct s;
    } arg_kernel;

    arg_user = (void __user *)argp;
    pr_info("cmd = %x\n", cmd);
    switch (cmd) {
        case LKMC_IOCTL_INC:
            if (copy_from_user(&arg_kernel.i, arg_user, sizeof(arg_kernel.i))) {
                return -EFAULT;
            pr_info("0 arg = %d\n", arg_kernel.i);
            arg_kernel.i += 1;
            if (copy_to_user(arg_user, &arg_kernel.i, sizeof(arg_kernel.i))) {
                return -EFAULT;
        case LKMC_IOCTL_INC_DEC:
            if (copy_from_user(&arg_kernel.s, arg_user, sizeof(arg_kernel.s))) {
                return -EFAULT;
            pr_info("1 arg = %d %d\n", arg_kernel.s.i, arg_kernel.s.j);
            arg_kernel.s.i += 1;
            arg_kernel.s.j -= 1;
            if (copy_to_user(arg_user, &arg_kernel.s, sizeof(arg_kernel.s))) {
                return -EFAULT;
            return -EINVAL;
    return 0;

static const struct file_operations fops = {
    .owner = THIS_MODULE,
    .unlocked_ioctl = unlocked_ioctl

static int myinit(void)
    dir = debugfs_create_dir("lkmc_ioctl", 0);
    /* ioctl permissions are not automatically restricted by rwx as for read / write,
     * but we could of course implement that ourselves:
     * */
    debugfs_create_file("f", 0, dir, NULL, &fops);
    return 0;

static void myexit(void)


Shared header:

#ifndef IOCTL_H
#define IOCTL_H

#include <linux/ioctl.h>

typedef struct {
    int i;
    int j;
} lkmc_ioctl_struct;
#define LKMC_IOCTL_MAGIC 0x33
#define LKMC_IOCTL_INC_DEC _IOWR(LKMC_IOCTL_MAGIC, 1, lkmc_ioctl_struct)



#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

#include "../ioctl.h"

int main(int argc, char **argv)
    int fd, arg_int, ret;
    lkmc_ioctl_struct arg_struct;

    if (argc < 2) {
        puts("Usage: ./prog <ioctl-file>");
        return EXIT_FAILURE;
    fd = open(argv[1], O_RDONLY);
    if (fd == -1) {
        return EXIT_FAILURE;
    /* 0 */
        arg_int = 1;
        ret = ioctl(fd, LKMC_IOCTL_INC, &arg_int);
        if (ret == -1) {
            return EXIT_FAILURE;
        printf("arg = %d\n", arg_int);
        printf("ret = %d\n", ret);
        printf("errno = %d\n", errno);
    /* 1 */
        arg_struct.i = 1;
        arg_struct.j = 1;
        ret = ioctl(fd, LKMC_IOCTL_INC_DEC, &arg_struct);
        if (ret == -1) {
            return EXIT_FAILURE;
        printf("arg = %d %d\n", arg_struct.i, arg_struct.j);
        printf("ret = %d\n", ret);
        printf("errno = %d\n", errno);
    return EXIT_SUCCESS;

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