Writing to a simple char device module from the user space with echo - Cannot see the write taking place

Question

I took some example in order to learn more about kernel modules.

So I took a basic example and tried it on my raspberry pi. The loading and the unloading is logged to the kernel log but I want to trigger the write of the device operation. In order to trigger it, I tried echo "hi" > /dev/hello and echo "hi" > sudo tee /dev/chardev. As far as I have understood, this should trigger the device_write and I should see a log entry in dmesg. But so far nothing. So is my approach wrong and the device_write is triggered in another way?

Here is my code. I tried also different other examples. All the same. the modules are loaded and unloaded when I do rmmod and I see no other error messages. Everything else seems to work fine.

/*
 *  chardev.c: Creates a read-only char device that says how many times
 *  you've read from the dev file
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <asm/uaccess.h>    /* for put_user */

MODULE_LICENSE("GPL");
MODULE_AUTHOR("User");
MODULE_DESCRIPTION("A simple kernel module");
MODULE_VERSION("0.1");
/*  
 *  Prototypes - this would normally go in a .h file
 */
int init_module(void);
void cleanup_module(void);
static int device_open(struct inode *, struct file *);
static int device_release(struct inode *, struct file *);
static ssize_t device_read(struct file *, char *, size_t, loff_t *);
static ssize_t device_write(struct file *, const char *, size_t, loff_t *);

#define SUCCESS 0
#define DEVICE_NAME "chardev"   /* Dev name as it appears in /proc/devices   */
#define BUF_LEN 80      /* Max length of the message from the device */

/* 
 * Global variables are declared as static, so are global within the file. 
 */

static int Major;       /* Major number assigned to our device driver */
static int Device_Open = 0; /* Is device open?  
                 * Used to prevent multiple access to device */
static char msg[BUF_LEN];   /* The msg the device will give when asked */
static char *msg_Ptr;

static struct file_operations fops = {
    .read = device_read,
    .write = device_write,
    .open = device_open,
    .release = device_release
};

/*
 * This function is called when the module is loaded
 */
int init_module(void)
{
        Major = register_chrdev(0, DEVICE_NAME, &fops);

    if (Major < 0) {
      printk(KERN_ALERT "Registering char device failed with %d\n", Major);
      return Major;
    }

    printk(KERN_INFO "I was assigned major number %d. To talk to\n", Major);
    printk(KERN_INFO "the driver, create a dev file with\n");
    printk(KERN_INFO "'mknod /dev/%s c %d 0'.\n", DEVICE_NAME, Major);
    printk(KERN_INFO "Try various minor numbers. Try to cat and echo to\n");
    printk(KERN_INFO "the device file.\n");
    printk(KERN_INFO "Remove the device file and module when done.\n");

    return SUCCESS;
}

/*
 * This function is called when the module is unloaded
 */
void cleanup_module(void)
{
    /* 
     * Unregister the device 
     */
    unregister_chrdev(Major, DEVICE_NAME);

}

/*
 * Methods
 */

/* 
 * Called when a process tries to open the device file, like
 * "cat /dev/mycharfile"
 */
static int device_open(struct inode *inode, struct file *file)
{
    static int counter = 0;

    if (Device_Open)
        return -EBUSY;

    Device_Open++;
    sprintf(msg, "I already told you %d times Hello world!\n", counter++);
    msg_Ptr = msg;
    try_module_get(THIS_MODULE);

    return SUCCESS;
}

/* 
 * Called when a process closes the device file.
 */
static int device_release(struct inode *inode, struct file *file)
{
    Device_Open--;      /* We're now ready for our next caller */

    /* 
     * Decrement the usage count, or else once you opened the file, you'll
     * never get get rid of the module. 
     */
    module_put(THIS_MODULE);

    return 0;
}

/* 
 * Called when a process, which already opened the dev file, attempts to
 * read from it.
 */
static ssize_t device_read(struct file *filp,   /* see include/linux/fs.h   */
               char *buffer,    /* buffer to fill with data */
               size_t length,   /* length of the buffer     */
               loff_t * offset)
{
    /*
     * Number of bytes actually written to the buffer 
     */
    int bytes_read = 0;

    /*
     * If we're at the end of the message, 
     * return 0 signifying end of file 
     */
    if (*msg_Ptr == 0)
        return 0;

    /* 
     * Actually put the data into the buffer 
     */
    while (length && *msg_Ptr) {

        /* 
         * The buffer is in the user data segment, not the kernel 
         * segment so "*" assignment won't work.  We have to use 
         * put_user which copies data from the kernel data segment to
         * the user data segment. 
         */
        put_user(*(msg_Ptr++), buffer++);

        length--;
        bytes_read++;
    }

    /* 
     * Most read functions return the number of bytes put into the buffer
     */
    return bytes_read;
}

/*  
 * Called when a process writes to dev file: echo "hi" > /dev/hello 
 */
static ssize_t
device_write(struct file *filp, const char *buff, size_t len, loff_t * off)
{
    printk(KERN_ALERT "Sorry, this operation isn't supported.\n");
    return -EINVAL;
}

I tried echo "hi" > sudo tee /dev/chardev or sudo bash -c 'echo "54" > /dev/chardev' and then tried to see the "Sorry, this operation isn't supported" in dmesg.

Answer 1

1. You must check is "/dev/chardev" presented after insmod
2. Try use dmesg and "cat /proc/kmsg"
3. Also try check error in process of works with file, the simple way is create small program and print errno.