07_reading_from_private_data

Reading from private_data

In the last example we read from a string that was generated on the fly, in the body of our read callback.

Now we will make some changes in our device struct so the data can be contained in there:

struct skull_d {
    int maxData;
    char data[256];
    struct cdev skull_cdev;
};

static struct skull_d skull = { .maxData = 256, .data = "hey! you are reading from the device!\n\0" };

As we saw, in the open callback we assigned this struct to our private_data field of of our file.

static int open(struct inode* inode, struct file* filp) {
    // Getting char device struct and adding it to private_data field
    struct skull_d* dev;
    dev = container_of(inode->i_cdev, struct skull_d, skull_cdev);
    filp->private_data = dev;
//...
}

This operation enable us to access that struct in any other callback, so we can now print the string stored in there:

static ssize_t read(struct file* filp, char __user* buf, size_t len, loff_t* off) {
    int result;
    size_t count;
    struct skull_d* dev = filp->private_data;
    char* data = dev->data;
    pr_alert("%s - READ TRIGGERED!\n", PREF);
    count = strlen(data);
    result = copy_to_user(buf, data, count);
    return result == 0 ? count : -EFAULT;
}

So if we compile, load the device and use head -n 1 to read one line: